Plagioclase


Plagioclase
Plagioclase

A photomicrograph of a plagioclase crystal under cross polarized light. The plagioclase crystal shows a distinct banding effect called a Polysynthetic twinning. (unknown scale)
General
Category feldspar, tectosilicate
Chemical formula NaAlSi3O8 – CaAl2Si2O8
Identification
Crystal system triclinic
Mohs scale hardness ~6
Streak white
Birefringence 1st order
Pleochroism none
Plagioclase displaying cleavage. (unknown scale)
In volcanic rocks, fine-grained plagioclase can display a 'microlitic' texture of many small crystals.

Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series (from the Greek "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic twinning or 'record-groove' effect.

Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the highlands of the Earth's moon.

Contents

Plagioclase series members

The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages.

Plagioclase minerals and their compositions
Name  % NaAlSi3O8(%Ab)  % CaAl2Si2O8(%An)
Albite2.jpg
Albite
100–90 0–10
Oligoclase-Sunstone from India2.jpg
Oligoclase
90–70 10–30
01722 Andesine.jpg
Andesine
70–50 30–50
Labradorite.jpg
Labradorite
50–30 50–70
Feldspar - Bytownite Sodium calcium aluminum silicate Crystal Bay Minnesota 2689.jpg
Bytownite
30–10 70–90
Anorthit Miyake,Japan.JPG
Anorthite
10–0 90–100

Albite

Albite is named from the Latin albus, in reference to its unusually pure white color. It is a relatively common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline and beryl.

Anorthite

Anorthite was named by Rose in 1823 from the Greek meaning oblique, referring to its triclinic crystallization. Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites.

The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties.

Oligoclase

Oligoclase is common in granite, syenite, diorite, and gneiss. It is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, in reference to the fact that its cleavage angle differs significantly from 90°. Sunstone is mainly oligoclase (sometimes albite) with flakes of hematite.

Andesine

Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.

Labradorite

Labradorite displaying typical iridescent effect termed labradorescence. (unknown scale)

Labradorite is the characteristic feldspar of the more basic rock types such as diorite, gabbro, andesite, or basalt and is usually associated with one of the pyroxenes or amphiboles. Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal. It is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland.

Bytownite

Bytownite, named after the former name for Ottawa, Canada (Bytown), is a rare mineral occasionally found in more basic rocks.

See also

References


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • plagioclase — [ plaʒjoklaz ] n. m. • 1899; du gr. plagios « oblique » et clasis « cassure » ♦ Minér. Feldspath contenant du calcium et du sodium, mais pas de potassium. ● plagioclase nom masculin (grec plagios, oblique) Aluminosilicate naturel de sodium et de… …   Encyclopédie Universelle

  • Plagioclase — Pla gi*o*clase, n. [Gr. pla gios oblique + kla^n to break.] (Min.) A general term used of any triclinic feldspar. See the Note under {Feldspar}. [1913 Webster] …   The Collaborative International Dictionary of English

  • plagióclase — s. f. [Geologia] Silicato de alumínio, sódio e cálcio, do grupo dos feldspatos. = PLAGIOCLÁSIO   ‣ Etimologia: grego plágios, a, on, oblíquo, enviesado + grego klásis, eos, quebra, fratura …   Dicionário da Língua Portuguesa

  • plagioclase — [plā′jē ō klās΄] n. [Ger plagioklas < Gr plagios (see PLAGIO ) + klasis, a cleaving, fracture < klaein, to break: see CLASTIC] any of a series of triclinic feldspars, ranging in composition from albite to anorthite and found in many rocks …   English World dictionary

  • plagioclase — plagioclastic /play jee euh klas tik/, adj. /play jee euh klays /, n. any of the feldspar minerals varying in composition from acidic albite, NaAlSi3O8, to basic anorthite, CaAl2Si2O8, found in most igneous rocks: shows twinning striations on… …   Universalium

  • Plagioclase — Grains fins de plagioclases dans roche volcanique Les plagioclases sont une importante série de silicates minéraux dans la famille des feldspaths. Plutôt que de se référer à un minéral particulier avec une composition chimique spécifique, le… …   Wikipédia en Français

  • plagioclase — noun A group of feldspar minerals that form a solid solution series from albite (sodium aluminum silicate) to anorthite (calcium aluminum silicate). The plagioclase feldspars are found in igneous, plutonic, and metamorphic rocks. They all have… …   Wiktionary

  • plagioclase — [ pleɪdʒɪə(ʊ)kleɪz, plagɪəʊ ] (also plagioclase feldspar) noun a form of feldspar consisting of aluminosilicates of sodium and/or calcium, common in igneous rocks and typically white. Origin C19: from plagio + Gk klasis cleavage (because orig.… …   English new terms dictionary

  • plagioclase — noun Etymology: Greek plagios oblique + klasis breaking, from klan to break more at clast Date: circa 1868 a triclinic feldspar; especially one having calcium or sodium in its composition …   New Collegiate Dictionary

  • plagioclase — pla·gi·o·clase …   English syllables


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.