Magnesioriebeckite

Magnesioriebeckite is a sodium-rich amphibole mineral in the riebeckite group, representing the magnesium-dominant end member of the riebeckite solid solution. In this series, magnesium substitutes for iron, with ferro-riebeckite as the iron-rich counterpart. The composition is commonly described as Na2(Mg,Fe)3Si8O22(OH)2, with the exact Fe2+/Fe3+ content varying across samples.

Crystallography and appearance: Magnesioriebeckite crystallizes in the monoclinic system and typically forms prismatic to fibrous aggregates. Colors range from colorless or pale to blue-green or green, and the mineral often exhibits a vitreous luster. It shares the characteristic two directions of cleavage common to amphiboles.

Formation and occurrence: It forms in alkaline igneous rocks and pegmatites, and can occur in metamorphic rocks derived from these sources. Magnesioriebeckite is commonly associated with other alkaline amphiboles such as riebeckite and arfvedsonite, as well as aegirine, natrolite, biotite, and quartz.

Distribution: The mineral has been reported in various localities worldwide wherever alkaline granitoids, pegmatites, or related metamorphic environments occur. As part of the riebeckite group, it is studied for its solid-solution behavior and its crystallography within amphiboles.

Etymology and classification notes: The name indicates its magnesium-rich relation to riebeckite. It is one of several as part of the broader amphibole supergroup, and like other members of the riebeckite group, may occur in asbestiform varieties in nature, which are of concern for health if fibers are inhaled. Magnesioriebeckite itself is primarily of interest to mineralogists and collectors rather than to industrial use.