Karlite (kar'-lite) is a silky white to light green orthorhombic, not to be confused with tremolite. It has a general formula of Mg7(BO3)3(OH)4Cl. Karlite is named in honor of Franz Karl (1918–1972), professor of mineralogy at Christian Albrechts University, for his studies of the geology.
Karlite possesses moderate optical relief, the degree to which the mineral grains stand out from the mounting medium. This mineral is orthorhombic and sphenoidal, exhibiting symmetry on 222. Karlite is also enantiomorphic and dihedral. It is a member of the P212121 space group. This mineral forms optically negative biaxial birefringent crystals, meaning that the 2V angle between the optic axes is bisected by the refractive index direction. Because this mineral possesses birefringence, we know it is anisotropic and will display double refraction; it breaks light into two different rays traveling at different speeds in the mineral.
Karlite is a relatively newly discovered borate mineral occurring in clinohumite in calcsilicate-carbonate lenses embedded in amphibolite. The amphibole at the original locale is situated between tectonic units “Zentralgneis” and “Schieferhulle”. Scholars assume that the high boron concentration needed for the formation of karlite is due to a contact metasomatism tonalitic, which make up the “Zentralgneis", although the boron content of karlite is not of commercial importance. This mineral was originally discovered in the Furtschaglkar near the Furtschaglhaus in Austria, but has also been found in Russia and France, and was probably formed during the alpine metamorphism