Andesine

(mineralogy) A plagioclase feldspar with a composition ranging from Ab₇₀An₃₀ to Ab₅₀An₅₀, where Ab = NaAlSi₃O₈ and An = CaAl₂Si₂O₈; it is a primary constituent of intermediate igneous rocks, such as andesites.

A plagioclase feldspar with composition Ab₇₀An₃₀ to Ab₅₀An₅₀ (Ab = NaAlSi₃O₈; An = CaAl₂Si₂O₈). Andesine occurs primarily in igneous rocks, often in a glassy matrix as small, chemically zoned, lathlike crystals known as microlites. The rock types may be called andesinites (if dominantly feldspar), andesites, andesitic basalts (or olivine-bearing andesites, as in Hawaiian lava flows), or pyroxene-, hornblende- or biotite-andesites (all are volcanic). See also Andesite; Feldspar; Igneous rocks.

The symmetry of andesine is triclinic, hardness on the Mohs scale 6, specific gravity 2.69, melting point ∼1210°C (2210°F). If quenched at very high temperatures, andesine has an albitelike structure, with aluminum (Al) and silicon (Si) essentially disordered in the structural framework of the crystals. But, in the course of cooling, most natural andesines develop an Al-Si ordered structure called e-plagioclases. See also Albite; Crystal structure.

Calcic andesines and labradorites (Ab₅₅An₄₅-Ab₄₀An₆₀) may exsolve into two distinctly intergrown lamellar phases whose regularity of stacking produces beautiful interference colors like those in the feathers of a peacock. Polished specimens of this material are called spectrolite in the gem trade, and at some localities (notably eastern Finland) crystals up to 10 in. (25 cm) are mined by hand. Smaller crystals are made into cabochons for jewelry. They may be abundant enough in the host rock to be valued as a decorative stone. See also Gem; Labradorite.

Andesine
Andesine
General
Category	plagioclase,feldspar,tectosilicate
Chemical formula	(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (% Anorthite) is between 30%-50%
Identification
Crystal system	triclinic
Streak	white
Density	~ 2.68

Andesine is a feldspar mineral, a part of the plagioclase series.

It is usually defined as having "%An" between 30 and 50.

Density is about 2.68.

See the plagioclase main article for more information.

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