KMgFe3AlSi3O10OH2

KMgFe3AlSi3O10OH2 is a potassium-rich mica belonging to the phyllosilicate mineral group. In practice, this composition sits within the annite–phlogopite solid-solution series, representing a highly iron-rich end member with notable magnesium content. End-members of this series are annite (Fe-dominant) and phlogopite (Mg-dominant); the formula KMgFe3AlSi3O10OH2 reflects a specific, Fe-rich intermediate composition in which Mg and Fe occupy octahedral sites alongside Al in the tetrahedral framework.

As with other micas, KMgFe3AlSi3O10OH2 features a layered 2:1 silicate structure. Each layer consists of a sheet of silicate tetrahedra (SiO4) sandwiching an octahedral sheet of Mg, Fe, and Al. Potassium ions sit in interlayer galleries, balancing charge and enabling the characteristic excellent basal cleavage. The OH groups in the structure are part of the mineral’s hydrous content, contributing to its general chemical behavior and physical properties. The mineral crystallizes in the monoclinic system and typically exhibits the glossy, sheet-like habit characteristic of mica minerals.

Occurrence of this composition typically reflects metamorphic environments with abundant iron and potassium, such as pelitic schists or migmatites, and can also be related to magmatic differentiation zones in granite-related complexes. It is commonly associated with other mica minerals, quartz, feldspars, chlorite, and sometimes amphiboles or garnet, depending on the rock type and metamorphic history.

Notes: The exact formula KMgFe3AlSi3O10OH2 is not a widely isolated mineral name on its own; rather, it is best understood as part of the mica solid-solution series between annite and phlogopite, illustrating the range of Fe–Mg substitution within a potassium-bearing 2:1 phyllosilicate.