Saussurite

Saussurite is a descriptive term used in geology and gemology for a dense, microcrystalline aggregate that forms by alteration of feldspars, especially plagioclase, in certain igneous and metamorphic rocks. It is not a mineral species, but a textural variety produced by saussuritization, the replacement of feldspar by a fine-grained mixture of silica-rich and carbonate phases.

Formation and composition: The alteration typically occurs under hydrothermal conditions or near metamorphic zones where silica- and carbonate-bearing fluids permeate feldspar-bearing rocks. The result is a smooth, compact mass with a microcrystalline texture, often with a waxy or greasy luster.

Appearance and properties: Saussurite is usually pale to light-colored, ranging from white and gray to pale green. Its matrix is microcrystalline and may include quartz, calcite, chlorite, and other alteration minerals; color depends on impurities. It is not a uniform mineral but a texture that can resemble jade or serpentine in hand specimen and carving.

Occurrence and use: It occurs in various terrains where feldspar-rich rocks have undergone alteration and has long been used in the gem trade as a jade-like carving material. Because of its resemblance to true jade or serpentine, saussurite has historically been subject to misidentification; identification relies on microtexture and mineral composition.

Etymology: The term is named after the historical practice of naming alteration textures after notable scientists, with saussurite linked to Horace-Bénédict de Saussure.