Abstract

Melanophlogite, found as single crystals and interlocking intergrowths on sulfur crystals from Agrigento (formerly Girghenti) and Palermo in Sicily, is a low temperature cubic polymorph of silica, space group P4232, a = 13.402± 0.004 Å, Z = 48 [SiO2]. The index of refraction is 1.425±0.002 and the calculated density 1.99 g/cc.

New chemical analysis reveals a [C+H+S] content of at least 6% in all samples. These elements are present in organic films trapped on the cube growth surfaces. The physically included organic matter is the brown pigmenting agent in melanophlogite, and is the material which turns black when heated, giving melanophlogite its name. The pigment is optically anisotropic, causing the cubic melanophlogite crystals to exhibit a weak birefringence. The optical anisotropy is destroyed on heating due to breakdown of the organic inclusions.

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