Silicate of calcium, magnesium, and iron. containing fluorine and hydroxyl. It is an arnphibole. oi the actinolite series.
Crystal system Monoclinic.
Appearance The amphiboles of the tremolite-actinolite series usually occur as elongated, parallel. radiating or even fibrous crystals; but the variety known as nephrite has a very compact, felted microcrystalline structure, which gives it the extreme tenacity characteristic of jade. The color varies from whitish—or. at any rate, very pale-for the magnesium end-member. to darkish green or gray for
members with a high iron content.
Physical properties It has a hardness of 5-6. The relatively easy cleavage ofthe amphibole group is not found in this microcrystalline material. The density ranges from 2.90 to 3.40 g/cms. The retractive indices are about nx 1.606. ny 1.632. but only one index can normally be established in this compact. microcrysta lline material; it is about 1 .61.
Genesis Nephrite occurs in both contact and regionally metamorphosed iron and magnesium-rich rocks.
Occurrence The amphiboles of this series are relatively common minerals, widely distributed in metamorphic and basic magmatic rocks. Even the densely felted form typical of nephrite is found in large deposits in the Soviet Union (Siberia. Turkestan) and New Zealand, but in small quantities almost everywhere.