Home > Mexico I – December 15 > MS1687 Goethite casts of Gypsum

Goethite casts of Gypsum


Goethite is an iron oxide hydroxide mineral, with the same crystal structure as diaspore. Lenz (1806) named the species after German poet, novelist, natural historian, and civil servant Johann Wolfgang von Goethe (1749-1832). Goethite here forms casts of gypsum. Gypsum is very unusual in its solubility properties, having retrograde solubility. Normally compounds become more soluble in hotter fluids, but in the case of gypsum, its solubility increases at lower temperatures, maxing out below 40°C. When ore forming fluids cool and recirculate, they redissolve early formed gypsum.

This is a specimen of black casts of goethite after former gypsum crystals. The delicate crystal casts reach 4 cm. Some attached calcite provides contrast. These specimens are vintage, usually dating to the 1960s and 1970s. This particular example was part of the Charles Noll collection, and carries his label.

Price: $75

Item code: MS1687

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