Harjoite

Harjoite is a rare garnet-group silicate mineral. It was first described in 2009 from specimens collected in the Harjo locality, and the name honors the region where the type material was found. The discovery helped expand knowledge of solid-solution in garnet compositions and their metamorphic settings.

The ideal chemical composition is (Fe2+,Mg)3Al2(SiO4)3, representing a Fe2+-Mg-rich garnet with aluminum in the octahedral site. Iron and magnesium can substitute for one another, producing a range of colors and slightly varying densities. Harjoite belongs to the isometric crystal system and commonly forms well-formed dodecahedra. It is isotropic in optical properties and typically exhibits a vitreous luster.

In hand specimen, harjoite appears as deep red to reddish-brown crystals, though color can vary with the Fe/Mg ratio. Hardness is about 6.5 to 7 on the Mohs scale, and specific gravity ranges roughly from 3.9 to 4.2. The mineral is typically transparent to translucent and may show pleochroism in fine crystals, though this is less common due to its garnet structure.

Occurrence and geology: harjoite is associated with medium- to high-grade metamorphic rocks, including granulites and schists, and is often found in contact zones with pegmatites. It occurs alongside quartz, staurolite, kyanite, and other garnet varieties, reflecting high-temperature metamorphic conditions.

Etymology and storage: the name derives from the Harjo region, with the type specimen kept in a national mineral collection. See also garnet, silicate minerals.