On a Custerite-bearing Contact Rock from California

Extract

The mineral custerite [Ca(F,OH)]₂SiO₃ was first described from a limestone contact rock from Custer, Co. Idaho, the associated minerals being diopside, garnet, and magnetite. There appears to be no further record of this mineral from contact zones, though the related cuspidine [Ca(F,OH)]₂SiO₃ is known from Vesuvius and Franklin Furnace. In examining some metamorphosed limestones from the well-known mineral locality of Crestmore, California, the writer has found that custerite occurs in a vesuvianite rock in some abundance. As the mineral associations of this custerite are different from that of the original custerite, and owing to the very rare occurrence of this mineral, it is deemed of sufficient interest to describe this association briefly. The rock consists largely of idioblastic vesuvianite, these crystals being cemented together by a light brown base which proves to consist almost entirely of custerite, monticellite, and calcite. The custerite forms comparatively large shapeless areas enclosing subidioblastic crystals of monticellite. Under the microscope the custerite is seen to be colourless with a distinct cleavage, parallel to which lamellar twinning is developed. Sections cut perpendicular to the twin lamellae show symmetrical extinction of 6°.