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.

Inclusions and Spectral Characterization of Demantoid from Baluchistan, Pakistan

“…The samples are almost pure andradite with a very low content of Al. Fe is the main component of demantoid and plays a crucial role in its coloration as an end member [23]. Mn was detected in each sample, Mn and V were rather constant, and Mn did not show any correlation with color in demantoid [6].…”

Characterization of natural silicate garnets by means of non-destructive testing methods

“…The mode associated with the rotation of SiO 4 tetrahedra R(SiO 4 ) appears in the lower frequency region 350-420 cm -1 [42]. Finally, the translation modes T(SiO 4 ) and T(M) usually can be observed at wavenumbers lower than 340 cm -1 [42,43,45].…”

“…Raman spectroscopy provides insights into the molecular structure and short-range ordering of garnets [37][38][39][40][41][42][43][44][45][46]. The Raman spectra of different garnets are displayed in The vibrational bands of the studied compounds can be divided into several groups: (i) internal vibrations of SiO 4 tetrahedra (symmetric stretching, ν 1 ; asymmetric stretching, ν 3 ; symmetric bending, ν 2 ; asymmetric bending, ν 4 ), (ii) translation of SiO 4 tetrahedra, T(SiO 4 ), (iii) rotation of SiO 4 tetrahedra, R(SiO 4 ) and (iv) translation of YO 6 octahedra (Y=Fe 3+ , Cr 3+ and Al 3+ ), T(M) [42][43][44][45]. The symmetric stretching vibrational mode of SiO 4 group (ν 1 ) appears as a medium-intense band in the frequency region 874-918 cm -1 .…”

Explaining Color Change in Gem-Quality Andradite Garnet

“…The Fe 2+ spin-forbidden transition ( 5 E(D) → 3 T 1 (H)) produces a broad absorption band in the visible-light range 550-650 nm, leading to two transmittance regions on both sides, which show a yellowish green to brown alexandrite-like effect along with the change in the spectral power distribution of the light source. Moreover, a broad band at about 856 nm was due to octahedral Fe 3+ [25,28,33].…”