saussuritite

Saussuritite is a rare rock texture associated with saussuritization, the alteration of plagioclase feldspar to a fine-grained, microcrystalline aggregate. In hand specimens it presents as a pale, waxy gray rock in which primary feldspar grains are partially replaced by a quartz-sericite–rich mosaic, giving a microcrystalline or cryptocrystalline texture.

The typical mineral assemblage includes quartz and sericite (fine muscovite), with possible minor chlorite, epidote, and calcite. Residual plagioclase grains may remain as small porphyroclasts. The defining feature is the replacement texture rather than a distinct mineral, so saussuritite is described by its textural appearance and associated minerals.

Formation and geological context involve metamorphic or hydrothermal processes that drive replacement of feldspar by silica- and mica-rich products. Saussuritization commonly occurs in environments where silica-rich fluids interact with feldspar-bearing rocks, and it is often linked to contact metamorphism of granitoid bodies or other mid- to high-temperature alteration zones. The result is a softly textured rock that can weather more readily than unaltered feldspathic rocks.

Classification and usage notes: saussuritite is primarily a historic petrographic term rather than a universally standardized mineral species. Modern petrography typically describes such rocks in terms of their mineral assemblage and textural features, rather than as a discrete mineralogical unit. The term remains encountered in older literature and in descriptions of specific Alpine and other metamorphic terrains where saussuritization has been observed.

Etymology: the name derives from saussurite, the alteration product of feldspar, with the suffix -ite forming a rock-name designation.

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