Hydrogrossular

Hydrogrossular is a mineral belonging to the garnet group. It is a calcium aluminum silicate with the general formula Ca3Al2(SiO4)3-x(OH)4x. The 'x' indicates that some of the SiO4 groups are replaced by (OH)4 groups. This substitution of hydroxyl groups for silicate tetrahedra is the defining characteristic of hydrogrossular and distinguishes it from grossular, another garnet mineral. Hydrogrossular can also be viewed as a solid solution series between grossular and a theoretical end-member, hibschite (Ca3Al2(OH)6).

Hydrogrossular typically occurs in metamorphic rocks, particularly those that have undergone hydrothermal alteration. It is often found in contact metamorphic zones or in skarns. The mineral can also form as a secondary mineral in various rock types. Its color can vary widely, ranging from colorless and white to pink, green, brown, and even black, depending on the presence of impurities. The luster is vitreous to greasy, and it is transparent to translucent.

The hardness of hydrogrossular is typically between 6.5 and 7 on the Mohs scale, similar to other garnets. Its density is also within the typical garnet range. Because of its variable composition and the presence of hydroxyl groups, hydrogrossular is sometimes considered a distinct mineral species, while at other times it is classified as a variety of grossular. It is not a particularly common mineral but is of interest to mineralogists due to its unusual composition within the garnet family.