pyralspite

Pyralspite is a garnet group term used to describe the solid-solution series among three end-members: pyrope, almandine, and spessartine. In this scheme, the garnet crystal’s X-site is occupied mainly by Mg, Fe2+, or Mn2+, while the Y-site is predominantly Al3+. The three end-members have the simple formulas pyrope Mg3Al2Si3O12, almandine Fe3Al2Si3O12, and spessartine Mn3Al2Si3O12. Natural pyralspite garnets occur with varying proportions of Mg, Fe, and Mn, producing a continuous range of compositions.

Crystallographically they belong to the garnet group with the isometric, often dodecahedral habit. They have a hardness of about 6.5–7.5 on the Mohs scale and a specific gravity roughly in the 3.5–4.3 range, increasing with iron and manganese content. Colors vary from red to pink, orange, or brown, depending on composition and trace elements.

Formation and occurrence: Pyralspite garnets form during regional metamorphism of pelitic and related rocks, especially at medium to high grades; pyrope-rich garnets are common in high-grade, high-temperature conditions and in eclogites, while spessartine dominates in Mn-rich metapelitic rocks. They are also found in skarns and as inclusions in some ultramafic rocks.

Geologic and gemological relevance: As a group, pyralspites are used by geologists as indicators of metamorphic conditions and can be fashioned into gemstone-quality material, with pyrope and almandine varieties common. Rhodolite is a well-known garnet variety composed mainly of pyrope–almandine. The term emphasizes the garnet’s mixed nature rather than a single mineral species.

History and naming: The name pyralspite derives from combining the initial syllables of pyrope, almandine, and spessartine, the three end-members of the solid solution.