hydrogarnets

Hydrogarnets are a group of minerals belonging to the garnet supergroup. They are characterized by the substitution of hydroxyl (OH) groups for some of the anions in the garnet structure. The general formula for garnets is X3Y2(ZO4)3, where X and Y are typically divalent and trivalent cations, respectively, and Z is usually silicon. In hydrogarnets, a portion of the [ZO4]³⁻ group is replaced by [OH]⁴⁻ or other hydroxyl-containing anions. This substitution can occur in various ways, leading to different specific hydrogarnets.

The most well-known hydrogarnets are members of the uvarovite-hydrogarnet series and the grossular-hydrogarnet series. Uvarovite is a calcium chromium garnet, and hydro-uvarovite incorporates hydroxyl groups. Similarly, grossular is a calcium aluminum garnet, and hydro-grossular represents the hydroxyl-substituted end-member. These substitutions significantly affect the physical and chemical properties of the minerals, such as their refractive index, density, and hardness.

Hydrogarnets are typically found in metamorphic rocks, particularly those that have undergone hydrothermal alteration. They can also occur in some igneous rocks. Their formation is often associated with conditions where water is present during mineral crystallization or alteration. The presence of hydrogarnets can provide valuable information about the geological conditions and fluid environments under which they formed. Their appearance can vary, but they are often found as granular aggregates or as idiomorphic crystals.