Ca2MgFe5Si8O22OH2

Ca2MgFe5Si8O22OH2 is a calcium-magnesium-iron amphibole, representing a composition at the iron-rich end of the tremolite-ferro-tremolite series. In mineralogy, it is commonly described as Ca2(Mg,Fe)5Si8O22(OH)2, indicating substantial iron (Fe2+) substituting for magnesium (Mg) within the tremolite group of amphiboles. This solid-solution series forms part of the larger inosilicate double-chain framework characteristic of amphiboles.

In terms of crystal chemistry, the mineral crystallizes in the monoclinic system and typically occurs as elongated prismatic crystals, though fibrous or acicular habits can appear in certain environments. It has the usual amphibole two-direction cleavage, near 56 and 124 degrees. Physical properties often include a vitreous to pearly luster, a hardness around 5 to 6 on the Mohs scale, and a specific gravity in the approximate range of 3.0 to 3.3. Color is commonly white to gray or greenish, with color variations influenced by the relative Mg–Fe content. Optical properties are anisotropic, with refractive indices and birefringence typical of calcic amphiboles.

Geologic occurrence is in metamorphic rocks formed under medium-grade conditions, especially calc-silicate rocks such as marbles and skarns, where magnesium and calcium are abundant. It can appear in regional metamorphic suites and in contact metamorphic zones, often alongside minerals like calcite, diopside, quartz, and chlorite. While not a primary ore, tremolite-group minerals (including ferric-rich variants) are notable for their association with asbestos-forming fibrous varieties when weathered or metamorphosed under particular conditions; such forms require appropriate health and safety considerations.

Overall, Ca2MgFe5Si8O22OH2 exemplifies the tremolite-family amphiboles, illustrating the chemistries possible within calcic amphiboles and their occurrence in metamorphic rock systems.