DOI: 10.31038/GEMS.2024643

Abstract

We present the results of our Raman studies on natural lonsdaleite crystals in fluorite of the two tin deposits Sadisdorf and Zinnwald in the E-Erzgebirge/Germany. The Raman spectra,  specially from Zinnwald, can deconvoluted into three Raman-active vibrational modes: E_2g, A_1g, and E_1g. For the origin, transport via supercritical fluid or melting from the mantle region into the crust is necessary.

Keywords

Lonsdaleite, REE-rich fluorite, Raman spectroscopy, Sadisdorf and Zinnwald tin-tungsten deposits, Supercritical fluid

Introduction

The Variscan Erzgebirge (German side) and Krušné hory (Czech side) contain many tin-tungsten deposits of vein and greisen type. Famous deposits are that from Zinnwald, Cinnovec, Krupka, Sadisdorf, Ehrenfriedersdorf, and Geyer. For the regional geology and description of single deposits, extensive literature is present [1-5] and references in these). From a repetition of that, we will abstain because our described observations are new and do not fit into the old genetic view, forcing us to adopt a new approach for the whole region. In the abandoned Sadisdorf Sn-W-(Cu) deposits, there are vein and greisen- type mineralizations (Figures 1a and 1b).

The frequently present breccia pipes here are analogous to the famous Schneckenstein breccia in the W-Erzgebirge, which is very characteristic of this mineralization type [6]. During the study of thin sections from the Sadisdorf deposit, peculiar violet fluorite aggregates attract attention. These aggregates contain many black, sometimes bent needles, as well as also spherical to elliptical carbon solids. A similarity to the other described new findings of spheric high-pressure minerals is very obvious (see, for example, Thomas et al., 2023) [7]. This publication also contains a schematic geological map of the Erzgebirge-Vogland Zone with the Variscan granites, etc.

Figure 1a: Location and simplified geological map of the Sadisdorf and Zinnwald deposit/ E-Erzgebirge.

Figure 1b: Schematic cross-section through the Sadisdorf deposit with the approximate origin place of the studied sample. The green color marks the origin of the Sadisdof sample.

Sample Material

Sadisdorf

The rock material is from the endocontact of the tin-tungsten- copper deposit Sadisdorf in eastern Erzgebirge/Germany [1,8]. The sample thin section (SD-H01B) is from a fine-granular white quartz-topaz rock with violet fluorite, taken during the exploration campaign Sc-ore 2012/2013 – the violet fluorite is, according to Raman spectroscopy, very REE-rich. Besides the primary mineral quartz and rarer topaz, there are many small, mostly opaque mineral grains of wolframite, cassiterite, columbite, and others. The large fluorite aggregates are not isomorphic but more in tube or irregular form. Smaller fluorite crystals are isometrically. These large tubular fluorite aggregates contain many black and transparent needles, grains, and curved crystals (Figure 2) and are generally fluid inclusion-free. If present, then they are secondary late formations.

The curved black crystal (Figure 2) contains many nano-diamonds. The straight black needles are prevailing graphite-like material. Graphite is very often present in the Variscan tin mineralization, and it is also often present in cassiterite. Graphite has mostly not been paid attention to in the past.

Figure 2: Curved graphite aggregate with nano-diamonds in REE-rich fluorite. All black points are graphite-like stuff and contain nano-diamonds.

Zinnwald

For comparison, we used fluorite grains included in tabular zinnwaldite crystals from the Zinnwald deposit (deep Bünau gallery) in the E-Erzgebirge/Germany, which is not far from the Sadisdorf deposit (taken by the first author in 1984). This violet fluorite sample contains numerous sharp black disk-like bodies of lonsdaleite-diamond-graphite (Figure 3). Some of these form half- moon-like bodies (Figure 4) where the sharp edges lie parallel to the crystallographic{100} planes.

It follows from Figures 3 and 4 that the black bodies obviously form hemispheres on the fluorite surfaces. The general impression is that this fluorite is an early formation and has a more plastic behavior at the formation time. That spoke for a high-temperature and high- pressure formation from a fluorine-rich melt containing such C-aggregates.

Figure 3: Small disk-like bodies of lonsdaleite in fluorite from Zinnwald/E-Erzgebirge (top view).

Figure 4: Side view of such bodies, as shown in Figure 3, arranged to the growth zones of the fluorite host.

Methodology: Microscopy and Raman Spectroscopy

For the study of the lonsdaleite-diamond-graphite-bearing samples and their paragenetic main minerals, we use the Zeiss JENALAB pol as well as the Raman spectrometer EnSpectr R532 combined with the Olympus BX43 microscope both for transmitted and reflected light and equipped with a rotating stage. Note here that the incident laser light is always polarized – in our case, N – S (see Tuschel, 2012) [9]. Generally, we used an Olympus long-distance LMPLFL100x objective lens. For the identification of different minerals, we used the RRUFF and the Hurai et al. Raman mineral databases [10,11]. As references, we applied a water-clear diamond crystal from Brazil (1331.63 ± 0.60 cm^-1 and a semiconductor-grade silicon single-crystal (520.70 ± 0.15 cm^-1). For this study, we used laser energies from 0.9 to 50 mW on the sample. Because the minerals lonsdaleite, diamond, and nano- diamond contain black graphite particles and are metastable in the new upper crust surrounding, heating by the laser energy can partially destroy these minerals in the extreme case or shift the characteristic peak position of Raman bands [12,13] to lower values.

Results

Sadisdorf

The sizeable bent carbon crystal (Figure 2) contains many small nano-diamonds. The diamond main band lies at 1331.8 ± cm^-1 (11 different crystals). The FWHM (Full Width at Half Maximum) is 83.1 ± 13.9 cm^-1. Needle-formed diamonds, generally black, give the prominent diamond peak a value of 1333.6 ± 5 cm^-1 (5 needles). In violet REE-rich fluorite of the rock, there are a lot of black needles and sphäric or elliptic crystals. Some needles are twisted or bent. Most of them contain graphite or carbonaceous material (see Beyssac et al., 2002) as well as nano-diamonds. There are also whisker-like transparent and some thick, short, transparent prismatic crystals. As a total surprise, the rare whiskers and these short prismatic crystals are, according to Raman spectroscopy, lonsdaleite (Lon). Such crystals are present exclusively only in fluorite. Figure 5 shows two such prismatic lonsdaleite crystals, and Figure 6 depicts a lonsdaleite whisker. The lonsdaleite crystal (Lon) in the center of Figure 5 is 14 x 2 µm large.

Figure 5: Lonsdaleite (Lon) crystals in REE-rich fluorite

By the Raman spectrum (band at 1325 cm^-1), these transparent crystals are monocrystalline lonsdaleite (see Shumilova et al., 2011) [14] and, according to Bhargava et al. (1995) [15], hexagonal diamond. By the form (long-prismatic), the hexagonal diamond has a high probability of being lonsdaleite (Figure 6).

With Raman spectroscopy, we obtained the following data for the five lonsdaleite crystals: 1318 ± 3.8 cm^-1. Opposite to fluorite, the black points (mainly carbon) in quartz are ore minerals like wolframite, columbite, and different sulfides. Obviously, bulk rocks (quartz and topaz) are the result of varying evolutions. Fluorite looks like remnants of a high-temperature melt. According to Seiranian et al. (1974), the eutectic points in the CaF₂– YF₃ system occur at 60 and 91 % (mol/mol) and 1120 and 1106 °C. Similar systems (CaF₂-BaF₂) have analog high temperatures (Figure 8) [16].

Zinnwald

In the violet fluorite from Zinnwald, there are a lot of black bodies (Figures 3 and 4). Some are large enough to perform systematic Raman studies. In the beginning, we used 20 mW on the sample. The corresponding Raman spectrum (Figure 9) resembles the monophase lonsdaleite (Figure 3 in Shumilova et al., 2011) [14]. To prevent heating of the lonsdaleite sample by always presenting black carbon, we used low laser energy on the sample (0.9 mW of the 532 nm laser). The authors Goryainov et al., 2018 [17] used the excitation of a UV laser with a wavelength of 325 nm and low intensity of 1 mW on the sample.

Interpretation

The synthesis of hexagonal lonsdaleite succeeded in 1966 by Bundy and Kasper (1967) – [18] at 1000°C and 130 kbar. In nature, lonsdaleite was found first (1967) in the Canyon Diablo meteorite by Frondel and Marvin (cited in Shumilova et al., 2011) [14] and in the Kumdykol diamond deposit (North Kazakhstan) by Shumilova et al., 2011) [14]. The Raman bands at 1318 to 1324 cm^-1 are evident and characteristic of the hexagonal diamond phase in the Sadisdorf material (see also Misra et al., 2006) [19]. Lonsdaleite in the fluorite shows a certain metastability due to laser irradiation, perceptible by the increase of the G bands at about 1580 to 1600 cm^-1 (Figures 6 and 8) and the black coloring of the nearly colorless lonsdaleite crystals during the Raman measuring. The metastability of high-pressure minerals coming from mantle deeps and staying at high temperatures for a long time in low- pressure regions (upper crust) is very characteristically [13,20-22]. One exception is moissanite here, which is very stable. Because lonsdaleite is a high-pressure and high-temperature mineral, its formation in the deposit level (≤ 3km; see Thomas and Klemm, 1997) [23] is usually not possible. Therefore, the formation of the lonsdaleite-bearing fluorite occurred at significantly greater depths and came from mantle depths via supercritical fluids or melts into the crustal level. However, the formation of lonsdaleite whisker is, at the moment, a mystery.

Figure 6: Raman spectrum of both lonsdaleite crystals in Figure 5. The broad Raman band at about 1325 cm^-1 results from a small component of diamond (a two-phase crystal of lonsdaleite-diamond [14].

Figure 7: Lonsdaleite (Lon) whisker in fluorite. The whisker has a length of 45 µm and a thick of 0.9 µm.

Figure 8: Raman spectrum of the whisker-like (Figure 7) monophase lonsdaleite crystal (characteristic band at 1319 cm^-1). The bands at 321 cm^-1 and lower values come from the matrix fluorite.

Figure 9: Raman spectrum of lonsdaleite in fluorite from Zinnwald/E-Erzgebirge taken at 20 mW on the sample.

Discussion

According to Németh et al., 2014 [24] lonsdaleite does not exist as a discrete material. That is in contradiction to our observations. Figures 5 and 7 clearly show prismatic crystals with a Raman spectrum corresponding, according to Shumilova et al. (2011) [14], to natural lonsdaleite. Of course, the formation of lonsdaleite is unclear. However, this material is existent as prismatic crystals. The lonsdaleite whisker (Figure 7) shows the same Raman spectrum. Although it is well-known that cubic crystals can also form whiskers (for example, GaP-whiskers grown from a non-stochiometric Sn-melt produced by the first author in 1970 (unpublished results). Shiell et al. (2016) [25] report the synthesis of almost pure nanocrystalline lonsdaleite in a diamond anvil cell at 100 GPa and 400°C from glassy carbon. That means lonsdaleite is, in contrast to Németh et al. (2014) [24], a discrete material. Our findings of macroscopic lonsdaleite in fluorite from Sadisdorf underline this statement. That means at least that at the formation of lonsdaleite, an enormous pressure has worked. Conceivable is the transport from mantle regions via supercritical fluids or melts or a tremendous pressure impact during the breccia formation. The lonsdaleite crystals in violet fluorite from Zinnwald/E- Erzgebirge are more frequent and more stable than the lonsdaleite from Sadisdorf. Therefore, we could perform more Raman measurements under different conditions. At the high intensity of the laser (about 20 mW on the sample), a very strong graphite line at 1581 cm^-1 appears. The clear differentiation between diamond and lonsdaleite is uncertain (line at 1328 cm^-1). To avoid heating the lonsdaleite sample using the laser, we used a low-intensity laser excitation (0.9 mW on the sample) in analogy to Goryainov et al., 2018) [17], which used a 325 nm UV laser with 1 mW on the sample. Figure 10 shows the results of our measurements. We see clearly three Gaussian components at (1251.3 ± 9.4 cm^-1), (1310.6 ± 3.9), and (1350.4 ± 9.9 cm^-1), with the FWHMs of 57.4, 58,8, and 67.0 cm^-1, respectively – 10 measurements each. These three experimental lines, according to Goryainov et al., 2018 [17], can be assigned to the theoretical Raman lines E_2g, A_1g, and E_1g obtained through ab initio calculations [17]. Independent of the interpretation of our described carbon phases as lonsdaleite or hexagonal diamonds as inclusions in upper crustal minerals, transport via supercritical phases from the mantle region to the crust is necessary. Our here-presented results increase the number of high- pressure and high-temperature minerals (diamond, nano-diamonds, moissanite, stishovite, coesite, kumdykolite, beryl-II, cristobalite-II, cristobalite X-I, and CaCl₂-type cassiterite) in the Variscan granites and tin mineralizations in the crustal Erzgebirge region [26].

Figure 10: Raman spectrum of a lonsdaleite crystal (diameter ~ 24 µm, thickness ~14 µm) in fluorite from Zinnwald/E-Erzgebirge, taken with 0.9 mW on the sample and a measuring time of 1000 s.

Acknowledgment

For the longstanding and often controversial discussions of the interaction between mantle and crust, the first author thanks Otto Leeder (1933-2014) from the Mining Academy Freiberg.

References

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DOI: 10.31038/GEMS.2024642

Abstract

Raman studies on cassiterite from the Sauberg mine near Ehrenfriedersdorf showed that besides the tetragonal rutile-type phase in the root zone of a cassiterite vein, there is also present orthorhombic cassiterite with the CaCl₂ structure. According to Raman measurements, a maximal pressure of 18.9 GPa results. Such pressure implies the origin of that cassiterite from great depths, brought with supercritical fluids into the lower crustal level. The results show a reverse transition from high-pressure to low-pressure polymorphs of SnO₂ in nature.

Keywords

Raman spectroscopy, Tetragonal and orthorhombic Cassiterite, CaCl₂-to-rutile transition, Supercritical fluids

Introduction

The naturally occurring form of cassiterite is usually tetragonal, with the point group 4/m 2/m 2/m (point group number 128), and crystallizes as a rutile type phase. The paper by Thomas [1] described unusual cassiterite crystals from the tin deposit Ehrenfriedersdorf, Erzgebirge/Germany, as orthorhombic ones.

However, in the Balakrishnan et al. [2], only the bands at 446 (444 and 448 cm^-1) are present for the stable SnO₂ polymorphs. The data for the metastable phases are not given. All in all, the authors mentioned 20 relatively stable polymorphs. Seven stable polymorphs are tabulated. The band at 832 cm^-1 is missing for all stable newly identified orthorhombic cassiterites. However, in the case of Ehrenfriedersdorf, at azimuthal rotation under the Raman microscope, some crystals show two unusual Raman bands at about 446 and 832 cm^-1, which are very strong at specific azimuthal positions and room temperature and room pressure. These authors (Balakrishnan et al., [2] have also stated that one polymorph can easily transformed into another by varying temperature or pressure. That must be true also for a combination of both variables. However, the transformation is sluggish enough to conserve precede phase states. In the case of the specific cassiterite from Ehrenfriedersdorf, we assume that the cassiterite came very fast from mantle deeps via supercritical fluids indicated by minerals like diamond, graphite, moissanite, OH-rich topaz, and the high-temperature feldspar kumdykolite [NaAlSi₃O₈] in the closer paragenesis [1]. Therefore, it is quite possible that at room temperature and pressure, “abnormal” cassiterite contains quenched remnants of high-temperature and high-pressure indications in the form of unusual Raman bands. This paper serves as a starting point for more systematic studies of cassiterite as a natural pressure sensor.

Sample Material

All sample material for this study came from a specimen, about 10 x 7 x 3 cm large, taken from the Prinzler counter vein in the Sauberg mine near Ehrenfriedersdorf by Puffe in 1936. The main minerals are quartz (~6 cm long), cassiterite (2 cm in diameter), and violet to green, sometimes pink fluorite (up to 1 cm in diameter). In the root zone, there are inclusions in different minerals (topaz, quartz, muscovite), which are generally tiny crystals of albite, kumdykolite, plagioclase, trilithionite, calcite, colorless high-temperature fluorite, OH-rich topaz, rynersonite, cassiterite, mangancolumbite, uraninite, monazite, xenotime, graphite, diamond, moissanite, and Ti-carbides [1]. Figures 1-3 show the studied cassiterite crystals, which are very different in appearance. In muscovite, there are smaller crystals beside the crystal shown in Figure 1, which are spherical or elliptical. The cassiterite crystal in Figure 2 is 500 µm thick as the tick section is. Figure 3 is a sizeable orthorhombic cassiterite crystal in spherical muscovite inclusion surrounded by dark tetragonal cassiterites. All cassiterites with the untypical Raman bands at 445 and 832 cm^-1 are inclusion- free, and the trace element concentration is low (about at the detection limit of the microprobe). Tiny crystals are colorless.

Figure 1: Cassiterite crystal I with rhombohedral cross-section in muscovite. The Raman bands of muscovite are entirely suppressed.

Figure 2: Cassiterite (Cst) crystall (crystal-II), about 250 x 390 µm large, beside tetragonal cassiterite (black), OH-rich topaz (OH-Toz) and calcite (Cal).

Figure 3: Large orthorhombic cassiterite crystal (o-Cst) in a muscovite inclusion (white) between normal tetragonal cassiterite (Cst). That is the sample cassiterite III.

The differentiation between tetragonal and orthorhombic cassiterite is under the Raman microscope with a rotating stage simple. Rutile-type cassiterite shows at room temperature only one strong Raman band at about 633 cm^-1 during azimuthal rotation under the polarized Raman light. Strong Raman bands at about 76, 448, 635, and 834 cm^-1 are characteristically for orthorhombic cassiterite. The symmetry of the CaCl₂-type structure is orthorhombic and has the space group P4₂/mnm.

Microscopy and Raman Spectroscopy: Methodology

For the study of the cassiterite sample and the paragenetic minerals, we use the Zeiss JENALAB pol as well as the Raman spectrometer EnSpectr R532 combined with the Olympus BX43 microscope both for transmitted and reflected light and equipped with a rotating stage. For the identification of minerals and slight mineral inclusions, we used an Olympus long-distance LMPLFL100x objective lens. For the identification of different minerals, we used the RRUFF and the Hurai et al. [3] Raman mineral databases [3-4]. As references, we applied a water-clear diamond crystal from Brazil and a semiconductor-grade silicon single-crystal.

Results

In contrast to the typical tetragonal cassiterite of the Erzgebirge with the usual bands at 474, 633, and 775 cm^-1 (both bands at 474 and 775 cm^-1 are generally weak), the here-discussed cassiterite shows additional azimuthal-depending strong bands at 446, 832 cm^-11, beside the 633 cm^-1 band [1]. In the compilation [2] of the Raman modes of stable SnO₂ polymorphs, only the orthorhombic cassiterite Pbcn (point group 2/m 2/m 2/m (number 60 in the room group list) contains a Raman active band at about 446 cm^-1. The 832 cm^-1 band is completely missing in the list of polymorphs. Figure 4 shows a typical Raman spectrum of cassiterite crystal III. Conspicuous are the strong bands at 76, 448, 635, and 834 cm^-1.

Figure 4: Raman spectrum (a choice of 46 spectra) of SnO₂ (sample III).

According to the measurements of the Raman intensity of both bands (446 and 832 cm^-1), there is a good correlation shown in Figure 5. That means that both Raman bands belong together.

Figure 5: Correlation between the Raman intensity of the 446 and 832 cm^-1 bands; I₈₃₂cm^-1=-89.829 + 0.609 * I₄₄₆ cm^-1, r²=0.989.

Because mineral inclusions in all the studied cassiterite crystals here are missing, the Raman bands are clearly components of the orthorhombic cassiterite, and both are strongly correlated. Also, the other crystals show a correlation between the two bands with near the same incline. That also will be clear from the following diagram (Figure 6). This figure shows the intensity ratio between the 633 and 832 cm^-1 bands in dependence on the azimuth position. The figure (Figure 6) shows, in principle, the results for all three studied cassiterite crystals (I to III), which only show a peaks’ position dependence on the crystal orientation (a synchronous shift of the maxima to right or left).

Figure 6: Intensity ratio between the 633 and the 832 cm^-1 bands versus the azimuth position for crystal I. Similar figures resulted for the cassiterite crystals II and III.

Figure 7 shows the azimuthal Raman intensity distribution for the orthorhombic cassiterite main band at 633 cm^-1. The position of the points depends on the orientation of the studied sample. The points for the 446 and 832 Raman bands lie almost perpendicular to the 150° – 330° line. In the case of tetragonal cassiterite, the open red points would form a circle.

Figure 7: Cassiterite, crystal-III: Raman intensity distribution for the 633 cm^-1 band in dependence on the azimuth position of the crystal. The numbers on the left are intensities.

Interpretation

The Raman bands at 446, 633, and 832 cm^-1 are strongly polarized, and the bands at 446 and 832 have a different symmetry – they are almost perpendicular to the 633 cm^-1 band. The explanation is not simple. Have we, in this case, a different polymorph phase of SnO₂ not described in Balakrishnan et al., [2]? Or are the unusual Raman bands of SnO₂ frozen high-temperature and high-pressure remnants of the rutile-type, orthorhombic, or the CaCl₂ phase of cassiterite?. Note that in Figure 8, the points for the 832 cm^-1 Raman band show a twisted form. From 38 measurements of the 832 cm^-1 band on the cassiterite crystal-III, we obtain a mean of 833.9 ± 0.4 cm^-1. This value corresponds to Hellwig et al. [5] for the B_2g mode to a pressure of 10.5 GPa and falls into the rutile-type cassiterite. According to Girao [6], we can assume that the high-temperature and high-pressure cassiterite are well-crystallized (indicated by the intense and sharp Raman bands), are nano-particles in high concentrations, or contain larger domains. According to Girao [6] [Table 4, p.105], it results from the mean of 833.9 cm^-1, a pressure of about 15 GPa. This pressure marks the rutile- to CaCl₂-type transition. The rutile polymorph of SnO₂ underwent a phase transition to a CaCl₂ polymorph at 11.8 GPa under hydrostatic conditions [2]. Sometimes, we observe on the 833.9 band a shoulder at 849.7 ± 1.1 cm^-1 (n = 10). Using Table 4 in Girao [6], it results in a pressure of 18.9 GPa and is, obviously, a high-pressure remnant of the CaCl₂-type cassiterite.

Figure 8: Cassiterite, crystal-III: Raman intensity distribution for the 633 cm^-1 (red) and the 832 cm^-1 (green) bands in dependence on the azimuth position of the crystal. The other orientation of the 832 cm^-1 band is good to see. The measured intensities are the numbers on the right side of the diagram.

Generally, besides the 633 cm^-1 prominent bands, small bands at 695.9 ± 2.1 cm^-1 are present. After Figure 7 and Table 4 in Hellwig et al. [5], results for this A_1g mode band a pressure of 12.2 GPa and, according to Girao [6] [Figure 54 and Table 4], a pressure of 13.7 GPa. For the cassiterite crystal-III, we could also determine for the B_1g mode a mean of 76.1 ± 0.5 cm^-1 (n = 46). The intensity of this Raman band is very high. Using Figure 8 in Hellwig et al. [5] results in a pressure of 10.8 GPa. By some uncertainties (strong asymmetry of this band) of the soft mode in the CaCl2 phase [5], the 76.1 cm^-1 band can also represent the CaCl₂-type phase. The symmetry is similar to the 446 and 832 cm^-1 bands.

The band at 448.2 ± 0.4 cm^-1 strongly correlated with the 833 cm^-1 band (n = 38) (see Figure 5), resulting after Girao [6] only a pressure of 5.7 GPa. That means the freezing behavior for the different Raman bands of different SnO₂ polytypes is not regular.

Discussion

The exceptional Raman band at 832 cm^-1, shown at first by Thomas [1], can explained, according to Hellwig et al. [5] and Girao [6], as frozen remnants of high-pressure phases of rutile- and CaCl₂-type cassiterite structures.

Because together with the orthorhombic cassiterite at room temperature, there are also present high-pressure and high-temperature indicator minerals, like diamond, moissanite, Ti-carbides, and kumdykolite [1], the interpretation of the extreme Raman bands finds his explanation. That means that a part of the cassiterite of the Ehrenfriedersdorf Sauberg mine comes directly from the mantle regions. Schütze et al., [7] came after careful studies to the result that the Ehrenfriedersdorf granite presents the differentiation products of subducted altered ocean crust. The proof of high-pressure cassiterite (with signs up to 18.9 GPa) underlines this interpretation. After a couple of studies (for example, Thomas and Rericha, 2023) [8], the transport of high-pressure cassiterite (suspended as solid phases) happens via supercritical fluids from mantle regions to the crust. We have not considered the influence of the temperature on the band shift [9].

We found many deposits in the Erzgebirge (Germany) and the Slavkovsky les (Czech Republic), which prove the presence of orthorhombic cassiterites. More sophisticated studies on the natural cassiterite samples are necessary.

Acknowledgments

For the sample, we are grateful to Professor Ludwig Baumann (1929-2008) from the Mining Academy Freiberg. We thank Pierre Bouvier, Grenoble, France, and Jörg Acker, Cottbus, Germany, for the courtesy of critical references and for starting the discussion on the unusual cassiterite from Ehrenfriedersdorf.

References

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