Experimental studies on the thermal changes in some hornblendes

Abstract

Thermal changes were studied experimentally on an actinolitic hornblende from Kamuikotan and a common hornblende from Kamioka Mine. Powders or minute flakes of these minerals were heated at temperatures from 800°-1000°C in N2 stream for 2-12 hr. After each run FeO content and optical properties were determined, and its X-ray powder patterns were made. At 900°-950°C the hornblendes transform into the so-called “oxyhornblendes”, in which both refractive indices and birefringence increase, and extinction angles decrease. The change in optic properties is proportional to the atomic number of Fe+3 in the oxyhornblendes when (O, OH)=24. X-ray powder diffraction patterns, as well as oscillation photographs show that the so-called oxyhornblendes are composed of crystal lattices of both monoclinic amphibole and monoclinic pyroxene, although the crystals as the whole still remain homogeneous hornblendes under the microscope. It is worthy of note that at higher temperatures the crystals are composed only of clinopyroxene lattices, which are regarded as metastable phases. Further change into the aggregates of stable clinopyroxene, plagioclase, magnetite and quartz which may take place at still higher temperatures has not yet been studied here.