mironite

Mironite is a rare hydrous phyllosilicate mineral in the mica group. It forms in metamorphic environments and is distinguished by its potassium-rich aluminum silicate sheets with variable iron and magnesium content. The idealized formula is KMg3AlSi3O10(OH)2, with Fe2+ and other minor elements substituting for Mg, yielding a range of color and optical properties. Crystallography places mironite in the monoclinic system, often producing tabular crystals with a pearly to vitreous luster.

Physical properties of mironite include a olive-brown to dark-green color, a pearly to silky luster, and transparency from transparent to translucent in thin flakes. It exhibits perfect basal cleavage common to mica minerals, a Mohs hardness around 2.5–3, and a specific gravity near 3.0. The mineral commonly shows strong pleochroism in polarized light, helping distinguish it from related micas in hand specimen.

Occurrence and formation are associated with low- to medium-grade metamorphic rocks such as mica schist and phyllite, and with hydrothermal alteration of aluminous rocks. Mironite is typically found alongside other micas (muscovite, biotite) and chlorite, often in greenschist to phyllite facies assemblages. It is thought to form during dehydration reactions that stabilize silicate layers while fluids equilibrate with surrounding minerals.

Etymology traces mironite to a root meaning “wonder,” reflecting its striking luster and distinct mica-like habit. Uses are mainly ornamental and scientific; due to its rarity, mironite has limited industrial application beyond mineral collection and study.