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.