Mexican Demantoid from New Deposits

Mikhail Ostrooumov

Gemologists have recognized three andradite varieties: melanite (black), topazolite (yellow-brown), and demantoid. Demantoid, the yellowish green to green variety, is the most important to the jeweler-gemologist. The traditional demantoid garnet sources are Russia (Ural Mountains) and Italy (Val Malenco and Sondrio). More than 20 years ago, small garnet crystals of pale yellowish green color were found in the Sonora andradite occurrence near the Mexican city of Hermosillo (Fall 1994 GNI, p. 194). This garnet has been classified as demantoid without any detailed gemological and mineralogical analysis.
 
It is very interesting to note that in some deposits of metamorphic skarns (e.g., Kamchatka, Russia; Val Malenco, Italy; and Arizona, USA), the demantoid crystals are associated with topazolite (E.P. Kievlenko, Geology of Gems, Ocean Pictures, Littleton, Colorado, 2003). In late 2014, our investigations showed that the same mineralogical association (figure 1, left) is characteristic for the Las Vigas Mexican topazolite source (Fall 2014 GNI, pp. 246–247). These green to yellow-green garnet crystals, measuring 1.5–5.5 mm, are also hosted by the Las Vigas skarn deposits (Cerro de la Concordia, in Las Vigas de Ramirez municipality and the Piedra Parada mine in Tatatila municipality). These deposits are located in Veracruz State, about 50 km southeast of the town of Valle de Veracruz. Studying crystals from these two new Mexican deposits with different analytical techniques, the mineral was identified as demantoid garnet (figure 1, right).
 
Three samples of rough demantoid (measuring 4.0–5.5 mm in the longest dimension) were characterized for this report, and the following gemological properties were determined: color—green; polariscope reaction—isotropic and weakly anisotropic; weak strain birefringence; RI—n_α = 1.888–1.889; very strong dispersion, at 0.055 (visual indicator identification); hydrostatic SG—3.82–3.88; fluorescence—inert to both long- and short-wave UV radiation.
 
The garnet composition was determined by means of electron microprobe analysis (EPMA), using a total of 15 analysis points. Standard conditions of 20 kV, 20 mA, and 1 µm beam size were used with a JEOL JSM-35c microprobe. The analyses showed little compositional heterogeneity or zonation. The structural formula was calculated on the basis of 12 oxygen atoms, yielding an average approximate composition of (Ca_2.96 Fe²⁺_0.08)_3.04 Fe³⁺_2.05 Si_2.94O₁₂. This structural formula has an abundance at the X-site and Y-site cations but is close to electroneutrality due to the sum of the positive and negative charges (W_c = +23.99; W_a = –24.00). The EPMA-WDS data showed that these green garnets were almost pure andradite (And 95.60 mol.%). No chromium was detected in the chemical composition of the studied crystals.
 
Infrared transmission spectra were measured with a Bruker Tensor 27 FTIR spectrometer, scanning from 4000 to 400 cm^–1 and using the KBr pellet method. The pellets were prepared by mixing approximately 3 mg of the sample with 300 mg KBr. OPUS software was used for the spectroscopic interpretation of the infrared spectra. Raman (figure 2, left), mid-infrared (figure 2, right) and X-ray analysis confirmed that the crystals belonged to the demantoid variety (http://rruff.info). The mid-IR spectra showed the presence of hydroxyl groups. The infrared spectra consisted of a prominent band at 3420 cm^–1, with a secondary band at about 3610 cm^–1 due to the fundamental OH stretching vibration of water molecules, as well as the water bending vibration at approximately 1650 cm^–1.

UV-Vis-NIR spectroscopy showed absorption bands at 403, 444, 574, 617, 854, and 1170 nm, which can be assigned to spin-forbidden crystal-field transition of Fe³⁺, substituted on the octahedral Al³⁺ site of the garnet structure. Our EPMA analyses of garnet generally indicated the presence of some divalent iron. The spectra showed absorption bands in the 900–1000 nm and 1150–1250 nm ranges, which are assigned to Fe²⁺, and corresponding absorption features were observed at about 860 and 1170 nm. The simultaneous presence of both Fe³⁺ and Fe²⁺ means that intervalence charge transfer is possible (in accordance with A.S. Marfunin, Advanced Mineralogy, Vol. 2, Springer-Verlag, Berlin, 1995, pp. 113–114), and the 574 nm band is therefore assigned to an Fe²⁺→Fe³⁺ intervalence charge transfer band. The typical absorption bands of Cr³⁺ in the visible region between 630 and 690 nm are absent, consistent with the EPMA results. Our investigations have also shown that some demantoid garnets from Russia do not show any chromium absorption, and their color is due to the presence of Fe³⁺ alone.
 
The color measuring system of the International Commission of Illumination (ICI) has been found useful for describing the color characteristics of minerals (K. Langer et al., “Optical absorption spectroscopy,” in A.S. Marfunin, Ed., Advanced Mineralogy, Vol. 2, Springer-Verlag, Berlin, 1995, pp. 119–122). The color of a mineral is assigned to a point in the x-y coordinates of the ICI color chart. Special computer programs are used to calculate the color parameters (x-y coordinates, λ—dominant wavelength, P—saturation or purity, Y—lightness) of a mineral directly from the measured optical absorption spectrum. The dominant wavelength λ, or hue, is the human eye’s psycho-sensory interpretation of wavelengths that are identified by the x-y coordinates of the ICI color chart. Preliminary colorimetric calculations showed that the colors of the Mexican demantoid have low saturation (P=20%) and lightness (Y=17%). Therefore, they are darker than pure green demantoid from other deposits (e.g., Bobrovka River in the Ural Mountains). The dominant wavelength of Mexican demantoid (λ=560 nm) is a slightly yellowish green. This combination of colorimetric parameters defines a color different from that of chromium-bearing demantoid. Measurements on the demantoid from the Urals showed higher values of saturation (P=44–50) and lightness (Y=20–37%) and a purer green hue, with λ=530–545 nm (M. Ostrooumov, “Colorimetry of minerals,” Priroda, No. 6, 1987, pp. 43–53, in Russian). Such demantoids are more attractive to the gem trade.
 
Thus, electron microprobe (EPMA-WDS) chemical analyses, various spectroscopic techniques, X-ray diffraction, and standard gemological testing have confirmed the discovery of demantoid in the Las Vigas skarn deposits of Veracruz State. The discovery could represent an interesting mineralogical and gemological opportunity. Although the full range and economic potential of this demantoid has not been determined, it may well have features that distinguish it from other important deposits worldwide.