nacrite

Nacrite is a clay mineral in the kaolinite group, a 1:1 phyllosilicate with the ideal formula Al2Si2O5(OH)4. It is one of several kaolinite polymorphs, distinguished from kaolinite, dickite, and halloysite by its distinct layer stacking and crystallographic symmetry. In nature, nacrite occurs as fine, platy aggregates; crystals are rare and typically too small to identify without X-ray diffraction.

Formation and occurrence: Nacrite forms through diagenetic and low-grade metamorphic alteration of aluminous rocks and in hydrothermal environments where feldspar-bearing rocks interact with silica-rich fluids. It is commonly found in soils and sedimentary rocks that have undergone alteration and is often found in association with other kaolinite-group minerals such as kaolinite, dickite, and halloysite, as well as quartz and mica-bearing rocks.

Physical properties: Nacrite shares many properties with other kaolinite-group minerals. It is white to gray in color, with a dull to earthy luster. It has a soft Mohs hardness of about 2 to 2.5 and a relatively low specific gravity near 2.6. It exhibits perfect basal cleavage and forms as microscopic to very fine platelets, making it resemble other clay minerals under ordinary inspection.

Uses and significance: Nacrite is not widely mined or used commercially, unlike kaolinite. It is mainly of interest to mineralogists and geologists for its diagnostic crystallography and for providing information about the temperature and pressure history of alteration environments. It helps in understanding diagenetic and metamorphic processes in aluminous rock systems.