Halloysite was originally depicted as a 1:1 layered aluminosilicate mineral of the kaolin group by Berthier [1
]. The chemical composition structure of halloysite is similar to that of minerals of the kaolin group (kaolinite, nacrite and dickite minerals) but the unit layers of halloysite are isolated by a monolayer of water molecules [2
]. Halloysite appears mainly in two different polymorphs: a chemical formula Al2
O when fully hydrated and Al2
when dehydrated [5
Halloysite can been found in a variety of particle morphologies, such as short-tubular, large-tubular and spheroidal, and platy shapes [6
]. However, nanotubular morphology is the most common shape of halloysite. The tubular shape can be considered as rolled kaolin sheets with an inner diameter of 1/30 nm, an outer diameter of 30/50 nm and a length of 100–2000 nm [7
]. The interior surface of halloysite is composed of siloxane (Si–O–Si) groups, while the external is a gibbsite-like array of aluminol (Al–OH) groups [9
Halloysite deposits have been discovered and exploited in different countries such as New Zealand, United States, Australia, China, Brazil, and Turkey [11
]. This mineral can be formed both by weathering of igneous rocks and their hydrothermal alteration [12
]. For instance, the Matauri Bay (New Zealand) halloysite deposit was formed by hydrothermal alteration at low temperature of rhyolite and dacite volcanic rocks [20
]. The large mass of halloysite at the Dragon Mine (UT, USA) was formed by irregular replacement of Early Paleozoic dolomite rock in contact with hydrothermal fluids channeled along the Dragon Fissure Zone [21
]. Halloysite at TePuke is a weathering product of volcanic rocks of rhyolite and andesite in the Bay of Plenty, New Zealand [22
]. The above literature and others have shown that halloysites from different areas also have different morphological and physicochemical properties [7
In the recent years, due to its superior properties such as tubular structure, non-toxicity, large surface area, high mechanical strength, lower cost compared to nanotubular carbon, halloysite has attracted considerable attention of scientists and many new possibilities of application [8
]. However, in many cases, differences in morphology, size, as well as other properties of halloysites may have certain impacts on their applicability in practice. For instance, Makaremi et al. [35
] used two different types of halloysite nanotubes to improve the properties of apple pectin bionanocomposites as potential films for food packaging applications. Results indicated that the short halloysites with 50–3000 nm length and 50–200 nm outer diameter had better ability for the encapsulation of salicylic acid into their lumen, while the long halloysites with 200–30,000 nm length and 40–55 nm outer diameter made the encapsulation process more difficult. Zheng and Ni [36
] prepared an efficient flame-resistant composite using the pentaerythritol-loaded halloysites for the UV-curable epoxy resin. In this study, halloysites have length 300–1000 nm, outer diameter 50–70 nm and BET surface area 36.40 m2
/g. The obtained composite showed a low moisture absorption and a good stability of the mechanical properties. Pasbakhsh et al. [37
] have studies the properties of some halloysites in the world, and have given orientations for their applications. For example, the long-tubular halloysite with 200–5000 nm length and 40–55 nm outer diameter are very suitable for use both an additive and a carrier. The halloysite tubes showing a wide variation in size may be well suited as microfiber filler. Thus, it can be seen that studying the properties of halloysites from different deposits or even in a deposit is necessary before using them for different applications.
This study aims to study the distribution and characteristics two types of halloysite nanotubes from a weathered pegmatite profile in the Thach Khoan area, Phu Tho Province. Different characteristics of these halloysites were determined using X-ray diffraction (XRD), scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FT-IR), thermal analysis (TG and DTG), and N2 adsorption-desorption isotherms. The results showed that halloysites from different depths of the weathered pegmatite in the study area have different morphological properties. This information is useful for the understanding of distribution and characteristics of halloysites in the deposit and helping for exploitation and use these nanotubular minerals effectively.
2. Materials and Methods
2.1. General Geological Setting of Study Area
The study area has many pegmatite bodies with different sizes related to the Late Paleozoic Tan Phuong granite Complex [38
]. The surrounding rocks of pegmatite bodies are the metamorphic Thach Khoan formation of Proterozoic age (Figure 1
). The composition of this formation consists mainly of mica quartz schist, mica schist, staurolite-bearing quartz, disten, sillimanite, and garnet.
The pegmatite bodies have the strike of 60° N–80° W, dipping to the southwest with a slope of 50°–80°. They vary from several hundred up to thousands of meters in length and from tens to hundreds of meters wide. All pegmatite bodies have a similar weathering profile with an upper brown yellow zone (15–20 m), a middle pink zone (5–10 m) and a lower white, light orange zone (5–15 m).
A typical outcrop about 40 m high that has the GPS position of 21°11′31″ N and 105°15′07″ E, was prepared for sampling. For comparison purposes, two samples were collected separately. The first sample, called UPS sample, was taken in upper zone, and the second one, LOS sample, was from the lower zone of the weathered pegmatite profile. The samples were taken from the top down, perpendicular to the weathering layers. Separated samples were mixed homogeneously before using for further steps.
The bulk samples were first dissolved in deionized water by repeated ultrasonic vibration. A portion of the <2 µm clay sample fraction was obtained using the decantation method. The clay fractions were then freeze dried and examined by different analyses.
X-ray diffraction (XRD) patterns of the samples were collected by using a D8-Advance Bruker diffraction (Bruker Corporation, Billerica, MA, USA) with radiation of CuKα (λ = 1.5406 nm) generated at 40 kV and 40 mA. The data were archived in the Bragg angle (2θ) range of 3°–70° with scanning speed of 2° min−1
. Minerals were defined by using the software of Evaluation 10.0 with database (PDF-2 2004) provided by the International Centre for Diffraction Data. Formamide (FA) treatment was used to estimate the content of halloysite and kaolinite in the samples [22
The Fourier transform infrared (FT-IR) spectra for each sample were achieved in transmission mode on pellets containing a pressed mixture of approximately 1.0 mg of the sample and 100 mg of KBr. The IR spectra were recorded in the range from 4000 to 400 cm−1 with a resolution of 2 cm−1 (Shimadzu IR Prestige-21 spectro-meter instrument, Kyoto, Japan).
Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) (Quanta 450, FEI Company, Hillsboro, OR, USA) were initially used to analyze the morphology of minerals and elements present in the samples. Transmission electron microscopy (TEM) images were obtained by a JEM 1010 operated at an accelerating voltage of 200 kV. The samples were suspended by using a drop-wise of ethanol and evaporated on 200 mesh copper grids covered with amorphous Formvar carbon.
Thermmogravimetric analyses (TG) were carried out on a SETERAM Instrument (Caluire-et-Cuire, France). Approximately 2–3 mg of the samples were heated from 50 to 1050 °C in a platinum crucible with a heating rate of 10 °C min−1, under an atmosphere of high purity N2.
The specific surface area of the samples was measured from N2 gas adsorption at 77 K by using a TriStar 3000 (Micromeritics Corp., Norcross, GA, USA). Surface areas were calculated from the linear part of the (Brunauer-Emmett-Teller) BET plot. The N2 isotherms and the Barret-Joyner-Halenda (BJH) method were used to calculate pore size distributions of halloysites.
In conclusion, two main types of halloysite were formed in the weathered pegmatite profile in the Thach Khoan area, Phu Tho, northern part of Vietnam. Analysis methods of XRD, SEM-EDS, TEM, FT-IR, TG and N2 adsorption-desorption isotherms were used to characterize these halloysites. The results showed that the short halloysite type is mainly distributed in the upper zone and long halloysite type can be found in the lower zone of the weathered pegmatite profile. The short halloysites have the length ranging mainly from 250 to 750 nm, the outer diameter of >100 nm (79.1%), the specific surface areas of 15.7434 m2/g and the average pore sizes of 18.9837 nm. Meanwhile, the length ranging mainly from 750 to 1250 nm (69.9%), the outer diameter of 50–100 nm (74.2%), the specific surface areas of 22.0211 m2/g, and the average pore sizes of 17.0281 nm are properties of the long halloysites. XRD after formamide (FA) treatment indicated that the halloysite contents are approximately 81% and 93% for the upper zone and the lower zone of the weathered pegmatite profile, respectively. The results provided useful information for the understanding of distribution and characteristics of different halloysites in the deposit and for exploiting and using these nanotubular minerals effectively.