märgistite

Märgistite, known in English as margarite, is a magnesium- and aluminum-rich mica, a member of the phyllosilicate mica group. It is a dioctahedral mica with a layered 2:1 silicate structure in which magnesium and aluminum occupy the octahedral sites. The sheets are bonded between layers by interlayer cations, giving the characteristic basal cleavage of micas.

Margarite forms in metamorphic settings from the alteration of magnesium- and aluminum-bearing minerals, commonly in carbonate rocks such as marbles and skarns, and also in hydrothermal veins. It is stable under relatively low to intermediate metamorphic conditions, typically greenschist to lower amphibolite facies, and often occurs with chlorite, calcite or dolomite, tremolite, quartz, and other micas such as phlogopite or sericite.

Physical properties: Margarite is usually colorless to pale green, yellowish, or brown. It has a vitreous to pearly luster and perfect basal cleavage. Crystals are micaceous and platy, and in hand specimens it forms flexible sheets. In thin section it shows the characteristic mica interference colors.

Occurrence and significance: Margarite is interpreted as an indicator of specific metamorphic environments, particularly carbonate-rich lithologies and low-grade metamorphism. Geologists distinguish margarite from other micas by chemical analysis and X-ray diffraction, as color and habit alone are not diagnostic.

Note: In Swedish mineral literature, margarite is referred to as märgistite.