plagioclase feldspar One of the most important rock-forming silicate minerals with the general formula (Na,Ca)(Al)_1–2(Si)_2–3 O₈. 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 spilites and solid solution 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 solid solution, good spilites; is similar to albite except in the percentage An it contains. Andesine; sp. gr. 2.66; triclinic; cleavage perfect basal solid solution, good spilites; 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 solid solution, good spilites; 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 solid solution, good spilites; 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 solid solution, good spilites; 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’.

Plagioclase

Plagioclase is a form of the mineral feldspar . All feldspars are crystals of aluminum, oxygen , and silicon , plus a major additive. In the case of plagioclase, the additive is calcium, sodium, or a blend of both. Miscellaneous impurities may also be present.

Like other feldspars, plagioclases have a vitreous (glassy) luster, are translucent or transparent, and are typically pink, gray, or white in color. Plagioclase cleaves along two planes that intersect at about 86 degrees, hence its name (from the Greek plagio, slanted, and clase, breaking). About 29% of the earth's crust consists of plagioclase.

There are two pure or extreme forms of plagioclase. Albite (NaAlSi₃O₈) contains sodium but no calcium, and anorthite CaAl₂Si₂O₈ contains calcium but no sodium. Many plagioclases consist of both minerals microscopically blended to form what is termed a solid solution. Plagioclase blends can occur along a linear continuum from albite to anorthite, intermediate minerals being mixtures of both. Plagioclase with 10–30% anorthite is defined as oligoclase; 30–50%, andesine; 50–70%, labradorite; and 70–90%, bytownite. The remainder in all cases is albite. A plagioclase with less than 10% anorthite is classified simply as albite, while one with less than 10% albite is classified simply as anorthite. A continuous, evenly represented series of plagioclases is not actually found in the field; oligoclases with anorthite percentages in the low twenties are most common, while some other points on the continuum are scarcely represented at all. The plagioclases are difficult to distinguish from each other without laboratory tests.

Many plagioclases are zoned. Zoning is the concentric or onion-skin structuring of an individual plagioclase crystal (often only a few millimeters long) with layers of varying sodium–calcium content. For example, a plagioclase crystal may consist of an andesine core (30–50% anorthite) surrounded by thin shells or zones of labradorite (50–70% anorthite) alternating with zones of andesine. Zoning records the chemical and thermal environment in which a plagioclase crystal formed.

Plagioclases are used for ceramics and glassmaking, and several gemstone varieties exist.

See also Bowen's reaction series; Mineralogy

plagioclase Type of feldspar. Plagioclase minerals occur in igneous and metamorphic rocks. Off-white, or sometimes pink, green or brown, they are composed of varying proportions of the silicates of sodium and calcium with aluminium. They show an oblique cleavage and have triclinic system crystals. Hardness 6–6.5; r.d. 2.6.

oligoclase See PLAGIOCLASE FELDSPAR.

bytownite See PLAGIOCLASE FELDSPAR.