The Scenario of Clays and Clay Minerals Use in Cosmetics/Dermocosmetics

: The use of clays in beauty care comes from ancient times, with therapeutic use since prehistory, and it is considerably relevant in the current cosmetic industry worldwide. In our review, we described types of clay and clay minerals used in cosmetics and dermocosmetics, compositions, usages as active compounds and cosmetic ingredients/starting materials, and observations about formulation techniques. From this review, we observed that although much scientiﬁc and specialized literature has reported the characterization of clays, only some involved efﬁcacy tests when incorporated into cosmetic products, mainly concerning haircare applications. Our review could be considered and encouraged in the coming years to provide scientiﬁc and technical information for the cosmetic industry regarding the multifunctional use of clays and clay minerals.


Introduction
The cosmetic industry has been using natural components in products since the beginning of history.The use of clays in beauty care is old, with therapeutic uses since prehistory [1][2][3].Reports of treatments with "medicinal earth", mainly constituted by clay minerals, are present in ancient civilizations' scriptures, such as ancient China, Egypt, and Greece [2,4].Nowadays, there is a demand for natural ingredients by consumers, mainly for cosmetic products [5].
Minerals used for cosmetic purposes are mainly natural clay minerals due to the high cost and difficulty in industrial production.Concerning topical use, they are applied either as cosmetics or dermatological protectors [2,3], acting as starting materials, adjuvants, or active ingredients, and as vehicles.
A cosmetic product is a preparation made for external topic application (skin, hair, lips, nails) or to be applied in the oral cavity (teeth and mucous membranes) aiming to clean, beautify, perfume, change its appearance, correct body odors, protect, and/or keep in good condition [6][7][8][9].According to our point of view, cosmetics and dermocosmetics can act further ahead as previously described, also contributing to the health maintenance of the consumers/patients.A broad range of cosmetic products can be formulated with clay minerals to clean, moisturize, and treat/improve skin conditions, like gynoid lipodystrophy, dandruff, seborrheic dermatitis, and acne [8].By improving such conditions, they act indirectly on consumers' health, life quality, and self-esteem.
There is an important advantage in using clays for cosmetic purposes as they are a lowcost, environmentally friendly, natural, and abundant component, which are easy to apply and remove, dry and harden fast, and present low toxicity risk when used in adequate conditions, respecting the specifications of official compendiums [8,[10][11][12][13][14].They also are used due to their high surface area, rheological properties, and excellent ion exchange capability [11,14,15].
Cosmetics with clays can be formulated in several forms, such as ointments, gels, creams (emulsions), and pastes.Those products must be formulated considering their adequate rheology profile (must be appropriate to maintain contact with application region) and sensory aspects (must be cosmetically acceptable), which are influenced by the clays' physicochemical properties (e.g., particle size) [16].Due to their composition, uses, and properties, clays are relevant in cosmetology.In such products, clays can act either as active or as cosmetic ingredients, influencing stability, rheology, color, etc.As actives, they are widely used for skin cleansing, oil reduction/control (skin and scalp), substance adsorption, antiaging, ultraviolet (UV) radiation protection (sunscreens), and ion exchange with skin, in the function of color, qualitative and quantitative mineralogical composition, particle size, and shape, structure, and ion exchange capacity.They can also be used in makeup products [2,[16][17][18][19].
Despite the importance of clays and clay minerals in the cosmetic industry, specialized literature about this theme is scarce.Therefore, in our review, we provided updated information concerning clays that are used in cosmetics/dermocosmetics, both as active and as starting materials, and gathered investigation findings to better understand future perspectives and deficiencies in this proficuous field.

Clays x Clay Minerals
The term "clay" does not have a consensus in the specialized/scientific literature.As such, it is difficult to define due to the variety of materials known as "clays" and for being used in several areas, such as chemistry, mineralogy, geology, etc. [20].Therefore, clays are classified as natural inorganic rock or soil materials, composed of finely divided particles (inferior to 2 µm) with some plasticity while mixed with water and which hardens after drying.They are formed by clay minerals, organic matter, salt impurities, feldspars, and other minerals, like quartz, dolomite, calcite, iron, and/or aluminum oxides [5,6,13,15,18,[20][21][22].It is applied to all small-sized particles found in soils or sediments including phyllosilicates, quartz, feldspars, carbonates, sulphates, iron and aluminum oxides, humus, and other mineral and/or organic components [6,13].Clays can be formed by one clay mineral, or (more commonly) a mixture of clay minerals with the prevalence of one.Those clay minerals give clays several properties that allow their use in many industries [13].

Types of Clay
Clays for cosmetic use are produced in many colors and the color difference is caused by the proportion of the minerals in it.As a raw material for cosmetic purposes, they are commercialized based on their color [11,15,16,18,20,21].The color, which is a reference for the purchaser, is mentioned during the marketing and is often used aligned to the clay's properties [1].
The presence and proportion of minerals in clays interfere with their color and formulation stability [21].Their colors also result from the crystalline structure in a certain state, being affected by composition, oxidation of structural cations, ionic charge, and ion position.The amount of water can also affect the color aspect.Color can come from matter associated with clay [1].According to Gubitosa and co-workers (2019), different clays can be found in nature depending on the presence of iron and its chemical state.For example, clays with bivalent iron present a green color, those with trivalent iron are red, and those that do not contain iron are white [5].
The clays of each color present different cosmetic attributes due to their composition, as shown in Table 1 [18,20,21].However, literature correlating color to cosmetic use and concerning chemical/mineralogical compositions of clays are infrequent [11,15].Matike and co-workers (2011) correlated clay colors with sunscreen ability and mentioned that most clay soils used as UV filters corresponded to hematite's and goethite's colors, which are iron oxides that are responsible for reddish and yellowish colors [16].Wargala and coworkers (2021) reported that the application of clays in cosmetics is directly related to their composition.For example, clays rich in Si present hydration properties, mitigate skin inflammatory processes, and can be used to contribute to skin regeneration/protection; and clays rich in Al provide hydration, pigment dispersion, and adsorption of melanin.For clays containing Si, Al, Ca, Ti, Fe, and K, the authors mentioned antiseptic, antibacterial, and regenerative activities, as well as cell renewal action, circulation activation, and adsorption of impurities [25].

Structure and Composition
Clays are composed by solid, liquid, and gas substances-solid particles form a skeleton and the spaces between these particles are filled by gas and/or liquid [18].Mineralogic composition, particle shape (laminar or fibrous), and particle granulometric distribution are the main factors that determine clay's physicochemical properties and properties of the final product obtained [18,21].Therefore, it is important to know these characteristics when developing a cosmetic to choose formulation components and preparation technique, culminating in a stable formulation [21].Other characteristics that differentiate clay types are ion exchange capability, the nature of the exchanged cations, specific area, dispersion viscosity, plasticity, among others [18].
Clays may be found in different types of soils due to their structure, colors, and metals that compose them, which contribute to their function.Clays used in cosmetics are formed by metal such as aluminum, iron, magnesium, and titanium, which contribute to their functions [20,21].
Clay minerals are the mineral constituents of clays, which are normally crystalline and formed by hydrated aluminum silicate [20].They contain in their composition silicon, aluminum, water, and frequently iron, alkali metals, and alkaline earth metals [18].According to Daneluz and co-workers (2020), in general, clay minerals contain Si, Al, Fe, Ti, Mg, Ca, K, Na, phyllosilicates, oxides, carbonates, kaolinite, chlorides, etc.According to the authors, these elements are important for cosmetology due to their effect on skin.Some examples include hematite (Fe 2 O 3 ) which acts as pigment, opacifier, antiseptic, and stimulates cell renewal; rutile (TiO 2 ) which provides photoprotection; kaolinite which provides renewal of the skin, hydration, and soothing effect; and ZnO and MgO which are invigorating [19].
According to Balduíno (2016), the complexity and amount of different clay minerals in clays makes it difficult to classify them and the author considers that there are no equal clays-each one will differ in at least one property.This variability is due to varied geology formation conditions [20].Crystalline clay minerals can be divided in two classes, which are composed by families, groups, and subgroups, as we can see in Table 2 [20,26].[6,13,18,20,21,26].

Family Group Subgroup
Silicates with lamellar structure (phyllosilicates) Note: The word 'bentonite' is employed for a plastic, colloidal, swelling clay consisting of a smectite mineral regardless from its origin [11].
There is also a specific third type of layer with intercalation of 2:1-type layer and an additional octahedral layer ("hydroxide layer").This occurs in the chlorite family.
These structures are based on a perfect model.However, natural clay minerals present defects on their ions concerning nature and position, which influences their properties.Disturbed zones favor ion and molecule trapping from the external medium, thus influencing their properties [1].
The proportion of tetrahedral and octahedral layers can vary between clay minerals.The bond between those crystallin layers determines the different clay mineral structures and families they belong to (Table 2) [20].
Chemical bonds between atoms inside the layer are covalent and, therefore, strong.Adjacent layers are connected parallelly one above the other by Van der Waals bonds and, therefore, are considered weak.The space between layers is called interlayer space.This allows layers separation when submitted to excess of water or under mechanical force [20,21].
These two structure types behave differently when dispersed in polar solvents [6,9,13,27]: • 1:1-type minerals do not swell in contact with polar solvents [6,13].• 2:1-type minerals swell in contact with polar solvents, creating structured systems with interesting rheological properties [6].They lead typically to gels with pseudoplastic behavior.After hydration, a tridimensional net is built leading to sharply higher viscosity.When tension is applied, most of the structure breaks as shear occurs [13].
Tetrahedral layers are formed by an atom of silicon in the center and four atoms of oxygen in the vertices (SiO 4 ).Silicon atoms (Si +4 ) may be replaced by aluminum (Al +3 ) and occasionally by iron (Fe +3 ), causing negative charges on the faces [18,20,21].
Octahedral layers are formed by six hydroxyl groups or oxygen atoms in the vertices of an octahedron, and an atom of either aluminum (Al +3 ), magnesium (Mg +2 ), or iron (Fe +2 ) in the center [20,21].The substitution of aluminum by magnesium or iron also causes charge deficiency and the particle's surface becomes negatively charged.This is compensated by the adsorption of interlamellar cations like Na + in the layers' faces [21].
Therefore, most clay minerals are negatively charged on the faces (as mentioned above) and have a pH-dependent charge on the edges (positive in acid or neutral solutions and may become negative with pH increase) [8,21].The fact that clay minerals are charged is one of the main reasons for their cation exchange capacity, which is one of the aspects responsible for their use in cosmetics [8].Based on charge, clay minerals can also be classified as cationic (most abundant) and anionic clay minerals (uncommon) [8].
High repellent potential in the layers' surfaces contributes to increasing the space among them, causing water penetration in the interlayer space.Therefore, some clay minerals (mainly smectites) have an expansive structure where all layer surfaces are open for hydration [21].
Several characterization techniques may be used to identify clay components, behavior, and structure.As examples, we can mention X-ray diffraction, X-ray fluorescence, scanning electronic microscopy, thermal analysis, and Fourier transform infrared spectroscopy.These techniques allow verification that most clay minerals are hydrated aluminum silicates, which present a defined crystalline structure and may contain non-clay materials, organic and inorganic substances, adsorbed cations, organic matters, and soluble salts.These components interfere with the mineralogical composition and properties of each clay [18].

Demands for Cosmetic Use
Among 4500 minerals known today, and only around 30 are used in the pharmaceutical and cosmetic industries (including kaolin, talc, smectites, and fibrous clays) due to safety requirements they must fulfill.According to the Cosmetic Ingredient Review (CIR) (2023), kaolin has the most reported uses in cosmetics, followed by bentonite [28].Clays must be submitted to a series of purification treatments and characterization tests to meet strict pharmacopeial specifications before cosmetic use [29].Clays, as actives and as starting materials, must [4,6,8,[12][13][14][15]18,20,27,30]: Fulfill chemical requirements-stability, purity, and chemical inertia.

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Fulfill physical requirements-texture, water content, particle size (must present fine granulometry), and be pH compatible with the region of application.Therefore, the most used clay minerals in this segment are smectites (montmorillonite, saponite, and hectorite), fibrous clay minerals (palygorskite and sepiolite), kaolinites, and talc [4,5,12,20].Clay minerals' high adsorption capacity allows them to adsorb toxins, impurities, oiliness, secretion, bacteria, and viruses.The high cationic exchange capability may offer vital chemical elements to the organism that are present in minerals, such as sulfur, phosphorus, sodium, potassium, magnesium, copper, iron, zinc, and manganese [4] The clay's cation exchange capacity, together with other formulation characteristics, may interfere with the percutaneous depth that ions may reach in the cutaneous tissue and, even, the absorption.This has a direct impact on clay's safety and efficacy [14].
Natural deposits of those clay minerals are rarely pure and may present chemical composition variations.They are composed of two or more clay minerals mixed with variable amounts of non-clay materials (for example quartz, feldspars, carbonates, oxides, amorphous materials, and organic matter).Thus, before use, clay raw materials are treated to increase and achieve quality patterns.The physical and chemical treatments to which clays are submitted may include desiccation, pulverization, bleaching, magnetic separation, size fractionation, chemical modification, and drying, among others.In some cases, it is necessary to remove specific associated substances that exceed pharmacopoeia requirements or modify appropriate properties (e.g., quartz, heavy metals, dolomite) [8,9,12,27].To exemplify clay purity requirements, "food-grade bentonite should contain no more than 5 mg/kg arsenic, no more than 15 mg/kg lead, and no more than 1000 colony-forming units (CFU)/g aerobic microbes.Bentonite should be negative for Escherichia coli in 25 g"; and "food-grade kaolin should contain no more than 3 mg/kg arsenic and no more than 10 mg/kg lead" [28].International regulations must be considered before selecting clays to be used in cosmetics aiming to fulfill safety requirements.Therefore, selecting suppliers is of utmost importance.According to Bastos and Rocha (2022), current studies evidenced the need to establish quality criteria and certification for clay-based products for topical/cosmetic use and to adopt methodologies for clay decontamination before incorporating them into formulations, to achieve the required limits assuring safety [14].
According to Silva (2011), the most important clay properties for their usage choice are mineralogical composition, particle shape and granulometric distribution, plasticity, mechanical resistance, linear drying retraction, compaction, thixotropy, reactive surface (absorption, ionic exchange, swelling), low toxicity, as well as therapeutic and viscosity dispersion [18].Also, they must be easy to apply and remove, and dry quickly in contact with skin [18].In addition to the several positive properties of clays to be considered for the cosmetic industry, it is not trivial to obtain a clay ingredient, regarding the relevant characteristics.Commercially available clays for cosmetic use must include the following information: substance identification (mineralogical and chemical composition), hazard identification, handling and storage, stability, reactivity, toxicology, and physicochemical properties [15].
Despite the abundance of clay minerals in nature, some have been synthetized to allow obtaining purer raw materials with a homogeneous structure and composition, as well as lower contaminations, thus meeting industrial requirements.Also, they can be enriched with mineral elements, such as Zn, Co, and Ni [8].In addition, clay minerals can be modified to improve performance and expand their applications.Modifications can be chemical alterations to promote surface reactivity (homoionic clays), interactions with organic substances to improve hydrophobicity (organoclays) and incorporation into polymers to create clay-polymer composites [8,9].
Natural deposits of clays used for cosmetics are rarely pure and may vary in chemical composition.Therefore, clays used as raw materials for cosmetics benefit from improvement techniques to eliminate accessory minerals, enhance physicochemical properties, and increase quality by physical and chemical treatments such as magnetic separation, flotation, drying, calcination, bleaching, size fractionation, among others [12].

Toxicological and Safety Aspects of Clays and Clay Minerals
Clays must present a zero to low toxicity profile for further use in cosmetics/ dermocosmetics, as well as being submitted to purification processes.Due to their high adsorbing capacity, they may accumulate toxic substances, heavy metals (such as Sb, As, Cd, Pb, Ni, and Tl), and micro-organisms, which must be well-verified before use.Also, the presence of organic matter is commonly associated with pathogenic microorganisms in clays; therefore, as a rule, before using clays in cosmetic products, they undergo fine processing to reduce potentially toxic elements and pathogens-for example: refining, beneficiation, and sterilization/decontamination [25,31].
Clays' and clay minerals' toxicology can be assessed by in vitro and in vivo assays.However, in vitro assays are mainly for screening purposes and are useful to obtain mechanism-derived information [32].Toxicologic assays' results may vary according to the type of clay, route of administration, dose, experimental times, etc.
Maisanaba and co-workers (2015) studied/reviewed toxicological aspects of clay minerals and derived nanocomposites frequently used in food packaging (mainly kaolinite, montmorillonite, and sepiolite), and concluded that toxicological evaluation is needed when taking into account that clays have distinct toxicological profiles and their modification can alter it.Toxicity results in the literature vary thoroughly, with most in vitro toxicity showing cell death and toxic effects (oxidative stress, etc.), although in vivo studies in humans and animals demonstrated lower toxicity [32].
The dose is an important issue regarding the safety of cosmetic ingredients.Amongst clays intended for cosmetic use, kaolin presents the highest maximum concentration usage in leave-in formulations (up to 53.2% for manicuring preparations, 16% in leave-on dermal formulations), followed by bentonite (8% for face and neck formulations) [28].Also, some clay ingredients have been reported to safely be used in formulations that may be incidentally ingested, like kaolin in lipsticks (up to 14.5%); and formulations that may get in contact with the eyes, like kaolin in eye shadows (up to 8.5%).As far as cosmetic regulations in the European Union regarding clays is concerned, there are no restrictions on attapulgite, clay, fuller's Earth, hectorite, illite, or montmorillonite.Bentonite and kaolin are listed in Annex IV (allowed colorants) as they contain calcium, magnesium, or iron carbonate, ferric hydroxide, quartz-sand, and mica, among others as impurities [28].Clays have been submitted to bioavailability toxicokinetic studies using human skin as a model membrane and diffusion cells to assess (trans)cutaneous permeation of heavy metals (such as lead, arsenic, chromium, and aluminum) after application of three clay pastes (white montmorillonite, kaolin, and clay).Diffusion cells were incubated for 24 h, and diffusion and storage liquids were analyzed for metal content.No detectable quantity of heavy metals was found, leading to the conclusion that traces of heavy metal in the clay pastes did not penetrate the skin [28].An acute inhalation assessment was performed following OECD TG 403, in which rats were exposed to 3.856 mg/L air of clay composed of illite (75%), kaolin (19%), and montmorillonite (6%) versus a control group (air).Both groups were exposed for 4 h and observed for 14 days.As a result, neither mortality nor clinical signs of toxicity were observed.Also, dermal sensitization was not reported in human repeated insult patch tests (HRIPTs) with a foot mask containing 3.5% bentonite, a clay mask containing 3.8% bentonite (108 subjects), a face cream containing 7.5% bentonite, a lip product containing 14.5% kaolin, and a clay mask containing 40% kaolin.However, one subject in a study of a clay mask containing 14.5% kaolin involving 103 subjects had moderate erythema and edema with papules through the induction and challenge phase.From this, it is considered that kaolin, attapulgite, bentonite, clay, fuller's Earth, hectorite, illite, and montmorillonite are safe to be used in cosmetics in the mentioned concentrations [28].
According to Gomes and co-workers (2021), topically applied clays may cause exposure to toxicity due to persistent skin adsorption of potentially toxic elements and compounds.Although rare, clays may harm human health when the particles are persistently inhaled (respiratory diseases, lung cancer, mesothelioma, or pneumoconiosis); persistently ingested, for example, in geophagy (which should not be the case for cosmetics); or dermally adsorbed (podoconiosis).The conditions' severity depends on the dose and the exposure duration.This justifies the importance of the sanitary safety of clays selected to be used in topical samples [25,31].
Heavy metals, such as Sb, As, Cd, Pb, Ni, and Tl, have been banned by the European Commission and restricted by the FDA with strict limits for cosmetics.Metals in cosmetics can accumulate on the skin and cause allergic reactions and internal organ damage (Hg and Pb)-topical and systemic side effects [25].
Another toxicological aspect that should be considered is related to the talc.Carcinogenic fibrous materials (asbestos) could be detectable in cosmetic-grade talc, which is used as a protective, abrasive, absorbent, anticaking, filler, opacifier, lubricating, and refreshing ingredient.Also, talc should not be applied on the skin when its barrier is not intact.Cosmetic talc's purity is required to be at least 90% [25,31].According to Wargala and co-workers (2021), the toxicity of the minerals is mostly related to the existence of asbestos or quartz from mining procedures [25].

Important Considerations When Formulating Clay-Containing Cosmetics
When formulating cosmetics with clays, parameters such as particle size and shape/ morphology, temperature, pH, agitation time, and speed, as well as other ingredients influence product stability and clays' dispersion in the medium [9,21].The formulation's pH is also important, concerning the effect and product safety on the skin-ideally, it should be compatible with the skin's value (slightly acid) in cleansing and beautification cosmetics, for example [16].
When preparing a face mask, for example, clay hydration (dispersion in water) is a critical step.It involves incorporation, powder humectation, and fragmentation of agglomerates.This step is affected by temperature, pH value, and agitation parameters (speed and time), as well as product composition.The stability of this dispersion may be affected by interactions between clay particles and between those particles with the liquid.The use of humectants (propylene glycol and glycerin, for instance), for example, helps retain water in the formulation and avoids its dehydration, thus enhancing product stability and improving product use [21].
When laminar clays are dispersed in polar solvents, a rigid network is formed by face-face and face-edge interactions.Also, laminar silicate gels are sensitive to electrolytes, which may influence the formed structure.As for fibrous clays, they form a 3D structure in water composed of interconnecting fibers, and they retain their stability in the presence of electrolytes in high concentrations [6,27].
Another important aspect is their swelling property.As previously mentioned, high repellent potential in layers' surfaces contributes to increasing the space between them, causing water penetration in the interlayer space.Therefore, when they are dispersed in the aqueous medium, the solvent is trapped between those layers by solvation.Swelling involves the separation of those layers until reaching balance [21].However, some clay gels may contract upon standing, expelling interstitial liquid (syneresis) [9].
Swelling is a required property to achieve high-viscosity systems.Smectite gels can swell by absorbing liquids and increasing volume, and the resulting material presents thixotropic behavior.Bentonite can swell to approximately 12-times its volume except in the presence of organic solvents [9,27].Clay's swelling degree can be influenced by several factors, such as present clay mineral types (expansive or non-expansive), the addition of electrolytes (may increase interaction between particles), and the presence of other hydrophilic substances that will compete with clay for water [21].
The formation of gel clays is influenced by the type of mineral and preparation technique.For example, the simple addition of water to bentonite does not lead to gel formation.To jellify it, bentonite must be sprinkled on hot water and the dispersion must rest for 24 h with occasional stirring after the clay has become wetted.Bentonite may also be dispersed in water after being triturated with glycerin or being previously mixed with a powder, like ZnO.To achieve dispersion in cold water, high-speed mixing equipment is required to enable swelling [9].Another relevant factor to consider is that formulated products should have adequate consistency to be suitable for cosmetic use.Viscosity must allow product application, and it must allow the product to remain in contact with the application area at least until achieving the desired effect.As far as concentration is concerned, the amount of clay applied to the formulation may vary from a small proportion until almost the total final mass [9].

Minerals
There are several minerals used as active ingredients in cosmetic products (Table 3).Their activity depends on their physical and physicochemical properties, as well as their chemical composition [30].Minerals with a high refraction index and good light scattering properties, like oxides, can be used in photoprotective formulations.Those with high sorption capacity and large surface areas may be used in powders and emulsions.The ones with proper hardness can be used as abrasives in toothpastes [8,30].Those with antiseptic properties (for example, borax, zincite, and goslarite, among others) are highly astringent, and astringency is controlled by their concentration.They may be incorporated in liquid (lotions) and solid formulations (powder) and can be used in deodorants/antiperspirants.They are toxic in high concentrations, so one should avoid continuous application on extensive skin areas or application on damaged skin [30].Minerals can be used as antibacterial agents depending on high sorption properties, large surface area, mineral content (release minerals that are toxic to bacteria), pH and oxidation state, and structure.The structure must ease the sorption of nutrients and/or disrupt bacterial envelope and/or impair bacterial metabolites' efflux.In general, the antibacterial effects from minerals come from the exchange of their soluble constituents that are toxic to bacteria [8].
Although kaolinite has a low cation exchange capacity and relatively small surface area compared to other groups, it can still absorb small substances such as proteins, bacteria, and viruses, justifying its use in cosmetics [27].
Minerals with high sorption capacity (kaolinite, talc, smectites) can be applied in dermatological protectors, which are solid or semisolid compositions that protect skin against external agents, exudations, and liquid excretions.Those minerals adhere to the skin, forming a film that provides mechanical protection against external agents, as well as taking up skin exudations.They also produce a water-poor medium unfavorable to bacterial growth and sorb bacteria, viruses, grease, and toxins, thus presenting some antiseptic activity [2,3,13,30].Also, minerals used in deodorant formulations can eliminate gases responsible for bad odor [6].The mentioned properties could also allow those minerals to be used in antipollution cosmetics, which is an increasingly used and desired claim in the market in skin and haircare products.In that case, minerals could be applied on skin to protect it against pollutants from the environment, which favor the skin aging process.This is a field that should be explored in future research.
Minerals with high refraction index and that also scatter light (rutile and zincite) are suitable as UV filters in photoprotective formulations.Their effectiveness as filters also depends on their stability against degradation by UV radiation.Natural rutile is not used; rutile's synthetic analogous (synthetic TiO 2 ) is used instead.Synthetic TiO 2 is a white powder with a high refraction index that reflects UV radiation and presents good photostability.Its light-scattering property depends on particle size-bigger particles (around 230 nm) scatter visible light, while smaller particles (around 60 nm) scatter UV rays and reflect visible light [8,30].As synthetic TiO 2 may give a white appearance on the skin due to bigger particle distribution, currently it has been used in very small particle sizes to avoid this undesirable effect.A size of 50 nm is considered an optimum particle size to provide good photoprotection without being white on the skin [30].
Iron oxides are commonly used in cosmetics depending on their color.Among them we can mention Fe 2 O 3 , which is the most used iron oxide in cosmetics.According to Wawrzynczak and co-workers (2016), iron pigments are considered stable and safe, mainly when synthesized.The authors mentioned that synthetic production allows the elimination of impurities normally found in natural minerals [33].
Regarding toothpastes, minerals can be used as desensitizing agents for sensitive teeth (niter), or as abrasive/polishing agents (calcite).Niter releases K + ions when dissolved in contact with saliva, which act on nerve endings in the dentine to inhibit pain sensation [30].Also, hydroxyapatite mineral particles in micro-or nanocrystalline forms can be added to kinds of toothpaste, where, through in vitro and in situ studies, they were deposited on and restored demineralized enamel surfaces.Hydroxyapatite toothpastes can remineralize enamel lesions, reduce/prevent demineralization, and present a caries reduction/prevention effect, without the risk of fluorosis [34,35].
Clay minerals with opacity and high sorption capacity (for example, palygorskite, sepiolite, kaolinite, smectites, and talc) are applied to cosmetic compositions (solid and semisolid) as opacifiers, mattifiers, and for imperfection coverage.They also form a protective film on the skin, adsorb excessive oiliness and toxins, and increase adherence to the preparation [6,13,30].Talc is widely used in a diversity of applications in cosmetic products.It is odorless and can be micronized to an ideal particle size, becoming a white powder.As it absorbs grease, it can be incorporated into makeup face products as a mattifier and/or oil control component [27].Talc is also widely used in children's cosmetics for its sorption and fluidity properties to absorb humidity and sweat in the diaper's zone.It cleans, deodorizes, lubricates skin surfaces, and acts as an antiseptic.It also keeps skin folds lubricated, avoiding friction [6,13].Micas are used in makeup cosmetics, like lipsticks and eyeshadows due to their high reflectance and iridescence.They are also applied to moisturizers to provide a luminous effect on skin [13,27,30].

Clays
Clay use depends on the type of clay mineral (mineralogical composition), type of layer (clay structure), and chemical composition.Also, differences in texture may affect rheological properties and adsorption capacity, even among identically structured clays [6,15].Application temperature also affects their use.When using clay minerals as facial masks, they can be applied directly on the skin at room temperature.However, when used to treat acne, it is advisable to apply at higher temperatures, as the heat increases perspiration and opens pilosebaceous orifices, thus favoring efficacy [11].On inflamed areas, the application temperature should be lower than body temperatures, so that the mixture of water and clays will cool the inflamed treated area, acting as an anti-inflammatory agent [2].
As examples of cosmetic products that may contain clays, we can mention exfoliants, masks, sunscreens, soaps, shampoos, toothpastes, deodorants, makeups (foundations, eye shadows, lipsticks, etc.), and facial skincare products, among others [4,5,36].According to Velasco and co-workers (2016), "clays are mostly used in face masks due to their high absorbency levels on skin surface, such as greases, toxins and even bacteria and viruses".They are also used for cleansing and lifting effects.Still, there are few studies concerning the impact of clay masks on skin biomechanical properties [11].Face masks can contain more than 25% of solid phase dispersed in liquid and are applied on the skin during 10 to 25 min in a layer of 1 to 2 mm thick.After water evaporation, the mask hardens and contracts, causing mechanical tension, slight physical exfoliation, and astringency [13,21].
Another important characteristic that allows clays to be used in cosmetics is their detergent property.Some clays behave like detergents when wet with water, as well as remove impurities, what makes them an excellent choice for products like soaps and shampoos [5].This property also enables their use as emulgents or emulsifiers [36].
Topical formulations using clays as active components, like facial masks, applied to skin during a certain period, trigger a flow that transports metabolic products, cell particles, and bacterial toxins out of the skin to adhere to the clay.In addition, clay particles absorb excess of sebum, impurities and skin exudates, cleanse pores, and improve blood flow, thus enhancing oxygen and nutrients' supply to skin [2,6,38].
The absorption capacity of cutaneous exudates may be related to particles' porosityporous particles from minerals with a large surface area can adhere to the skin, forming a film with mechanical protection and oil retention properties [22].Meier and co-workers (2012) studied the efficacy of facial masks with clay and jojoba oil against mild acne.The results showed that the proposed treatment reduced acne lesions, such as papules, pustules, and comedones.For the study, participants used the product 2-3 times a week for 6 weeks with 15-20 min contact per application and further removal.Lesions were counted before and after treatment for comparison [38].
As clays have high absorption power, they can adhere to the skin and form a pellicle that protects against external chemical and physical agents [5,10].This property is important for the retention of skin oiliness, contributing to the skin regenerating potential [22].Due to their capacity to eliminate excessive oiliness and toxins from the skin, they are considered effective against several dermatological conditions, such as acne [2,37].
As they are rich in sulphidric acid, they present bactericide, and fungicide properties [4].Lately, clays have received special attention concerning their potent antimicrobial properties.Studies demonstrated the in vitro broad-spectrum antibacterial activity of an iron-rich clay that was applied to treat Buruli ulcers.However, studies revealed that only a few deposits showed antibacterial properties [39].In 2020, Gomes and co-workers published an overview of antibacterial clays.The authors reported that clays did not present bactericidal characteristics when in a dry state, i.e., bactericidal activity only occurs in hydrated clays.Also, not every clay presents such activity, and only some have this function and against certain types of bacteria.Clays containing illite and smectite, that bear ferrous iron in the structures, and clays bearing one or more ferrous iron-rich-associated phases (pyrite, marcasite, magnetite, pyrrhotite, and goethite) present antibacterial activity, since they can inhibit microorganisms' growth or disrupt cell membranes.In addition to iron, other metals/ions contribute to antibacterial function of clays, such as Ag, Cu, Zn, and Au [40].
Clays with high amounts of silicon mean they can be used for skin hydration and to contribute to skin renewal/regeneration [37].Clays with high sorption capacity may be used in cosmetics as an opacifier, mattifier, and for skin imperfection coverage [5].Massage practices with clays suspended in liquid vehicles explore their slight abrasive effect for physical exfoliation of the skin.This property is also used in shampoos and soaps.For toothpastes, clays can also be used as abrasives, as well as for their properties of impurity absorption [1].
They may also be used as physical filters in photoprotective formulations [10,16,17].To be used as filters, they must have a high refraction index and optimal light dispersion properties [36].This property is also affected by particle size.Small particle sizes allow better skin coverage, reducing the intensity of UV radiation reaching the skin [16].
In addition, they are capable of invigorating tissues, activating microcirculation, presenting lifting effect, softening skin, and reducing oiliness due to absorption properties [18].
As clays are rich in iron, silicon, magnesium, titanium, and potassium, they present antibacterial, antiseptic, and regenerative efficacy; contribute to cell renewal; absorb impurities; and activate microcirculation, therefore, are suitable to be used as active compounds for numerous cosmetic products.The importance of those minerals in cosmetology stems from their assumed effects on skin, e.g., iron is an antiseptic and catalyzes cell renewal; silicon helps to renew/regenerate and hydrate the skin; zinc and magnesium are invigorating; potassium acts on circulation and tissue invigoration; titanium is used as a UV filter [18,22].Therefore, they have been successfully used in haircare and hair therapy through application protocols on the scalp in patients with seborrheic dermatitis, psoriasis, dandruff, and seborrhea [5,41].In those cases, clays may be associated with essential oils for synergic effect to clean, nourish, and revitalize the scalp [23].
The application of clays on the scalp (for example, as hair treatment masks) allows the removal of dead cells (slight physical exfoliation); stimulates local cutaneous microcirculation (thus nourishing the scalp); eliminates impurities, dirt, excessive oiliness, and toxins by adsorption; and also acts as a seborregulator [23,41].Damazio and Makino (2017) [23] published several hair therapy protocols with clays associated with essential oils to treat different scalp conditions, for example:

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Treatment for scalps affected with dandruff and seborrhea-after cleaning the scalp with a neutral shampoo, apply a hair mask on the scalp composed of 10 mL of the same shampoo, 3 drops of peppermint (Mentha piperita), 3 of lemon (Citrus limon), and 3 of petitgrain (Citrus aurantium) essential oils and 5 g of yellow clay thoroughly mixed; then, cover with plastic film and leave for 20 min.Rinse completely after that and apply hair conditioning [23].

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Protocol for chemically treated hair (bleached or straightened, for instance)-after cleaning the scalp with a neutral shampoo, apply a hair mask on the scalp composed of 10 mL of the same shampoo, 3 drops of lavender (Lavandula angustifolia), 3 of chamomile (Chamomilla recutita), and 3 of copaiba (Copaifera langsdorffii) essential oils and 5 g of white clay thoroughly mixed; then, cover with plastic film and leave for 20 min.Rinse completely after that and apply hair conditioning [23].
Clays incorporated in cosmetics protect the skin against external damaging agents, like UV radiation, acting as a physical barrier and increasing sun protection factor (SPF) [6].This is improved by its high surface area, which allows effective skin coverage.Therefore, when applied to sunscreens, this property offers a great advantage.Still, the magnitude of UV protection depends on its mineralogical composition [10,37,42], as well as the type of the vehicle.
Studies showed that smectite and kaolinite clays incorporated into sunscreens were effective in reflecting/scattering/absorbing UV radiation between the wavelengths of 250 to 400 nm.This is probably related to their composition, as clays' UV protection capability was shown to depend on iron oxides' concentration among their components; the higher the amount of iron oxides (Fe 2 O 3 ) in the minerals, the better the protection against UV rays [10,17].
Clays were also found to contain other physical protectors, such as titanium dioxide (TiO 2 ), zinc oxide (ZnO), and silicon oxide (SiO).The amount of these protectors in clay's composition may also present a positive effect on clay's photoprotective efficacy [10,17].Also, clays' particle size was shown to influence their photoprotective efficacy [17].On the other hand, clays containing higher iron concentrations present stronger colors, affecting the final product aspect [10].
Dusenkova and co-workers (2015) researched the use of Latvian illite clays in sunscreens.They proved them to be effective in improving protection due to high iron content, mentioning the advantage of clay's brown color which allows its use as a pigment in facial sunscreens [42].Hoang-Minh and co-workers (2010) assessed the UV protection of several types of clays (some types of kaolins, bentonites, among others), discussing the influence of mineralogical parameters on photoprotective efficacy.According to the authors, clays have UV protection potential due to absorbing or reflecting radiation, which could be influenced by particle size and chemical composition.They measured UV transmission of cream samples containing clays using an Analytik Jena AG Specord 50 photometer.They found that samples had different levels of UV transmission, which varied across UVA and UVB spectral ranges.They concluded that the hematite (Fe 2 O 3 ) content of clay minerals significantly affected the samples' UV protection behavior.They observed different patterns of relations between Fe 2 O 3 content and photoprotection efficacy, comparing expandable and non-expandable clay minerals, which could be explained by their arrangement in the cream samples.Therefore, UV protection of clay minerals was found to be dependent on their hematite content and expandability [43].
Not only are clays used in cosmetics as actives but also as starting materials.In this case, they are added to formulations to improve stability and rheology, as thickening or suspending agents, and to carry active substances, allowing the development of formulations with active controlled liberation [1,13,18,21].Their functionality depends highly on particle morphology and surface electric charge [1].

Clays Used as Formulation Starting Materials
Cosmetic starting materials are components incorporated into formulations to improve the physicochemical characteristics of the active substances and improve/allow the formulation process and application [8].Clays in cosmetics can be used as raw materials for solid, liquid, and semisolid samples based on their properties.Among those properties, we can mention high adsorption and swelling, high cation exchange capacity, large surface area, water miscibility and hydration ability, dispersity, thixotropy, opacity, and color [8,19,44].They can be used as lubricants, grease absorption agents, carriers, inert bases, protectors, heat release controllers, and so on [5].
Clay minerals dispersed in polar solvents tend to form gels with characteristics between solid and liquid, whose rheological behavior differs depending on the type of clay mineral used, concentration, and presence of other molecules/ions in the composition [9,13].It can be dilatant (less frequent), pseudoplastic, or thixotropic.As they form a tridimensional structure that is easily deformed and rearranged, they are usually incorporated into semisolid cosmetic preparations, like dental gels or mascara.This is because those formulations need to be easily deformed as a liquid during application, and then restore to their initial shape (more solid) at rest [13,27].
They are often used to stabilize emulsions or suspensions, increase their viscosity, modify systems' rheological behavior, carry active substances, and are used as adsorbents or absorbents [5,19,36,44].To improve the stability of suspensions, clays may be used as agents to delay sedimentation.The same is applied to emulsions [44], to avoid phase separation in a short period of time.Stabilization of those formulations occurs because of the gel-forming capacity of clay minerals and due to their presence on interface boundaries, which occurs because of their colloidal size, surface charges, and high surface areas [9].
In semisolid cosmetics, clay minerals may be used for two main reasons: to stabilize dispersed systems and to modify rheology.This is related to the presence of charge on their surface, their colloidal dimension, and their capacity to form different structures when dispersed in polar media [9,44].Also, clay minerals are adsorbed and act as a physical barrier in the interface, thus preventing stability issues, like flocculation and coalescence, as well as acting as an emulsifier [13,21].
Clay minerals may also be used to formulate Pickering emulsions.In Pickering emulsions, colloidal solid particles (colloidal surfactants) act as stabilizing agents in the interface between two liquid phases.It is possible to formulate stable emulsions using only solid particles as emulsifiers, and some clay minerals may be used with this aim.The characteristics of the obtained Pickering emulsions will depend on the properties of the chosen solid particles [45][46][47].Ashby and Binks (2000) studied emulsions stabilized by laponite (synthetic smectite clay with uniform particle size) [45].Lu and co-workers (2014) prepared Pickering emulsions with fibrous palygorskite clay particles, which formed a three-dimensional network to stabilize the formulation [48].Kpogbemabou and co-workers (2014) formulated oil-in-water Pickering emulsions stabilized using three different phyllosilicates-kaolin, halloysite, and palygorskite [49].
Clay minerals' fine texture and plasticity ease the application of makeup products and increase their durability over the skin.Their oil control property also improves makeup's water resistance without making the skin dry.As they provide excellent coverage, sorption, and adhesion, they have been used in facial treatments to hide imperfections and fine lines [8].
Phyllosilicates may have several functions in cosmetic formulations, such as: thickening or suspending agents, binders, anti-caking agent, emulgent, adherent, diluent, lubricant, and stabilizers in emulsions.They can also be used to facilitate the incorporation of hydrophobic actives in formulations, as they enable their dispersion [13].In Table 4, we list different uses of clays as raw materials in cosmetics/dermocosmetics.** Used at 1.0-2.0%(w/v) to slightly increase viscosity and 10.0% (w/v) for accentuated increase [13].

Clays Used as Delivery Systems
Components, including clay minerals, can be used in formulations to target active release [50].Currently, clays have been explored as active ingredients/drug delivery systems.They can be used as vectors to transport substances to their targets in an organism, thus benefiting pharmaceutical and cosmetic industries [1,26].They can interact with formulation components and affect bioavailability by influencing on actives' liberation and stability [3,6,13].
Clay minerals can be used as auxiliary components to maintain the active dose in the treated area due to viscosity increase, better skin adhesivity, and active concentration on the treated site [6].Clay minerals can interact with organic molecules by different mechanisms, such as hydrophobic interactions (kaolinites, smectites, and others with neutral sites), hydrogen bonding (clays with oxygen surfaces), cation exchange (smectite, vermiculite, illite), etc.Based on these interactions and on their swelling properties, clay minerals are effective in delaying and targeting drug release, as well as in improving solubility.So, they can be used to increase active stability and alter drug delivery patterns, creating extended and/or site-specific delivery systems [50].
One example of clays used as a delivery system in cosmetics is in sunscreens.In those products, clays improve the stability of organic UV filters, as well as allowing the slowing of the filters' release, avoiding close contact with the skin, therefore, preventing cutaneous reactions and allergies, and improving water resistance [3,5].Sepiolite and smectites can form complexes with organic UV absorbers, enabling their use in sunscreens.This allows the use of lower concentrations of active components [2], which contributes to improving formulation efficacy and safety.
Another example is the combination of vitamins (e.g., B and C) and antioxidants with clay minerals to deliver these actives to the skin for cosmetic purposes [8].
Halloysites have been used in nanotechnology due to their tubular morphology, which allows their use as drug carriers in the dermatological field.Their nanotubular structure can accommodate active molecules inside, enabling the controlled delivery of substances to the target area [27].
Clay nanotubes have also been used for haircare cosmetics, as shown by Panchal and co-workers (2018).The authors developed halloysite clay nanotubes formed by rolled sheets of aluminosilicate kaolin, a natural abundant biocompatible clay with low cost, which were loaded with dyes for hair coloring, allowing an efficient coloring procedure.The halloysite clay nanotubes were used as a coating on hair via physical adsorption and self-assembly not involving any additional chemical treatment.The same mechanism can be used to transport other actives in haircare formulations [51].
Clay nanoparticles are a promising alternative to nanomaterials for cosmetic use.Clay minerals, like kaolinite and halloysite, have been used for the encapsulation of essential oils, obtaining nanohybrids for several applications.An example for cosmetic use is a bio-based antimicrobial mosquito repellent composed of Curcuma aromatica and Zanthoxylum limonella essential oils onto montmorillonite clays dispersed in a methyl ester of the castor oil.Also, the mixture of clay and essential oils can be used as a flavor and fragrance nanodelivery system [52].

Clays and Clay Minerals Used in Spas and Aesthetic Medicine
Clay minerals, mainly smectites and kaolinite, are widely used in spas and aesthetic medicine in geotherapy, pelotherapy, and paramuds, even before the industry started incorporating them in cosmetic products [2,3,27].Polymineralic clays are also used [3].
Geotherapy corresponds to clay minerals mixed with water and applied directly on the skin, as a layer.It is used mainly as a facial treatment against boils, acne, ulcers, seborrhea, blackheads, and spots, among others.In spas, when the treated area is extensive, it can be applied as mud baths, where the whole area is immersed in the mixture [2,3].
Pelotherapy consists of clay minerals mixed and matured with sea or salt-lake water, or minero-medicinal water resulting in a peloid [2].Carretero (2020) recently wrote a review about clays in pelotherapy (should be read for further information).The article mentions a new definition of a peloid: "a peloid is a maturated mud or muddy suspension (more precisely, a muddy dispersion) with . . .cosmetic properties composed of a complex mixture of fine-grained materials (mineral and/or organic), with mineral water, seawater, salt-lake water, and commonly organic compounds from biological metabolic activity" [53,54].The maturing process alters some clay minerals' properties, thus improving their efficacy; it cools more slowly, decreases grain size, increases plasticity, and improves absorption capacity.It is normally applied on the skin for 20-30 min in layers covered with an impermeable material to preserve the heat.This causes vasodilatation and perspiration, favoring its effect [2].However, depending on the purpose, it can also be applied cold [3].
Paramuds consist of a mixture of paraffin with an inorganic material which is usually clay.They are applied hot for 20-30 min in layers covered with an impermeable material to preserve temperature.This therapy moisturizes skin and enhances penetration of other active substances, as pores are dilated, and superficial circulation is stimulated.All these actions are due to the provided heat [2,3,53].The presence of paraffin allows paramuds not to adhere to the skin, so they can be conveniently removed after treatment.However, it has reduced efficacy since, unlike peloids, there is no exchange between paramuds and skin apart from heat exchange [53].
These approaches are used in spas for their properties of (1) high sorption capacity-to eliminate the excess of grease and toxins from the skin; (2) high cation exchange capacityto enable the exchange of nutrients; (3) rheology-must be adequate to enable the formation of a consistent paste with good plastic properties for easy application and skin adhesion; (4) grain size-ideally must be small and soft for application to be pleasant; (5) cooling index/heat retention capacity-in some therapies they are applied hot to treat dermatological conditions; and (6) adequate pH-must be compatible with the skin to avoid irritation [2,3].
Among those properties, we can highlight high cation exchange capacity, which is presented mainly by peloids containing clays with smectites (minerals sensitive to ion exchange).This property enables ion exchange with minero-medicinal water, modifying the composition of the liquid part of the peloid, thus enhancing bioavailability of ions to the treated skin [53].
Geotherapy and pelotherapy can be applied as face masks (mainly in beauty treatment), cataplasms (applied on small areas), or mud baths (on extensive areas), depending on the treatment area.They can be applied hot (40-45 • C) or cold, depending on the aims.Paramuds are always applied hot (40-45 • C) and as cataplasms [3].
In beauty treatment, geotherapy, pelotherapy, and paramuds should be applied hot for the following reasons: (1) it allows moisturizing of the skin since perspiration cannot evaporate during therapy application due to the impermeable material, thus remaining retained on the skin; (2) it increases cutaneous absorption of active ingredients; (3) it stimulates local circulation and acts as an anti-inflammatory; and (4) it improves cutaneous cleaning and treatment of dermatological conditions (as acne, ulcers, and seborrhea) since it increases perspiration and sebaceous secretions, as well as opens pilosebaceous orifices, thus improving clay mineral's sorption properties and, therefore, efficacy [2,3,11].

Conclusions
A review of clays and clay minerals used in cosmetology was carried out, evidencing and discussing the numerous attributes of such substances, from ingredient processing and characterization to safety and efficacy establishment.They can be used in several types of cosmetics/dermocosmetics, both as active ingredients and/or as starting materials, due to their properties, like sorption, cation exchange capacity, physical exfoliation, and swelling, among others.In addition, they can be formulated in distinct types of cosmetic/pharmaceutical forms.However, most specialized literature justifies their efficacy based on theories, and only a few report proven efficacy using specific assays to suggest or demonstrate cosmetic attributes.Most papers that showed efficacy tests concerning clays used in photoprotection performed comparative in vitro spectrophotometric methods, which are not accepted for product registration, for instance.As far as haircare is concerned, scarce literature was found.Therefore, we believe our review, as organized with the discussed issues, could be considered in the coming years in the cosmetic field to provide information for the related industry.e Tecnológico, CNPq, for the doctorate scholarship.R.M.M. acknowledges the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Brasil, CAPES, for the doctoral scholarship.A.R.B. is extremely thankful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq, for the Research Productivity Scholarship, and to the PrInt USP/CAPES Program.The authors are thankful to the reviewers of this research work for their high-quality contributions.

Table 2 .
Classification of clay minerals

Table 3 .
Minerals used as actives in cosmetic formulations.