STAUROLITE with ALMANDINE GARNET, Pond Hill near Pearl Lake Lisbon New Hampshire USA (large cabinet)

Staurolite is a nesosilicate mineral that ranges in color from reddish-brown to black and is primarily opaque, exhibiting a white streak. It crystallizes within the monoclinic system and possesses a Mohs hardness of 7 to 7.5, with the chemical formula Fe2+2Al9O6(SiO4)4(O,OH)2. Iron can be substituted by magnesium, zinc, and manganese, while trivalent iron may replace aluminum.

This mineral is frequently found in a distinctive cross-shaped twinning known as cruciform penetration twinning. In hand samples, staurolite crystals are typically prismatic and can form porphyroblasts. Under thin section examination, staurolite often displays twinning and exhibits lower first-order birefringence akin to quartz, with optical continuity in the twinning. It is identifiable in metamorphic rocks by its characteristic "swiss cheese" appearance, often accompanied by poikilitic quartz and a mantled porphyroblastic structure. The name staurolite is derived from the Greek words for cross and stone, reflecting its common twinning pattern.

Almandine, also referred to as almandite, is a mineral within the garnet group. Its name derives from "alabandicus," a term used by Pliny the Elder for a stone sourced from Alabanda, a town in ancient Caria, Asia Minor. Characterized by its deep red to purplish hue, almandine is an iron alumina garnet often shaped with a convex surface, known as carbuncle when polished. Under strong light, it typically reveals three distinct absorption bands when examined with a spectroscope.

As one end-member of a solid solution series, almandine pairs with pyrope as the other end member. Its chemical formula is Fe3Al2(SiO4)3, with magnesium capable of substituting for iron in compositions that lean towards pyrope. Almandine, represented as Fe2+3Al2Si3O12, is a significant ferrous iron end member among garnet minerals, which are vital components of rock-forming silicates found in the Earth's crust, upper mantle, and transition zone.

Former John Betts Collection NY #12171