File:Labradorite plagioclase feldspar (Nain Anorthosite, Mesoproterozoic, 1.29-1.35 Ga; Nain, Labrador, Canada) 2.jpg

English: Plagioclase feldspar from the Precambrian of Canada.

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5800 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). All other silicates have silica + impurities. Many silicates have a significant percentage of aluminum (the aluminosilicates).

Feldspar is a group of common silicate minerals. Feldspars are silicate minerals having one-fourth of all the silicons in SiO2 replaced by aluminum (Si4O8 to (Si3Al)O8). When this happens, the (Si3Al)O8 has a -1 electric charge. The charge is satisfied by the addition of one or more metals. The (Si3Al)O8- structure has relatively large holes, and the only metals that tend to stay in these holes are: K (potassium), Na (sodium), Ca (calcium), Cs (cesium), Ba (barium), Sr (strontium), and Pb (lead). Of these, K & Na & Ca are the most common metals that enter the matrix. Sometimes, several different metals enter the structure, resulting in "garbage can minerals".

Chemical analyses of feldspars show that they range in composition from K-feldspar to Na-feldspar and from Na-feldspar to Ca-feldspar. Mineralogists have thus established two "families" of feldspars. There is no chemical gradient between K-feldspar and Ca-feldspar.

The sodium- to calcium-feldspars are called plagioclase ("plag" for short). Six different mineral names are available for the plagioclase feldspars: albite, oligoclase, andesine, labradorite, bytownite, and anorthite. Albite is ~pure sodium feldspar (NaAlSi3O8) and anorthite is ~pure calcium feldspar (CaAl2Si2O8). The other mentioned minerals are plagioclase feldspars having a mix of sodium and calcium. The pure end-members are whitish-colored. The plagioclase feldspars having a mix of sodium and calcium tend to be light gray to dark gray to mottled gray. Some have a spectacular play of color.

Plagioclase has a nonmetallic luster, a white to light gray to dark color, a white streak, a hardness of 6 on the Mohs Hardness Scale, and breaks along two cleavage planes meeting at 90 degrees. Cleavage planes often show striations (= very thin, straight, parallel grooves).

Seen here is a piece of labradorite plagioclase feldspar from the Nain Anorthosite in eastern Canada. Anorthosites are uncommon intrusive igneous rocks almost exclusively composed of plagioclase feldspar. There’s usually a blackish pyroxene component as well. Some famous anorthosite occurrences include Wyoming's Laramie Range, New York State's Adirondacks Mountains, and the Moon.

Geologic unit: Nain Anorthosite (Nain Plutonic Suite), mid-Mesoproterozoic, 1.29 to 1.35 Ga

Locality: unrecorded site at or near the town of Nain (possibly a quarry), coastal Labrador, eastern Canada

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Photo gallery of labradorite:

www.mindat.org/gallery.php?min=2308