scapolite, Tanzania, gems

Two faceted scapolites from central Tanzania, one yellowish and the other colourless, were investigated to characterize their composition, physical properties and abundant inclusions, and to infer aspects of their formation and subsequent alteration. Optical measurements, X-ray powder diffraction and electron-microprobe analyses identify both specimens as intermediate members of the marialite–meionite series, traditionally termed mizzonite, with approximately 70.3% and 59.5% meionite respectively. The yellowish specimen has refractive indices of 1.557 and 1.587 and birefringence of 0.030; corresponding values for the colourless specimen are 1.556, 1.582 and 0.026. Measured densities are approximately 2.716 and 2.709 g/cm³. Both stones contain channels parallel to the crystallographic c-axis and black, metallic, pseudohexagonal plates, but their relative abundance differs markedly: channels dominate the yellowish stone, while plates predominate in the colourless specimen. Healed fractures cross or extend from the channels, sometimes forming broad yellow haloes. Microprobe examination shows that yellow and brownish-red scales filling the channels are overwhelmingly iron-bearing. The yellow lamellae are interpreted as lepidocrocite on the combined evidence of colour, habit, pleochroism, estimated optic axial angle and an approximately 64° intersection between characteristic crystal planes. The reddish scales are assigned less certainly to hematite or maghemite. Metallic plates contain iron and sulphur with a small copper component and are identified as copper-bearing pyrrhotite. The authors propose that originally iron-rich fluids occupied the growth channels and, during cooling, deposited scapolite along their walls followed by lepidocrocite. Subsequent changes in physical and chemical conditions caused fracturing, fluid redistribution and partial dehydration of lepidocrocite to iron oxides. This process is invoked to explain irregular channel outlines, reddish scales, transparent yellow areas and surrounding haloes. Similar inclusion types suggest broadly comparable formation conditions for the two specimens. Using the inferred syngenetic copper-bearing pyrrhotite, the authors estimate a lower formation-temperature limit near 700 °C at relatively low pressure. They relate later lepidocrocite dehydration to temperatures above approximately 350 °C, with the distinction between maghemite and hematite potentially reflecting whether temperatures remained below or exceeded about 500 °C. These thermal interpretations remain dependent on the proposed identities and transformation sequence of the inclusions.