Hibschite - Rare member of the Garnet group containing hydroxyl (OH) in its structure in place of some silica (SiO₄). Its formula is Ca₃Al₂(SiO₄)_{3 - x}(OH)_4x
(where x is 0.2 to 1.5) . The mineral Katoite is the same as Hibschite, but contains more hydroxyl replacing the silicate (In Katoite, x is 0.2 to 1.5). Hibschite and Katoite are white to light yellow, and only occur in tiny crystals, which may be packed together very densely. Hibschite and Katoite are called hydrogarnets, or Garnets with hydroxyl in their structure. They may also be called hydrogrossular, for they are, in essence, the mineral Grossular with a slight replacement of silica for hydroxyl. Hibschite and Katoite are scientifically classified as individual minerals, but some classify them as a variety of Grossular.
For more information, see the Garnet group