Ferroaxinites from the Feather River area, northern California, and from the McGrath and Russian Mission quadrangles, Alaska

In the Feather River area, California, and in the McGrath quadrangle, Alaska, axinite-bearing veins occur as fracture fillings along or near the fault zones, suggesting that boron was introduced along the fractures. An unusual occurrence of axinite as a possible primary constituent of a plutonic rock is in the Russian Mission quadrangle, Alaska. The four analyzed axinites from these widely different localities and from different host rocks are surprisingly similar in chemistry and optics. All are ferroaxinites, having high iron and low manganese and magnesium contents. The number of calcium ions is very close to two, which is in agreement with the idealized formula Ca₂ (Fe,Mn,Mg)Al₂BSi₄O₁₅(OH). The indices of refraction increase slightly with increasing FeO:MgO ratio over the small range studied.