AEGIRINE ON MICROCLINE (miniature)

Aegirine belongs to the clinopyroxene group of inosilicate minerals and serves as the sodium endmember of the aegirine–augite series. Its chemical composition is represented by the formula NaFeSi2O6, where iron exists as the Fe3+ ion. Within the aegirine–augite series, sodium can be partially substituted by calcium, while iron(II) and magnesium can replace iron(III) to maintain charge balance. Additionally, aluminum can substitute for iron(III). A notable variety of this mineral is acmite, which is characterized by its fibrous green appearance.

 Aegirine typically forms dark green monoclinic prismatic crystals, exhibiting a glassy luster and perfect cleavage. Its hardness on the Mohs scale ranges from 5 to 6, and it has a specific gravity that falls between 3.2 and 3.4.

Microcline (KAlSi3O8) is a significant tectosilicate mineral that contributes to the formation of igneous rocks. As a potassium-rich alkali feldspar, it often contains trace amounts of sodium and is frequently found in granite and pegmatite formations. Microcline develops during the gradual cooling of orthoclase and exhibits greater stability at lower temperatures compared to orthoclase. In contrast, sanidine represents a polymorph of alkali feldspar that remains stable at even higher temperatures. The mineral can appear in various colors, including clear, white, pale yellow, brick red, or green, and is typically identified by its distinctive cross-hatch twinning, which results from the transformation of monoclinic orthoclase into triclinic microcline.