Micro-Raman—A Tool for the Heavy Mineral Analysis of Gold Placer-Type Deposits (Pianu Valley, Romania)

Abstract

In the current study, different heavy minerals typical of gold placer deposits were identified by means of micro-Raman spectroscopy, and their chemical composition analyzed and discussed (garnet, kyanite, staurolite, zircon, allanite, monazite, xenotime, rutile, anatase, cassiterite, titanite, barite). Even complex solid solution series, such as those of garnets, can be deciphered with the aid of systematic trends observed in Raman line frequencies. The ν₁ mode in garnets will shift from high to low frequencies as a function of the ionic radius of the X²⁺ cation, from Mg²⁺, to Fe²⁺ and Mn²⁺, while the presence of Ca²⁺ will make the band to be shifted strongly to even lower wavenumbers. This approach has successfully been taken to differentiate between polymorph triplets such as kyanite-sillimanite-andalusite and rutile-anatase-brookite. Minerals under consideration with high contents of REE, U and Th are affected by intensive metamictization, particularly zircon and titanite. Raman peak features, such as shape, symmetry and intensity, respond to this radiation damage of the lattice and enable fine-tuning of these heavy minerals, such as in the case of fluorite (fetid fluorite).

1. Introduction

Detrital heavy minerals provide insights into the mineral chemistry of the entire source area, and when cast in the role of provenance markers, environment indicators and ore guides, can be successfully used in applied and genetic geosciences [1,2,3,4,5].

Placers are clastic sedimentary deposits formed by mechanical and chemical processes operative in aeolian, fluvial and coastal-marine environments. In particular, weathering of metamorphic, igneous or reworked sedimentary source rocks, sorting on transport and in situ alteration at the trap site play paramount roles during formation of placer-type deposits [6,7,8]. When it comes to an economic assessment of placer deposits, it is especially gold, platinum group elements (PGE), platinum group minerals (PGM), Ti minerals, zircon and rare-earth elements (REE) minerals that attract the attention of economic geologists and mineralogists [9,10]. There is an ever-increasing demand for heavy minerals worldwide in various branches of the industry, mainly those dealing with new green technologies and high-tech applications, in addition to the traditional producers of pigments, ceramics, automobiles, glass, nuclear energy, abrasive steel [6,11]. It is not only the quantity of heavy minerals in placer-type deposits, but also their quality that is decisive of whether the raw material can be used for high-end products or only as a high-value commodity. This is where Raman spectroscopy comes in.

High-resolution Micro-Raman spectroscopy is a sensitive technique commonly used in mineral “fingerprinting”, but it also extends to structural, chemical and physical information, which are crucial in genetic and applied geosciences [12,13]. It is a low-destructive method capable of distinguishing the minerals in associations where common polymorphs such as rutile and anatase or calcite and aragonite occur side-by-side with each other [14]. In this case, microprobe analysis has reached its limits, since it cannot differentiate between minerals with the same chemical composition. Moreover, Raman spectroscopy can broaden our knowledge of bond properties that modify even upon slight changes in the chemical composition on a semi-quantitative basis and by considering systematic band shifting [15]. Important structural information can be deduced from the Raman spectra of various classes of minerals: silicates [16,17,18,19], phosphates [20,21], sulfates [22,23], sulfides [24,25,26], or oxides [27,28,29].

The fundamental vibrational modes of silicate minerals can be described as four types of motions of the main structural units—SiO₄ tetrahedra: (a) nondegenerate ν₁ symmetric stretching, (b) doubly degenerate ν₂ symmetric bending, (c) triply degenerate ν₃ antisymmetric stretching, and (d) triply degenerate ν₄ antisymmetric bending [30]. Similar modes are also observed in other classes of minerals where the structural units are of tetrahedral type. Besides these fundamental modes, other vibrations appear in the Raman spectra of minerals, such as external or lattice vibrations that are usually noticed in the low region of the spectra.

The aim of the current study is to investigate heavy mineral assemblage typical of placer deposits by means of Raman spectroscopy in order to shed some light on the strong correlation between chemical composition and structural data offered by this technique. Taking this approach requires single-crystal chemical analyses that can confirm certain substitutions and structure deformations that affect the Raman spectra. The results obtained from this information directly translate not only into further investigations regarding source and formation of placer deposits, but also into the economic potential of such mineral accumulations.

2. Geological Setting

The Pianu Valley is renowned in Romania for its important alluvial gold deposits. The area has a long-lasting history of gold exploration and exploitation reaching back to antiquity. Traces of panning gold by rudimentary and traditional methods can still be observed today in the environs of Pianu de Sus [31]. Pianu Valley takes an outstanding position as the only site where native platinum has been reported in Romania [32]. The following heavy minerals were identified as major components within the Pianu Valley: garnet, zircon, titanite, magnetite, tourmaline, kyanite, hornblende, Cr-spinel, monazite [32,33,34].

The Pianu Valley is located in the north-eastern part of the Sebeș Mountains and is made up of Upper Cretaceous and Quaternary sedimentary rocks that rest on the Getic Domain or Median Dacides, according to Săndulescu [35]. In terms of the lithofacies, it belongs to the Sebeș-Lotru variegated litho-group, which is characterized by gneisses, amphibolites, ultrabasic magmatic rocks, quartzites, migmatites, kyanite-bearing micaschists, meso- and epimetamorphic schists, granites, pegmatites and quartziferrous porphyries [31,36]. The Getic Realm is intersected by several shear zones [37,38]. These structures are considered as the primary source of the Pianu Valley gold and rare-elements accumulations [32,36,39].

The alluvial–fluvial drainage system spread along the Pianu Valley is Cretaceous in age, and its rocks are denominated as conglomerate, breccia, sandstone, shale, marl, and clay, while the Pleistocene to Holocene deposits include sand, gravel with quartzite boulders [40].

3. Materials and Methods

Heavy minerals were sampled in the Pianu Valley area at different locations at which gold occurrences had been identified. The samples were pre-concentrated in the field by manual panning with a 35 cm diameter plastic pan in order to remove as much as possible the light minerals (quartz, feldspars, carbonates). The samples were subsequently enriched in the laboratory using diiodomethane heavy liquid (CH₂I₂), density of 3.325 g/mL. After the separation procedure with heavy liquid, the resulting fractions consisted of mineral grains with high specific gravity. These heavy mineral fractions were embedded in epoxy resin and prepared as polished sections.

All samples were examined by means of micro-Raman spectroscopy and Electron Probe Microanalysis (EMPA) at Slovak Academy of Sciences from Banská Bystrica, Slovakia. The microprobe measurements were performed on carbon-coated polished sections using the JEOL JXA-8530F. The device operated under an acceleration voltage of 15 kV, 20 nA sample current, 2–5 μm beam diameter, 10–30 s counting time for peaks and 5–15 s for the background. The chemical composition was measured with the following standards and X-ray lines: Nb (Lα, LiNbO₃), Si (Kα, albite), Ti (Kα, rutile), Al (Kα, albite), Y (Lα, YPO₄), La (Lα, LaPO₄), Ce (Lα, CePO₄), Pr (Lß, PrPO₄), Nd (Lα, NdPO₄), Sm (Lß, SmPO₄), Gd (Lß, GdPO₄), Fe (Kα, hematite), Mn (Kα, rhodonite), Mg (Kα, diopside), Ca (Kα, diopside), Na (Kα, albite), K (Kα, orthoclase), and F (Kα, fluorite), Si (Kα, quartz), Mg (Kα, olivine), Al (Kα, kyanite), Ti (Kα, rutile), Cr (Kα, Cr₂O₃).

The Raman spectra were obtained on polished sections using the micro-Raman Spectrometer LabRAM HR800 (Horiba Jobin-Yvon, Kyoto, Japan) coupled with an Olympus BX51 optical petrographic microscope (100× /0.80 magnifying objective) and a CCD detector (1024 × 256 pixels). The samples were irradiated using the 632.8 nm excitation line of a He-Ne laser with a power of 17mW. Several density filters were used to prevent from heating, deterioration and fluorescence effect of the samples. The micro-Raman spectra were recorded in the range of 50–1400 cm⁻¹ with an exposure time of 2–20 s per frame, 10–20 acquisitions, in order to improve the signal-noise ratio. Spectral accuracy was calibrated with Rayleigh line and the 520.7 cm⁻¹ line of a Si standard. The system resolution was 2 cm⁻¹, and the wavenumber accuracy was ±1 cm⁻¹. The band fitting and was done by combined Gaussian/Lorentzian amplitude function. The Raman spectra were reduced in intensity for proper display in stacked graphs and better comparison.

4. Results and Discussion

Raman investigations of the heavy minerals from the Pianu Valley placer deposit revealed the presence of the following minerals: garnet, kyanite, staurolite, zircon, allanite, monazite, xenotime, rutile, anatase, cassiterite, titanite and barite. All minerals were confirmed by chemical analyses through electron microprobe measurements (EPMA). Their vibrational features are discussed in detail in the next sections.

4.1. Garnets

The garnet group is characterized by the general formula X₃Y₂(SiO₄)₃, where X = Fe²⁺, Mg²⁺, Ca²⁺, Mn²⁺ and Y = Al³⁺, Fe³⁺, Cr³⁺. Substitutions with other cations may occur in each of these sites, including for Si⁴⁺, which can be partially substituted by Al³⁺, Ti³⁺, and Fe³⁺ [41]. The structure of these nesosilicates consists of isolated SiO₄ tetrahedra, together with YO₆ octahedra and distorted XO₈ dodecahedra (Figure S1). Besides these three cationic sites, there is also an anionic site for O²⁻, OH⁻, F⁻ [41]. The garnets are divided into two series with six more common members, based on the cation sites occupancy: pyralspites, where Y = Al³⁺, and X = Mg²⁺ (pyrope-Pyr), Fe²⁺ (almandine-Alm), Mn²⁺ (spessartine-Spe); and ugrandites, where X = Ca²⁺, and Y = Cr³⁺ (uvarovite-Uva), Al³⁺ (grossular-Gro), Fe³⁺ (andradite-And) [41,42].

The chemical analyses show that most of the studied garnets have an almandine-rich composition with variable contents of pyrope, spessartine and grossular (Table S1). The main cation in Y site is Al³⁺ (20.65–22.59 wt.% Al₂O₃), while the X sites are occupied mostly by Fe²⁺ with values of FeO ranging from 20.91 to 37 wt.%, with an average of 31.61 wt.%. MgO reaches up to 8.47 wt.%, and MnO has a maximum value of 12.61 wt.%. Variable concentrations of CaO were also observed, with the highest being 9.96 wt.%. Therefore, the analyzed garnets have a calculated composition of Alm_41–81, which is supplemented either by Pyr_1–32, Spe_0–30, or Gro_0–28. This gives a variety of intermediate member compositions useful in Raman interpretation of garnet spectra.

Representative Raman spectra are shown in Figure 1, and the Raman bands are listed in

Table 1. Raman bands (cm⁻¹) of selected garnet compositions (mol%) and their mode assignments.

Samples/Assignment	01	02	03	04	05	06	07	08	09	10	11
T(X2+)	164	168	162	168	167	164	168	170	167	167	170
T(SiO4)	225		251	214	215	215	214	218	211	215	212
R(SiO4)				316	314			318	314	314
	350	353	355	351	347	343	347	347	345	345	347
					369	370	373	373	371	371	373
ν2	476		466	477	478	475	476	479	478	478	482
ν4	500	503	504	501	500	499	501	501	499	499	501
ν2	555	556	557	556	555	555	556	558	555	555	558
ν4			639	634	630	630	636	633	633	632	633
ν3	852	853	854	858	861	860	862	863	861	864	867
ν1	907	910	912	915	912	913	916	916	915	917	918
ν3	1002			1002			1002			1001
ν3	1028	1034	1038	1039	1035	1035	1041	1041	1038	1041	1047
ν3	1110		1111				1113
Pyr%	2	2	32	18	11	1	13	12	16	18	22
Alm%	67	47	41	66	74	69	61	79	78	80	71
Spe%	4	23	1	2	2	30	23	2	0	0	5
Gro%	27	28	24	14	12	0	3	6	5	1	2

T(X²⁺)—translation of X cation; T(SiO₄)—translation of SiO₄; R(SiO₄)—rotation of SiO₄.

together with their assignment. The spectra were selected from a total of 27 acquired on different garnet compositions.

The Raman spectra of the studied garnets show three typical regions of frequencies: low region between 162 and 379 cm⁻¹, a middle interval at 466–639 cm⁻¹, and high frequency bands between 867 and 1113 cm⁻¹ (Figure 1).

Although the minerals from garnet group have 25 Raman-active modes-3A_1g + 8E_g + 14T_2g [43], only 14 vibrational modes were observed in the obtained Raman spectra. The bands from the low region of the spectra are attributed to external vibration modes, such as: the peak at 162–170 cm⁻¹ is assigned to translational modes of X²⁺ sites, the band at 211–251 cm⁻¹ to translational motions of SiO₄ tetrahedra, while the 313–318, 343–355 and 369–379 cm⁻¹ peaks are attributed to rotational modes of SiO₄. The medium- and high-frequency regions of the spectra show the internal vibrations of SiO₄ tetrahedra: O-Si-O bending modes at 466–482, 553–559 cm⁻¹ (ν₂ symmetric bending) and 498–504, 611–639 cm⁻¹ (ν₄ antisymmetric bending); Si-O stretching modes at 907–918 cm⁻¹ (ν₁ symmetric stretching) and 852–867, 997–1003, 1028–1047, 1110–1113 cm⁻¹ (ν₃ antisymmetric stretching). The Raman mode assignments were made according to Bersani et al. [16], Mingsheng et al. [43], Hofmeister and Chopelas [44], Kolesov and Geiger [45], and Kos et al. [46].

A shifting behavior was observed for all Raman peaks based on the chemical composition. These variations are quite complex, since we are dealing with hexary systems such as garnet groups. Furthermore, we will discuss in detail the most intense peak dominating the garnet spectra, which was attributed to ν₁ vibrational modes of SiO₄, as being mostly representative in behavior also of the other bands.

The ν₁ mode shifts from 918 cm⁻¹ in Alm₇₁Pyr₂₂ composition, to 913 cm⁻¹ in Alm₆₉Spe₃₀, to 907 cm⁻¹ in Alm₆₇Gro₂₇ (Figure 1). Since the Alm% content remains constant, the Raman line basically decreases in frequency from pyrope to spessartine to grossular. Moreover, intermediate compositions of three end-members place their Raman line between these frequencies: 916 cm⁻¹ for Alm₆₁Spe₂₃Pyr₁₃, 912 cm⁻¹ for Alm₄₁Pyr₃₂Gro₂₄ and 910 cm⁻¹ in the case of Alm₄₇Gro₂₈Spe₂₃ (Figure 1). The frequencies of ν₁(SiO₄) mode seem to be significant correlated in a positive way with MgO (r = 0.48) and FeO (r = 0.51) concentrations, and negatively with CaO (r = −0.78), suggesting a shifting to higher wavenumbers as MgO and FeO contents increase, while CaO strongly decreases the line frequency. The same correlations were observed for the calculated Alm% (r = 0.46), Pyr% (r = 0.51) and Gro% (r = −0.79) compositions.

For a better understanding of these peak variations, it has to be considered that line shifting in pyralspites is caused by atomic mass, atomic structure, ionic radius and polarization of X²⁺ cations, which strongly affect the unit-cell size and bonds force constants in the crystal structure [43,46]. Out of all these factors, the ionic radius has the strongest influence on the position of the Raman peaks. In pyrope–almandine–spessartine–grossular series, the ionic radii increase from Mg²⁺_(0.89) to Fe²⁺_(0.92), Mn²⁺_(0.96) and Ca²⁺_(1.12) Å. Therefore, the ν₁ mode will shift from high to low frequencies from Mg²⁺, to Fe²⁺ and Mn²⁺, while the presence of Ca²⁺ will cause the band to be shifted strongly to even lower wavenumbers because of the much larger ionic radius [17]. This decrease of the wavenumbers with increasing ionic radius is also supported by the correlations with the chemical results discussed above.

Since in the case of garnets solid solutions we are dealing with four different cations that can substitute each other and form mixed intermediate members, it is important to consider how much each cation contributes with its ionic radius to the X²⁺ site occupancy. By calculating a weighted average of the ionic radii and molar ratios for each garnet composition, it can be observed that it strongly correlates with the measured Raman ν₁ mode wavenumbers (r = −0.86). This value can be calculated as (r_Mg·x_Pyr + r_Fe·x_Alm + r_Mn·x_Spe + r_Ca·x_Gro), where r is the ionic radius and x represents the mol% of each end-member. Another similar approach is to use the ν₁ mode frequencies of pure end-members as standard values weighted by their molar ratio. The ν₁ band appears at 926 cm⁻¹ in pyrope, 915 cm⁻¹ in almandine, 907 cm⁻¹ in spessartine, and 879 cm⁻¹ for grossular [16]. Within this study, by using these values together with the end-members’ mol%, a very good correlation with the real frequencies of the ν₁ mode was observed (r = 0.88). This confirms that by applying this procedure backwards and taking into account up to six peaks with unimodal behavior, one can roughly estimate the chemical concentration of any garnet intermediate member using only the Raman analyses, as previously discussed in more detail by Bersani et al. [16] and Smith [17].

As for the other vibrational modes that are present in the Raman spectra of the studied garnets, it was noticed that ν₃(SiO₄) modes at 852–867, 1028–1047 cm⁻¹, ν₂(SiO₄) modes at 466–482, 553–559 cm⁻¹, and T(X²⁺) mode at 162–170 cm⁻¹ show a similar trend as the already discussed ν₁(SiO₄) vibrations. An inverse behavior was noticed for the peaks at 211–251, 343–355 and 369–379 cm⁻¹, which are assigned to R,T(SiO₄). The other peaks show no direct correlation with the chemical data, most likely having a multi-modal character.

4.2. Kyanite and Staurolite

Kyanite (Al₂SiO₅) and staurolite (Fe₂Al₉Si₄O₂₃(OH)) are both nesosilicate minerals (Figures S2–S3). Kyanite is the high-pressure phase of the three Al₂SiO₅ polymorphs (sillimanite, andalusite, kyanite), and its structure involves Al³⁺ octahedra linked by Si⁴⁺ and Al³⁺. In the case of staurolite, Al³⁺ also occupies octahedral sites, and Fe²⁺ and Si⁴⁺ have tetrahedral coordinations [47].

The Raman spectra of kyanite (Figure 2) show a large number of clear and sharp peaks, especially in the low wavenumber region. The bands from the 892–1001 cm⁻¹ interval can be attributed to the ν₁ and ν₃ stretching modes of SiO₄ (symmetric and antisymmetric). Two extra peaks were observed in the studied samples at 1113 and 1185 cm⁻¹ (Figure 2—kyanite 2). As for the ν₂ and ν₄ bending modes, these appear in the spectra between 607 and 669 cm⁻¹ [19,47]. The strong peak at 485–487 cm⁻¹ can be assigned to bending vibrations of Al-O-Si bonds, as suggested by Zhai et al. [19]. In the low region of the spectra, below 450 cm⁻¹ there are several Raman bands that probably also overlap with the external modes. Zhai et al. [19] assigned these bands to Al-O bending modes and explained it as a cause of poor symmetry and degeneracy in the kyanite structure.

Some differences were observed between kyanite Raman spectra from different analyzed samples. For instance, all peaks from the 200–564 cm⁻¹ region seem to be shifted ~3 cm⁻¹ (Figure 2—kyanite 1 and 2). Instead, the stretching modes showed an opposite trend: from 892 to 900 cm⁻¹ and 952 to 962 cm⁻¹. Although the last-mentioned peak might be in fact two overlapping bands with varying intensities (sample orientation), the clear shifting of the other lines might be explained by a slight change in chemical composition. As the EPMA results showed (Table S2), the kyanite 2 sample has a higher content of FeO (0.29 wt.%) and TiO₂ (0.11 wt.%), which might cause structure deformations. Nevertheless, the Raman spectroscopy proved again to be very useful in identifying kyanite out of the other polymorphs (sillimanite and andalusite) based on structural features, since the chemical analyses were not enough to clearly differentiate between them.

The Raman spectra of the staurolite samples show less peaks than kyanite and have a lower quality with rather broad bands and background noise (Figure 2). This is due to the overlapping modes and the low degree of crystallinity of the analyzed staurolite samples. A similar spectrum was reported previously by Makreski et al. [47], but with some differences in peak positions. According to the assignment of those authors, the peaks observed at 895, 939 and 1000 cm⁻¹ can be attributed to the stretching modes of SiO₄ tetrahedra, while the bending vibrations occur at 639 and 796 cm⁻¹. The peaks from the low region of the spectra appear between 304 and 500 cm⁻¹ and might be assigned to Al-O vibrational modes.

The analyzed staurolites suggest a shifting of the 939 cm⁻¹ band to lower wavenumbers in samples with increasing TiO₂ concentrations correlated with a decreased ZnO, FeO and MgO content, while the other peaks maintain the same frequencies.

4.3. REE Minerals

Among the minerals enriched in rare earth elements (REE), zircon, allanite, monazite and xenotime were identified in the heavy mineral fractions of Valea Pianu samples.

Zircon (ZrSiO₄) has a nesosilicate structure with chains of alternating SiO₄ tetrahedra and ZrO₈ triangular dodecahedra along the c axis (Figure S4) [48,49]. Although zircon is not a true REE mineral, it can accommodate high quantities of REE, Li, P, Y, Ti, Hf, U, and Th in its structure [50]. Allanite-(Ce) belongs to epidote group and has the chemical formula CaCe(Al₂Fe²⁺)(Si₂O₇)(SiO₄)O(OH). Similar species are allanite-(La), allanite-(Y) and allanite-(Nd), depending on the main rare earth element from its structure (Figure S5) [51]. Most allanites can also incorporate the radioactive elements Th and U. Monazite ((Ce, La, Nd, Sm)PO₄) and xenotime (YPO₄) are both phosphate minerals that, in places, preferentially accommodate light rare earth elements (LREE), in the case of monazite, and heavy rare earths (HREE), in the case of xenotime, as well as significant amounts of Th and U, in their lattice [20,52]. The monazite structure consists of alternating (REE)O₉ polyhedra and PO₄ tetrahedra chains parallel to c axis [20], while xenotime is isostructural with zircon (Figures S6–S7) [52].

4.3.1. Zircon

The Raman spectra acquired on the studied zircons show some differences, as indicated in Figure 3. The most intense peaks appear at 352–358, 437–440 and 1002–1009 cm⁻¹. The 1002–1009 cm⁻¹ band is attributed to the ν₃ stretching mode of SiO₄ tetrahedra, while the ν₁ mode was observed at 970–974 cm⁻¹. Another internal mode was identified at 437–440 cm⁻¹ that was assigned to the ν₂ bending vibration [18,48,49,53,54]. The 352–358 cm⁻¹ line was previously assigned to the ν₄ bending mode [55], but more detailed studies based on group-theory analysis of the total number of Raman-active modes and structural investigations concluded that this line is due to external rotational modes of SiO₄ [18,48,49,53]. The bands from the low region of the spectra, 202, 214 and 224 cm⁻¹ are also attributed to the external modes [49]. Additional bands were observed (Figure 3) which are not consistent with those reported in the literature and might be caused by sample impurities.

There are some differences to be observed in the Raman spectra of the zircons under study that are considered a result of metamictization. Since natural zircon contains significant amounts of U and Th, the structure can strongly be damaged in the course the bombardment of alpha particles during the radioactive decay [18,48,49]. The metamict zircons have a high degree of structural disorder and consist of a mixture of defect-rich clusters with amorphous regions on a submicrometric scale [18]. These structural changes occur as point defects, SiO₄ tetrahedra polymerization, appearance of ZrO₂, and even a complete transformation to an amorphous state. The metamict zircons were extensively studied by means of Raman spectroscopy [18,48,49], and it was observed that the degree of damage is reflected by a broadening of the Raman bands, lower peak intensities and lines shifting to smaller wavenumbers. Fresh zircons display sharp and well resolved peaks as those represented in Figure 3—zircon 1 and 2. Therefore, zircon 3 crystal (Figure 3) might be affected by a certain degree of metamictization due to its broader Raman bands observed at lower frequencies. Looking at its chemical composition (Table S3) the U and Th contents are relatively low (0.07 wt.% UO₂ and 0.01 wt.% ThO₂). Out of all analyzed zircons, the one with the highest U and Th values (0.16 wt.% UO₂ and 0.10 wt.% ThO₂) shows shifted peaks at lower frequencies as well (i.e., ν₃ mode—1005 cm⁻¹). Judging from its very clear spectrum and sharp Raman lines, it can be considered that its structure is less damaged. This might suggest that the line shifting may appear even in non-metamict zircons, but it’s actually due to substitutions in cationic site which are causing structural deformations. This radiation damage of the crystal structure can also be observed in the Raman spectra of common ore mineral called fetid fluorite, a heavy mineral scattered in the clastic apron around U-bearing fluorite deposits [56]. The name has been derived from native fluorine in the lattice which spreads a garlic-like smell.

4.3.2. Allanite

Allanite was identified in the present study as a single crystal with three different compositional zones. The mineral is of allanite-(Ce) type, with Ce being partially substituted by La and Nd. The literature data is very limited concerning the Raman studies of allanite. Lopez and Frost [51] reported the Raman spectrum of an “allanite” sample, but their results are considerably different from those obtained in the present study. Most likely, the authors misidentified their sample, since no chemical or X-ray analyses can confirm their assumption. The Raman spectra of allanite identified in Valea Pianu samples (Figure 3) better resemble a Cr-rich allanite-group mineral reported by Varlamov et al. [57], and the Raman frequencies fit perfectly with those reported by Chukanov and Vigasina [58] for allanite-(Ce). A tentative assignment of the Raman modes can be made by comparing it with epidote spectrum [59]. Therefore, the observed peaks in the high-frequency region (872, 961 and 1046 cm⁻¹) can be attributed to the symmetric and antisymmetric stretching modes of Si-O_nb (O_nb—non-bridging oxygens). The stretching vibrations of Si-O_b-Si (O_b—bridging oxygens) appear in the spectrum at 689 cm⁻¹. The two strong bands at 417 and 453 cm⁻¹ and one at 489 cm⁻¹ could be assigned to the bending modes of Si-O_b-Si bonds. It can be considered that the Raman lines below 400 cm⁻¹ are due to the external modes of M-O interactions. These modes are observed at 130, 194, 221, 269, 317 and 358 cm⁻¹ (Figure 3). No peak shifting was observed in the spectra that might be due to different chemical composition.

4.3.3. Monazite

The Raman spectrum of monazite shows typical features of phosphate minerals (Figure 3). The spectra are dominated by the intense line at 972–975 cm⁻¹ that is assigned to symmetric stretching ν₁ modes of PO₄ units while the antisymmetric ν₃ stretch is observed at 1055–1060 cm⁻¹. The bending vibrations are attributed to the Raman bands from 620 and 570 cm⁻¹ (ν₄ modes) and from 464–472 cm⁻¹ (ν₂ modes). The remaining lines that appear below 450 cm⁻¹ are due to the external or lattice modes that involve M-O movements: 171, 222, 396 and 415 cm⁻¹ [20,21,60,61].

Some differences were observed in the Raman lines position for the measured monazites, especially for the symmetric modes ν₁ (972–975 cm⁻¹) and ν₂ (464–472 cm⁻¹). The studied samples are monazite-(Ce) with additional La, Nd and Th. It seems that there is a systematic shifting caused by the dominant cation that occupies M site. The stretching bands tend to shift to higher wavenumbers with the decrease of ionic radius of the main cation or with its increasing atomic number in the case of REE [20]. For the monazite with the highest frequency of the ν₁ mode (975 cm⁻¹), it was measured to have the highest content of Y (2.04 wt.% Y₂O₃), which has a smaller ionic radius (Table S4).

4.3.4. Xenotime

The Raman measurements of the xenotime samples revealed two slightly different types of spectra (Figure 3). The symmetric stretching ν₁ is observed at 997–999 cm⁻¹ and the ν₃ mode at 1057 cm⁻¹. The Raman bands at 569, 601–613 and 649–661 cm⁻¹ can be attributed to ν₄ antisymmetric bending modes, while the symmetric ones appear at 422–431, 474 and 519 cm⁻¹. The external modes are observed below 400 cm⁻¹ [60,62].

The Raman spectrum of xenotime 2 type (Figure 3) shows the ν₁ mode at a decreased intensity and lower frequency (997 cm⁻¹), while the ν₃ mode is completely diminished. Similar behavior was reported by Švecová et al. [52] for an altered xenotime affected by radiation damage with a high sub-micrometer porosity. An alteration state of xenotime 2 might explain the different shape of the spectrum. Moreover, the xenotime 2 sample has lower concentrations of U, Th and Y, most likely being leached due to its advanced degree of alteration.

4.4. Oxides

The oxides identified in the Valea Pianu gold deposit by means of Raman spectroscopy are rutile, anatase and cassiterite (Table S5). Titanium dioxides occur in nature as three polymorphs: the stable phase rutile, and metastable anatase and brookite (TiO₂). Rutile and anatase are both tetragonal with different space groups (Figures S8–S9). Tin dioxide has a rutile-like crystalline structure (Figure S10) and it is known as cassiterite (SnO₂).

The Raman spectra of the studied oxides show well-resolved and intense peaks (Figure 4). These bands can be assigned in terms of factor group analysis. Therefore, rutile has four Raman-active modes with symmetries B_1g, E_g, A_1g and B_2g, out of which the B_2g mode has a very low intensity, and it can appear as a weak band at 827 cm⁻¹ [28,29,63]. These modes are associated with symmetric stretching (E_g), symmetric bending (B_1g) and antisymmetric bending (A_1g) vibrations of O-Ti-O bonds [28]. The B_1g mode is observed in rutile at 144 cm⁻¹, E_g mode at 447 cm⁻¹, while the 612 cm⁻¹ peak is assigned to the A_1g mode (Figure 4). An additional broad band appears in rutile at around 243 cm⁻¹, which might be attributed to disorder-induced scattering or the second-order effect [28]. The spectrum of anatase shows six Raman-active modes as 1A_1g, 2B_1g and 3E_g symmetries. The bands at 142, 195 and 637 cm⁻¹ are assigned to the E_g modes, while the 394 cm⁻¹ peak is attributed to the B_1g mode. The Raman line at 514 cm⁻¹ in anatase is a doublet of the A_1g and B_1g vibrational modes [29,63,64].

In the case of cassiterite, group theory predicts four Raman-active modes. Several Raman bands are observed (Figure 4), out of which the most intense one at 634 cm⁻¹ is assigned to the A_1g mode. The other modes appear at 125 cm⁻¹ (B_1g mode), 452 cm⁻¹ (E_g mode), and 775 cm⁻¹ (B_2g mode) [27,65,66]. Two additional bands are observed at 499 and 833 cm⁻¹. The Raman lines at 243 and 286 cm⁻¹ are attributed to the transverse (TO) and longitudinal optical (LO) modes, respectively [27].

4.5. Titanite and Barite

Titanite (CaTiSiO₅) is a nesosilicate with a structure composed of SiO₄ tetrahedra, TiO₆ octahedra and CaO₇ polyhedra (Figure S11). The Ti atoms are in an off-centered position within the octahedra. TiO₆ octahedra share corners, forming zig-zag chains that are linked by SiO₄ units. The entire framework contains large cavities that enclose the Ca atoms [67,68].

The Raman spectrum of titanite (Figure 5) shows multiple bands that are rather broad and sometimes asymmetric. The band assignment considers the strong influence of the Ti-O bonds, which show a stretching mode at 608 cm⁻¹. The vibrational modes of SiO₄ tetrahedra are visible at 859 and 871 cm⁻¹ for the symmetric stretching ν₁ and at 913 cm⁻¹ for the ν₃ antisymmetric stretch. Three other intense lines appear at 423, 468 and 541 cm⁻¹ that can be assigned to the ν₄ bending modes [69,70]. Up to 10 more bands appear in the low region of the spectrum, which are attributed to the external modes (Figure 5).

As in the case of zircon, titanite can be affected by metamictization due to its relatively high contents of REE, U, and Th. Zhang et al. [68] showed that the Raman spectrum of titanite is strongly dependent on the degree of metamictization and it changes by broadening peaks, shifting frequencies, decreasing overall intensity and new additional bands.

Barite (BaSO₄) is a sulfate mineral with SO₄ tetrahedra as fundamental unit that surround the Ba²⁺ cations (Figure S12). The Raman spectrum of barite (Figure 5) shows very clear and sharp bands that can be attributed to the four fundamental vibrational modes of SO₄ tetrahedra. Therefore, the most intense line at 987 cm⁻¹ is assigned to the ν₁ stretching mode and the very weak one at 1142 cm⁻¹ is due to the ν₃ mode. The bending vibrations occur at 461 cm⁻¹ for the ν₂ symmetric type and at 616 and 628 cm⁻¹ for the ν₄ antisymmetric modes [22,23].

5. Conclusions

The micro-Raman technique proved to be a powerful tool for identifying a large number of heavy minerals in a typical gold placer deposit from Pianu Valley, Romania. The following minerals were observed: garnet, kyanite, staurolite, zircon, allanite, monazite, xenotime, rutile, anatase, cassiterite, titanite and barite. The results were confirmed by EPMA chemical analysis. Even if Raman spectroscopy can be used as a stand-alone technique for most of the minerals, in some cases it is recommended to use additional techniques for extra caution in structural and chemical interpretation of the minerals.

Systematic shifting of the Raman bands is very sensitive to structural deformations, even at a low degree of cation substitutions, especially for solid solution series. Such shifting behavior was observed in garnets, kyanite, staurolite and zircon. Several Raman bands in garnets spectra are strongly correlated with the average ionic radius in the X²⁺ site and therefore peaks positions are dependent to the molar ratios of each end-member. The chemical composition of intermediate members can be estimated by considering up to six peaks with unimodal behavior only by means of Raman spectroscopy.

The distinction between polymorphic minerals can be realized only by structural investigations, micro-Raman being the fastest way to obtain such information. This approach was successfully used complementary to chemical analyses in order to differentiate between kyanite-sillimanite-andalusite and titanium dioxide polymorphs (rutile-anatase-brookite).

Metamictization processes can affect some important accessory minerals due to their relatively high contents of U and Th radioactive elements. The degree of radiation damage can be assessed by evaluating the Raman spectra. Features like broad peaks, frequency shifting and lower intensities are directly correlated with the metamictization degree in zircon, titanite and monazite, even if the latter is more resistant to radioactive damage.