Plagioclase

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Ancient Greek for "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 crystal 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. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.[3]

Plagioclase
Plagioklas1
A photomicrograph of a plagioclase crystal under cross polarized light. The plagioclase crystal shows a distinct banding effect called polysynthetic twinning.
General
CategoryFeldspar mineral group, tectosilicate
Formula
(repeating unit)
NaAlSi3O8 – CaAl2Si2O8
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupC1
Identification
ColorWhite, gray, bluish white, e
Mohs scale hardness6 - 6.5
LusterVitreous
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.62 (albite) to 2.76 (anorthite)[1]
Optical propertiesBiaxial (+) albite, biaxial (-) anorthite[1]
Refractive indexAlbite: nα 1.527, nβ 1.532 nγ 1.538
Anorthite: nα 1.577 nβ 1.585 nγ 1.590[1]
SolubilityAlbite insoluble in HCl, anorthite decomposed by HCl [1]
References[2]
PlagioclaseFeldsparUSGOV
Plagioclase displaying cleavage. (unknown scale)
LvMS-Lvm
In volcanic rocks, fine-grained plagioclase can display a 'microlitic' texture of many small crystals.

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 % CaAl2Si2O8
% NaAlSi3O8 Image
Anorthite
90–100
10–0
Anorthit Miyake,Japan
Bytownite
70–90
30–10
Feldspar - Bytownite Sodium calcium aluminum silicate Crystal Bay Minnesota 2689
Labradorite
50–70
50–30
Labradorite
Andesine
30–50
70–50
01722 Andesine
Oligoclase
10–30
90–70
Oligoclase-Sunstone from India2
Albite
0–10
100–90
Albite2

Endmembers

  • Anorthite was named by Gustav Rose in 1823 from the Ancient 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.
  • 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 as the variety Cleavelandite with rarer minerals like tourmaline and beryl.

Intermediate members

The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member (albite 2.62) increases 0.02 per 10% increase in anorthite (2.75).

  • Bytownite, named after the former name for Ottawa, Ontario, Canada (Bytown), is a rare mineral occasionally found in more basic rocks.
Labradorite detail
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.
  • Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.
  • 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.

See also

References

  1. ^ a b c d Klein, Cornelis and Cornelius S. Hurbut, Jr.; Manual of Mineralogy, Wiley, 20th ed., 1980, pp.454-456 ISBN 0-471-80580-7
  2. ^ Webmineral data
  3. ^ Milam, K. A.; et al. (2010). "Distribution and variation of plagioclase compositions on Mars". Journal of Geophysical Research: Planets. 115 (E9). doi:10.1029/2009JE003495.
Albite

Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence its name from Latin albus. It is a common constituent in felsic rocks.

Amphibolite

Amphibolite ( ) is a metamorphic rock that contains amphibole, especially the species hornblende and actinolite, as well as plagioclase.

Amphibolite is a grouping of rocks composed mainly of amphibole and plagioclase feldspar, with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

Amphibolite need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals entirely based upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolite (tremolite-schist, grunerite-schist, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalt creates ortho-amphibolite and other chemically appropriate lithologies create para-amphibolite.

Tremolite, while it is a metamorphic amphibole, is derived most usually from highly metamorphosed ultramafic rocks, and thus tremolite-talc schist is not generally considered as 'amphibolite'. A holocrystalline plutonic igneous rock composed primarily of hornblende amphibole is called a hornblendite, which is usually a crystal cumulate rock. Igneous rocks with >90% amphiboles, which have a feldspar groundmass, may be a lamprophyre.

Andesite

For the extinct cephalopod genus, see Andesites.

Andesite ( or ) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts they are a major component of the Martian crust. The name andesite is derived from the Andes mountain range.

Anorthosite

Anorthosite ( ) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Anorthosites are of enormous geologic interest, because it is still not fully understood how they form. Most models involve separating plagioclase crystals based on their density. Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.Anorthosite on Earth can be divided into five types:

Archean-age anorthosites

Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth

Layers within Layered Intrusions (e.g., Bushveld and Stillwater intrusions)

Mid-ocean ridge and transform fault anorthosites

Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts)Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins.Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

Basaltic andesite

Basaltic andesite is a volcanic rock containing about 55% silica. It is distinct from basalt and andesite in having a different percentage of silica content. Minerals in basaltic andesite include olivine, augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America and the Andes of South America.

Crust (geology)

In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle).

The crusts of Earth, Moon, Mercury, Venus, Mars, Io, and other planetary bodies formed via igneous processes, and were later modified by erosion, impact cratering, volcanism, and sedimentation.

Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types: continental crust and oceanic crust. These two types have different chemical compositions and physical properties, and were formed by different geological processes.

Diorite

Diorite ( ) is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. Diorite is usually grey to dark-grey in colour, but it can also be black or bluish-grey, and frequently has a greenish cast. It is distinguished from gabbro on the basis of the composition of the plagioclase species; the plagioclase in diorite is richer in sodium and poorer in calcium. Diorite may contain small amounts of quartz, microcline, and olivine. Zircon, apatite, titanite, magnetite, ilmenite, and sulfides occur as accessory minerals. Minor amounts of muscovite may also be present. Varieties deficient in hornblende and other dark minerals are called leucodiorite. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makes the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than 10 percent, the rock type grades into monzodiorite or granodiorite. A dioritic rock containing feldspathoid mineral/s and no quartz is termed foid-bearing diorite or foid diorite according to content.

Diorite has a phaneritic, often speckled, texture of coarse grain size and is occasionally porphyritic.

Orbicular diorite shows alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus, within a diorite porphyry matrix.

Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from the partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building, such as in the Andes Mountains, as large batholiths. The extrusive volcanic equivalent rock type is andesite.

Feldspar

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of sedimentary rocks.

Felsic

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Gabbro

Gabbro ( ) is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".

Granodiorite

Granodiorite ( ) is a phaneritic-textured intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. According to the QAPF diagram, granodiorite has a greater than 20% quartz by volume, and between 65% to 90% of the feldspar is plagioclase. A greater amount of plagioclase would designate the rock as tonalite.

Granodiorite is felsic to intermediate in composition. It is the intrusive igneous equivalent of the extrusive igneous dacite. It contains a large amount of sodium (Na) and calcium (Ca) rich plagioclase, potassium feldspar, quartz, and minor amounts of muscovite mica as the lighter colored mineral components. Biotite and amphiboles often in the form of hornblende are more abundant in granodiorite than in granite, giving it a more distinct two-toned or overall darker appearance. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals. Minor amounts of oxide minerals such as magnetite, ilmenite, and ulvöspinel, as well as some sulfide minerals may also be present.

Labradorite

Labradorite ((Ca, Na)(Al, Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality. (The third direction is poor.) It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

Monzonite

Monzonite is an igneous intrusive rock. It is composed of approximately equal amounts of plagioclase and alkali feldspar, with less than 5% quartz by weight. It may contain minor amounts of hornblende, biotite and other minerals. If quartz constitutes greater than 5%, the rock is termed a quartz monzonite.If the rock has a greater percentage of alkali feldspar, it grades into a syenite. With an increase in calcic plagioclase and mafic minerals the rock type becomes a diorite. The volcanic equivalent is the latite.

Moon rock

Moon rock or lunar rock is rock that is found on the Earth's Moon including lunar material collected during the course of human exploration of the Moon, or rock that has been ejected naturally from the Moon's surface (and which has then landed on the Earth as meteorites).

Phenocryst

A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than .5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts.

Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics. The largest crystals found in some pegmatites are often phenocrysts being significantly larger than the other minerals.

Quartz monzonite

Quartz monzonite or adamellite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende constitute the dark minerals. Because of its coloring, it is often confused with granite, but whereas granite contains more than 20% quartz, quartz monzonite is only 5–20% quartz. Rock with less than five percent quartz is classified as monzonite. A rock with more alkali feldspar is a syenite whereas one with more plagioclase is a quartz diorite. The fine grained volcanic rock equivalent of quartz monzonite is quartz latite.Quartz monzonite porphyry is often associated with copper mineralization in the porphyry copper ore deposits.

Rhyodacite

Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium-rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate-colored matrix.

Rhyodacite is a high silica rock containing 20% to 60% quartz with the remaining constituents being mostly feldspar. The feldspar is a mix of alkaline feldspar and plagioclase, with plagioclase forming 35% to 65% of the mix.

Rhyodacite often exists as explosive pyroclastic volcanic deposits.

Rhyodacite lava flows occur, for example, in northwestern Ferry County (Washington), and at An Sgùrr on the island of Eigg in Scotland.

Rhyolite

Rhyolite ( RY-ə-lyte, RY-oh-) is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2 – see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 – see the QAPF diagram). Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.

Trachybasalt

Trachybasalt is a volcanic rock with a composition between trachyte and basalt. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and likely very small amounts of leucite or analcime. Trachybasalt is a basalt with high alkali content (5 to 7% Na2O + K2O, see TAS diagram).

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