Calcium Borate Minerals in the CaO-B2O3-H2O System at Fuka, Okayama Prefecture, Japan.

Calcium-borate minerals such as takedaite and parasibirskite have been found in a vein in crystalline limestone near gehlenite-spurrite skarns at Fuka, Okayama Prefecture, Japan. The minerals are plotted in the CaO:B2O3 molar ratio range from 1 : 1 to 3 : 1 and up to 0.25 in the B2O3 ratio in the CaO-B2O3-H2O system. This paper reviews the author's studies on physical and chemical properties and genesis of the borate minerals. The properties show the following characteristics with an increase of water content in the minerals: (1) the refractive indices, birefringence, hardness and density of the minerals decrease; (2) the endothermic peaks on the DTA curve shift remarkably at low temperature; (3) the intensity of IR absorption bands near 3400 cm-1 increases, and the band near 1000 cm-1 shifts toward the lower frequency region. The formation of the minerals may be considered as follows: (1) takedaite was primarily formed by a reaction of boron-bearing fluids with limestone; (2) sibirskite and parasibirskite were formed by hydrothermal alteration of takedaite; (3) a late-hydrothermal solution converted sibirskite and parasibirskite to olshanskyite, frolovite and nifontovite.