Gem-quality chrysoprase from Haneti-Itiso area, Central Tanzania

• Main Authors: K.A. Kinnunen, E.J. Malisa
• Format: Article
• Language: English
• Published: Geological Society of Finland 1990-12-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: gems; Chrysoprase; chemical composition; X-ray diffraction data; spherules; fluid inclusions; crystallization; Archaean; Tanzania
• Online Access: http://www.geologinenseura.fi/bulletin/Volume62/sgs_bt_062_2_pages_157_166.pdf

Gem-quality, apple-green, Ni-bearing chalcedonic quartz occurs as near-surface veins in silicified serpentinite in the Haneti-Itiso area, Central Tanzania. AAS determinations revealed a high Ni content, 0.55 wt.%, and low Co and Cr contents of 120 and 1 ppm respectively. NAA determination revealed near chondritic REE contents. X-ray diffraction determinations showed that the chrysoprase consists mainly of alpha quartz with some opal-CT. The gemmological properties are: refractive indices from 1.548 to 1.553 (± 0.002), mean specific gravity 2.56, hardness about 7 on Moh's scale, inert to ultraviolet radiation, green through Chelsea filter, and absorption in the deep red and violet part of the optical absorption spectrum. The results confirm the identity of the material as chrysoprase. Microscopically the Tanzanian chrysoprase consists of spherules which are highly disordered, concentric, and composed of bipyramidal quartz, chalcedony, quartzine, and opal-A. They were classified into four main types according to the shell arrangement. The diameter of the spherules ranged from 40 μm to 77 μm. Fluid inclusion types in the bipyramidal quartz were monophasic, low-temperature type. The spherules, silica types and REE contents suggest that this chrysoprase was deposited by evaporation of surface waters connected with the silicification of the serpentinites. Genetically analogous formations, common in Africa, include M-fabric type, weathering profile silcretes.