Metamorphic minerals in the ultramafic rocks from Kitakami mountainland (1)

Forsterite

Abstract

Some parts of the ultramafic bodies in the Kitakami mountainland have been metamorphosed by the granitic intrusions. The forsterite, which is presumably a metamorphic mineral, in the ultramafic rock metamorphosed by the Hitokabe granitic body at Daito-machi, Iwate Prefecture, has been studied. The chemical formula of the forsterite is determined as (Mg_1.87 Fe²⁺_0.11 Fe³⁺_0.01 Ni_0.11)_2.00 Si_1.00 O_4.00. Chemically no difference is observed between this forsterite and the forsterite in the ordinary ultramafic rocks. Some consideration of the chemical similarity of the two types of olivines is discussed. This forsterite is probably regenerated together with talc by thermal metamorphism of a serpentine at the P-T condition corresponding to the hornblende hornfels facies.