plagioclase feldspar

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One of the most important rock-forming silicate minerals with the general formula (Na,Ca)(Al)1–2(Si)2–3O8. There is a solid solution series (see plagioclase series) between the two end-members albite (Ab) (NaAlSi3O8) and anorthite (An) (CaAl2Si2O8), and the percentage of calcium end-member present is used to subdivide the series into a number of individual minerals: albite (0–10 mol % An); oligoclase (10–30 mol % An); andesine (30–50 mol % An); labradorite (50–70 mol % An); bytownite (70–90 mol % An); and anorthite (90–100 mol % An). Albite; sp. gr. 2.61; hardness 6.0–6.5; is whitish, vitreous; tabular or irregular; with two cleavages {010} and {001} meeting at almost right angles on the (100) face; and occurs in acid igneous rocks and spilites. Oligoclase; sp. gr. 2.64; triclinic; cleavage perfect basal {001}, good {010}; is similar to albite except in the percentage An it contains. Andesine; sp. gr. 2.66; triclinic; cleavage perfect basal {001}, good {010}; has properties similar to those of albite but tends to occur in more intermediate igneous rocks. Labradorite; sp. gr. 2.67; crystals thin and tabular, flattened parallel to (010); cleavage perfect basal {001}, good {010}; is greyish-white but may show iridescence due to lattice imperfections on cleavage faces and occurs in basic igneous rocks. Bytownite; sp. gr. 2.72; triclinic; crystals often tabular prismatic but normally form irregular grains; cleavage perfect basal {001}, good {010}; also greyish-white, is a constituent of basic and ultrabasic igneous rocks. Anorthite; sp. gr. 2.75; triclinic; crystals tabular and prismatic but normally form irregular grains; cleavage perfect basal {001}, good {010}; also greyish-white, occurs in basic and ultrabasic igneous rocks and in metamorphosed limestones. These plagioclase feldspars cannot be distinguished from one another in hand specimens, but under the microscope their extinction angles vary; this is a useful property in their identification, together with the nature of the multiple twinning (see crystal twinning) which is a very characteristic feature and serves to distinguish them from alkali feldspars. Exsolution of potassium-feldspar in a plagioclase feldspar host is called ‘antiperthite’.

Subjects: Earth Sciences and Geography.

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