Tephroite in California manganese deposits

Abstract

Recent studies of Mn deposits and mineral specimens in the Sierran belt of sedimentary rocks as well as in the Klamath Mountains to the N. and in the Rand Mountains to the S., have shown the presence of tephroite, the orthosilicate of Mn (Mn 2 SiO 4 ), at numerous localities. Earlier studies of these deposits have shown that the original layered deposits of carbonate and hydrous silicates of Mn have been widely altered by metamorphism to spessartite, (Mn 3 Al 2 (SiO 4 ) 3 ), rhodonite (MnSiO 3 ), and piedmontite (Ca 2 (Al,Mn9, Fe9) 3 Si 3 O 12 (OH)). Recent work shows that layered manganese oxides also were present in the original sedimentary rocks. It shows also that tephroite has formed widely in the original assemblage of carbonate, silicate, and oxide of Mn; in several localities, a little alleghanyite (2Mn 2 -SiO 4 .Mn(OH,F) 2 ) has been formed. Rhodonite, spessartite, and piedmontite uniformly follow the tephroite; in places, bementite and neotocite are present. In one deposit a little pyroxmangite ((Mn, Fe, Ca)SiO 3 ) has been noted and in another, some crystals that are probably pyrophanite (MnTiO 3 ), not yet recorded in the United States. By contrast, the restudy of large collections of material from deposits in sedimentary rocks of the Franciscan formation in the Coast Ranges indicates the tephroite is very uncommon; it has been recognized with assurance at only one locality - Alum Rock Park, Santa Clara County. In the Coast Ranges, rhodonite and spessartite, the high Mn garnet, are very uncommon. The assemblages of manganese silicates indicate that the layered deposits of Mn minerals in the Sierra belt have been metamorphosed to a higher degree than those of the Coast Ranges. This review also shows the presence of axinite, the boro-silicate of Al, Ca, and Mn, in 3 deposits.