Gem-Quality Green Cr-Bearing Andradite (var. Demantoid) from Dobšiná, Slovakia

Abstract

Andradite, variety demantoid, is a rare gem mineral. We describe gem-quality garnet crystals from serpentinized harzburgites from Dobšiná, Slovakia which were faceted. Both the andradite samples were transparent, with a vitreous luster and a vivid green color. They were isotropic with refractive indices >1.81. The measured density ranged from 3.82 to 3.84 g·cm⁻³. Andradite var. demantoid appeared red under Chelsea filter observation. Both samples contained fibrous crystalline inclusions with the typical “horsetail” arrangement. The studied garnet had a strong Fe³⁺ dominant andradite composition with 1.72–1.85 apfu Fe³⁺, Cr³⁺ up to 0.15 apfu, Al³⁺ 0.03 to 0.04 apfu, V³⁺ up to 0.006 apfu substituted for Fe³⁺, Mn²⁺ up to 0.002 apfu, and Mg up to 0.04 apfu substituted for Ca. Raman spectrum of garnet showed three spectral regions containing relatively strong bands: I—352–371 cm⁻¹, II—816–874 cm⁻¹, and III—493–516 cm⁻¹. The optical absorption spectrum as characterized by an intense band at 438 nm and two broad bands at 587 and 623 nm and last one at 861 nm, which were assigned to Fe³⁺ and Cr³⁺. Transmission was observed in the ultraviolet spectral region (<390 nm), near the infrared region (700–800 nm), and around 530 nm in the green region of visible light, resulting in the garnet’s green color.

1. Introduction

Slovakia has only a few occurrences which produce gem-quality minerals. For a long time in history, precious opal from Červenica-Dubník was the only mineral considered as a gemstone [1,2,3,4,5,6,7]. Only a few Slovak minerals, including quartz var. smoky quartz [8,9], sphalerite [8,10], pyrite [8], garnet var. almandine [8], hematite [8], and corundum var. sapphire [11,12,13] are suitable for faceting.

Andradite garnet, Ca₃Fe₂Si₃O₁₂, is a rock-forming mineral typically found in metamorphic rocks such as skarns and rodingites. The andradite crystal structure consists of alternating SiO₄ tetrahedra and FeO₆ octahedra, and Ca with eight-fold coordination. Several studies have been carried out on the crystal chemistry and the thermodynamic properties of andradite garnets [14,15,16,17,18,19,20,21,22]. Green to yellow-green andradite garnets, demantoid variety, represent one of the most appreciated and precious gemstones among the garnet-group minerals, due to their color, brilliance, and rarity [23,24,25]. Gem-quality andradite-garnet sources occur in Russia [26,27,28,29], Italy [30], Iran [31,32,33], Pakistan [25,34,35,36,37], Namibia [38,39,40,41], Madagascar [42,43,44,45,46,47], Afganistan [24], Switzerland [24], Canada [48], Mexico [49], and China [50].

Our goal is a mineralogical and crystal-chemical investigation of andradite var. demantoid from a serpentinite body in Dobšiná, Slovakia using various analytical methods.

Geological Setting

The serpentinized harzburgites from Dobšiná are part of the Jurassic mélange complex (Meliata Unit). This inner Western-Carpathian unit forms small nappes over the Central Western Carpathians [51]. The Meliata Ocean formed after the continental break up in the Anisian [52], probably as a back-arc basin, due to northward subduction of the Paleotethys oceanic crust beneath the Eurasian continent [53]. The back-arc basin (BAB) basalts and mid-ocean ridge basalts (MORB) were dominant in the Meliatic oceanic rift [54,55]. The association of eupelagic and deepwater pelagic sediments, basalts, dolerite dikes, gabbros, and serpentinized mantle peridotites found in the Meliata Unit clearly indicates an oceanic suite [56,57]. The geochemistry of the basalts reveals a normal mid-ocean ridge basalts (N-MORB) [(La/Sm) n = 0.6–0.9] composition [54,55]. Traces of obducted oceanic crust (blocks of basalts alternating with radiolarite layers) were also identified from deepwater sedimentary breccias with a radiolaritic matrix [58]. The Neotethyan Meliata Oceanic back-arc basin closed in the Late Jurassic, according to ⁴⁰Ar–³⁹Ar ages of white mica from the blueschists and U/Pb LA-ICP-MS age of metamorphic-metasomatic perovskite [59,60]. The Meliatic subduction–accretionary wedge [61] was incorporated into the Cretaceous orogenic wedge of the Central Western Carpathians [62] between 130 and 50 Ma dated by ⁴⁰Ar–³⁹Ar method on white micas [63] in the form of mostly small nappe-sheeted bodies, thrust over the Gemeric and Veporic units of the Central Western Carpathians [64,65]. The zircon (U-Th)/He data recorded three evolutionary stages: (i) cooling through the ~180 °C isotherm at 130–120 Ma related to the startling collapse of the accretionary wedge, following exhumation of the high-pressure slices in the Meliatic accretionary wedge; (ii) postponed exhumation and cooling of some fragments through the ~180 °C isotherm from 115 to 95 Ma due to ongoing collapse of this wedge; and (iii) cooling from 80 to 65 Ma, postdating the thrusting (~100–80 Ma) during the Late Cretaceous compression related to formation of the Central Western Carpathians orogenic wedge [66].

The Dobšiná quarry (48°49.622′ N, 20°21.977′ E) (Figure 1) is in a tectonic fragment of the Meliata tectonic unit overlying the Gemeric tectonic unit. The locality is famous for the occurrence of yellow-green Cr-rich andradite (demantoid) in serpentinite veins [67,68,69]. Andradite-bearing serpentinites are part of the Late Jurassic (to Early Cretaceous) serpentinite–blueschist mélange complex. The mélange complex contains talc–phengite–glaucophane schists, marbles, metaconglomerates, blueschists of magmatic and sedimentary origin, and serpentinites. All rocks form decimeter- to 100 m-sized fragments within the Late Jurassic very low-temperature greenschist-facies metasediments [70].

Partly-preserved magmatic structures of harzburgite consist of olivine (~70 vol. %), orthopyroxene (~25 vol. %), accessory spinel (~4 vol. %), and rare clinopyroxene (~1 vol. %). Magmatically corroded and partly dissolved porphyric orthopyroxene (±clinopyroxene) is enclosed in olivine matrix with spinel. Both pyroxenes show exsolution lamellae, of clinopyroxene in orthopyroxene, or orthopyroxene in clinopyroxene, respectively. Orthopyroxene often forms sigmoidally rotated porphyroclasts, including clinopyroxene exsolution lamellae. The serpentinized harzburgite contains serpentinite group minerals (antigorite replaced by chrysotile and lizardite), Cr-Fe spinel, perovskite, pyrophanite, Ti andradite, magnetite, and hematite, and rarely carbonates with talc, quartz, and characteristic relics of olivine, orthopyroxene, clinopyroxene, and spinel [70,71].

2. Materials and Methods

2.1. Sample Description

Two pieces of transparent andradite var. demantoid were separated from the host rock and faceted into round brilliant cuts with dimensions of 2.80 × 2.80 × 1.64 mm (0.11 carat) and 1.71 × 1.74 × 1.14 mm (0.025 carat) dimensions (Figure 2).

2.2. Analytical Methods

The faceted samples were examined by standard gemmological methods at the Gemmological Institute, Faculty of Natural Sciences, Constantine the Philosopher University in Nitra, to determine their optical properties (optical character and refractive indices), hydrostatic specific gravity, ultraviolet fluorescence, and microscopic features. The refractive indices were measured with a refractometer Kruss and Presidium Refractive Index Meter II. A Kern ABT 120-5DM with extension KERN ABT A01 hydrostatic balance was used to determine the density of the gemstones. The ultraviolet fluorescence was investigated with a short (254 nm) and long (366 nm) wavelength ultraviolet lamp. We used a Chelsea filter.

The chemical composition of andradite garnet was measured using a Cameca SX100 electron microprobe operated in wavelength-dispersion mode at the Masaryk University, Brno, Czech Republic, under the following conditions: accelerating voltage 15 kV, beam current 20 nA, and beam diameter 5 μm. The samples were analyzed with the following standards: fluorapatite (PKα), sanidine (SiKα, AlKα, KKα), titanite (TiKα), chromite (CrKα), vanadinite (VKα), YAG (YLα), almandine (FeKα), spessartine (MnKα), forsterite (Mg Kα), gahnite (ZnKα), wollastonite (CaKα), albite (NaKα), and topaz (FKα). Lower detection limits of the measured elements varied between 200–400 ppm except for Na (530–570 ppm), Fe (670–730 ppm), Zn (880–900 ppm), and Y (470–500 ppm). The measured content of Y, Mn, Zn, Na, and K was always below the detection limit. Analytical times were 15 to 40 s during measurement depending on the expected concentration of the element in the mineral phase. Major elements were measured using shorter times, whereas longer times were applied for elements with a low concentration. The andradite chemical formula was calculated based on 12 anions assuming all Fe as ferric.

Instrumentation for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Department of Chemistry, Masaryk University, Brno, consists of a UP 213 laser ablation system (New Wave Research, Inc., Fremont, CA, USA) and Agilent 7500 CE ICP-MS spectrometer (Agilent Technologies, Santa Clara, CA, USA). A commercial Q-switched Nd-YAG laser ablation device worked at the 5th harmonic frequency corresponding to 213 nm wavelength. The ablation device was equipped with a programmable XYZ-stage to move the sample along a programmed trajectory during ablation. Target visual inspection and photographic documentation were achieved by the built-in microscope/CCD-camera system. A sample was enclosed in the SuperCell and was ablated by the laser beam, which was focused onto the sample surface through a quartz window. The ablation cell was flushed with a helium carrier gas which transported the laser-induced aerosol to the inductively coupled plasma (1 L·min⁻¹). Sample argon gas flow was admixed with the helium carrier gas flow after the laser ablation cell to 1.6 L·min⁻¹ total gas flow. NIST SRM 610 silicate glass reference material was used to optimize the gas flow rates, the sampling depth, and MS electrostatic lens voltage in LA-ICP-MS conditions. This provided maximum signal-to-noise ratio and minimum oxide formation (ThO⁺/Th⁺ count ratio 0.2%, U⁺/Th⁺ counts ratio 1.1%). Laser ablation required a 100 μm laser spot diameter, 8 J·cm⁻² laser fluence, and a 20 Hz repetition rate. The fixed sample position during laser ablation enabled 60 s hole-drilling duration for each spot. All element measurements were normalized on ²⁸Si in the investigated andradite. Analyses were gained from 10 spots in a line across the crystal.

Raman analyses were performed on the LabRAM-HR Evolution (Horiba Jobin-Yvon, Kyoto, Japan) spectrometer system with a Peltier cooled CCD detector and Olympus BX-41 microscope (Department of Geological Sciences, Masaryk University). Raman spectra excited by blue diode laser (473 nm) in the range of 50–4000 cm⁻¹ were collected from each stone using 50× objectives. The acquisition time of 10 s per frame and 2 accumulations were used to improve the signal-to-noise ratio.

Optical absorption spectra of cut stones in the region (400–750 nm) were measured with the GL Gem SpectrometerTM at room temperature in the Gemmological Institute of Constantine the Philosopher University, Nitra. Both Raman and UV/Vis/NIR spectra were processed in Seasolve PeakFit 4.1.12 software. Raman and absorption bands were fitted by Lorentz function with the automatic background correction and Savitsky-Golay smoothing.

3. Results

Both the andradite samples were transparent, with a vitreous luster and a yellow-green color, which defines the andradite variety. They were all isotropic with refractive indices >1.81. The measured density ranged from 3.82 to 3.84 g·cm⁻³, and the gemstones were inert in shortwave and longwave ultraviolet radiation. Andradite var. demantoid appeared red under Chelsea filter observation, which is typical for Cr-bearing gemstones. Both samples contained fibrous crystalline inclusions with the typical “horsetail” arrangement (Figure 3).

The studied garnet had a strongly Fe³⁺ dominant andradite composition with 1.72–1.85 apfu Fe³⁺ (

Table 1. Electron microprobe analyses of demantoids, first provided in wt % of elements and then recalculated on the basis of 12 anions.

	1	2	3	4	5	6	7	8
P2O5	0.02	0.02	0.02	0.02	0.04	0.00	0.02	0.02
SiO2	35.21	35.41	35.38	35.33	35.47	35.43	35.74	35.72
TiO2	0.05	0.05	0.04	0.03	0.00	0.03	0.08	0.04
Al2O3	0.33	0.34	0.31	0.32	0.40	0.34	0.37	0.32
V2O3	0.09	0.07	0.07	0.08	0.07	0.09	0.06	0.05
Cr2O3	1.24	1.50	2.31	2.14	1.54	1.34	0.97	0.63
Fe2O3	29.15	29.15	27.69	28.34	28.64	28.82	29.16	29.65
MgO	0.20	0.23	0.20	0.17	0.34	0.23	0.20	0.21
CaO	33.88	33.80	33.74	33.72	33.85	33.85	33.74	33.65
F	0.08	0.11	0.09	0.11	0.09	0.08	0.09	0.10
O=F	−0.04	−0.05	−0.04	−0.06	−0.04	−0.04	−0.05	−0.05
Total	100.19	100.63	99.80	100.20	100.40	100.16	100.38	100.34
P5+	0.002	0.001	0.001	0.001	0.003	0.000	0.001	0.001
Si4+	2.975	2.977	2.993	2.981	2.984	2.990	3.003	3.004
F−	0.021	0.028	0.023	0.030	0.023	0.022	0.024	0.026
Σ	2.997	3.006	3.018	3.012	3.010	3.012	3.029	3.031
Ti4+	0.003	0.003	0.003	0.002	0.000	0.002	0.005	0.003
Al3+	0.033	0.034	0.031	0.032	0.040	0.034	0.037	0.032
V3+	0.006	0.005	0.005	0.006	0.005	0.006	0.004	0.003
Cr3+	0.083	0.100	0.155	0.143	0.102	0.089	0.064	0.042
Fe3+	1.845	1.837	1.757	1.793	1.806	1.823	1.840	1.873
Σ	1.969	1.979	1.950	1.975	1.953	1.954	1.950	1.952
Mg2+	0.025	0.029	0.026	0.022	0.043	0.028	0.025	0.026
Ca2+	3.067	3.044	3.059	3.049	3.052	3.060	3.037	3.032
Σ	3.092	3.073	3.084	3.071	3.095	3.089	3.063	3.058

and Figure 4). Only Cr³⁺ (up to 0.15 apfu), Al³⁺ (0.03 to 0.04 apfu), and Mg (up to 0.04 apfu) substituted for Fe³⁺. From the trace elements, Cr was the most abundant, along with moderate to minor concentrations (in relative decreasing order of abundance) of V, Ti, and Mn. The other trace elements measured (Sc, Ge, As, Y, and Gd) were near or below their respective detection limit (

Table 2. The trace-elements contents (in ppm) in demantoid from Dobšiná measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). (LOD—limit of detection).

	K	Sc	Ti	V51	Cr53	Mn	Ge72	As75	Y89	Mo95	Gd157
Min	392	16	103	305	3814	103	4	11	2	0	0
Max	787	40	389	572	13331	224	13	22	5	1	2
Average	523	25	244	444	9199	143	9	16	3	1	1
LOD	48.7	2.1	11.4	0.7	9.2	7.8	3.8	9.1	0.1	0.1	0.1

). There was no specific chemical zoning of crystal observed in major or trace elements.

From the calculated crystal-chemical formulae, it is obvious that Z site occupancy by Si is slightly lower than three. It could be filled to three by Al or Fe³⁺, but their content is sufficient only to accommodate the Y site. Even after adding all possible octahedral cations, the Y-site sum is always slightly below two which, however, could result from the analytical error. However, the small deficiency at the Z site can be justified by the presence of F. The X site is almost uniformly occupied by Ca with a very small admixture of Mg.

Raman spectrum of garnet (Figure 5 and

Table 3. Raman shifts (cm⁻¹) of Raman-active bands in the studied sample. Assignment of band modes was made according to Reference [72].

Symmetry	Assignment	Demantoid Dobšiná	Andradite [72]
T2g + T1u	T(M)	174	173
T2g + T1u	T(M)	236	235
T2g	T(SiO4)mix	265	264
Eg	T(SiO4)	297	296
T2g + T1u	T(SiO4)	311	311
?	?	343	-
Eg	ν2	352	352
A1g	R(SiO4)	371	370
T2g + T1u	ν2	452	452
Eg	ν2	493	494
A1g	ν2	516	516
T2g + T1u	ν4	552	553
Eg	ν4	576	576
T2g + T1u	ν3	816	816
T2g + T1u	ν3	843	842
Eg	ν3	874	874
T2g + T1u	ν3	995	995

) showed three spectral regions containing relatively strong bands: I—352–371 cm⁻¹, II—816–874 cm⁻¹, III—493–516 cm⁻¹. The other bands exhibited relatively lower intensities. Measured bands were assigned to various molecular motions (Table 3): translation of SiO₄—T(SiO₄), rotation of SiO₄—R(SiO₄), translation of YO₆ (Y = Fe³⁺, Cr³⁺)—T(M), symmetric band ν₂, asymmetric stretching frequency ν₃, asymmetric bending frequency ν₄ of SiO₄ group. Our spectrum is consistent with published andradite spectra [72], except the 343 cm⁻¹ band, which is not present in published data. Only bands with similar Raman shift are in almandine and pyrope spectra associated with rotation of SiO₄ [72].

The optical absorption spectrum (Figure 6 and

Table 4. The optical spectral bands (nm) in the studied sample. Assignment of bands was made according to Reference [22] and references therein.

Chromophores	Demantoid Dobšiná
Fe3+	438
Fe3+	470
Cr3+	488
Fe3+	587
Cr3+	623
Fe3+	861
Fe3+	964

) is characterized by small absorption features below 370 nm, an intense band at 438 nm with shoulders at 470 and 488 nm, and two broad bands at 587 and 623 nm, and 861 nm with a shoulder at 964 nm. These spectral features were all assigned to Fe³⁺ and Cr³⁺. Transmission was observed in the near the infrared region (700–800 nm), and around 530 nm in the green region of visible light resulting in the garnet’s green color.

4. Discussion and Conclusions

Chromium-bearing garnets are relatively rare and are associated with specific geological environments and geochemical conditions. In the Western Carpathians, only a few occurrences have been described, including Pezinok–Rybníček [73], Čierna Lehota [74], and Dobšiná [67].

The garnet from Dobšiná occurs as the three following types [67]. The first type is present as relatively evenly dispersed minute grains in the rock. It originated preferentially at the expense of pseudomorphs after rhombic pyroxene. The garnet also forms lenticular or elongated layers 1 to 2 mm thick and several centimeters long. The second-type of garnet occurs as infilling of chrysotile veinlets. The veinlets’ thickness is up to 3 mm. The third type of garnet forms monomineral fills in 0.3 to 3 mm-thick fissures. The garnet crystals display usually euhedral habit. However, only the third type of garnets are emerald-green color. All three garnet types occur in pale-colored, pale-green varieties of serpentinite.

Andradite color differences are caused by the variable Cr content. The studied garnet belongs to the third type which is generally enriched in Cr compared to the other two types. In some crystals of andradite, the determined value exceeded 6 wt % Cr₂O₃ [67], but the studied crystals only contain up to 2.3 wt % Cr₂O₃. However, this content is still significantly higher than in other demantoid occurrences around the world [21,30,47]. Moreover, demantoid from Dobšiná is enriched in V (up to 0.09 wt % V₂O₃ from EMPA, 305–572 ppm from LA-ICP-MS) compared to other localities with V usually below the detection limit of EMPA and below 270 ppm determined by LA-ICP-MS [21,30,47]. The As content between 10–22 ppm (although very close to the detection limit, were below detection limit on other localities [21]) and absence of REE (except very low Y), which are usually between 1–90 ppm [21] could also be a fingerprint feature for Dobšiná demantoid.

Raman mode analysis predicts the existence of 25 active Raman modes in all garnet species 3A_1g, 8E_g, and 14T_2g. The vibrational spectra of garnet clearly possess two different groups of vibrational modes. The first group extends from 816 to 995 cm⁻¹ in the Raman spectrum and possesses three strong bands and one weak one. In the garnet structure, these modes are commonly assigned to the ν₁ Si–O stretching motions within SiO₄ groups [72,75,76,77,78]. The second region extends from 174 to 576 cm⁻¹ and possesses three strong and ten weak bands. The bands at 352, 452, 493, 516, 552, and 576 cm⁻¹ are commonly assigned to the Si–O bending motions within SiO₄ groups [72]. The bands at 343 and 371 cm⁻¹ can be assigned to rotation of the [SiO₄]⁴⁻ tetrahedron R[SiO₄]⁴⁻ [72]. The bands at 174, 236, 265, 297, and 311 cm⁻¹ can be assigned to translation modes of the tetrahedra and octahedra [72].

The optical absorption spectra of studied garnet revealed a few absorption and transmission regions. The absorption feature at 438 nm and its shoulders are attributed to octahedral Fe³⁺, although a further contribution from Cr³⁺ cannot be excluded [22,50,75,79,80,81,82], but this was not very significant due to the low Cr content. The weak absorption band at 488 nm is related to Fe³⁺ [30,47]. Two bands at 587 and 623 nm belong to octahedral Fe³⁺ and Cr³⁺, respectively [22,50,75,79,80,81,82]. Presumed contribution of Cr³⁺ to the band at 438 nm can be assigned, according to the Tanabe–Sugano diagram [83,84,85], to the dominant ⁴A_2g → ⁴T_2g transition in Cr³⁺. The second ⁴A_2g → ⁴T_1g transition in Cr³⁺ was at 623 nm but was overlapping with the Fe³⁺ absorption band at 587 nm. Moreover, a broad band at about 861 nm was due to octahedral Fe³⁺ [22,50,75,79,80,81,82].