Plagioclase
Plagioclase | ||
---|---|---|
Specific gravity 2.62 (albite) to 2.76 (anorthite)[2] | | |
Optical properties | Biaxial (+) albite, biaxial (-) anorthite[2] | |
Refractive index | Albite: nα 1.527, nβ 1.532 nγ 1.538 Anorthite: nα 1.577 nβ 1.585 nγ 1.590[2] | |
Solubility | Albite insoluble in HCl, anorthite decomposed by HCl[2] | |
References | [3] |
Plagioclase (
Plagioclase is a major constituent mineral in 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 Moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.[5]
Its name comes from
Properties
Plagioclase is the most common and abundant
The ability of albite and anorthite to form solid solutions in any proportions at elevated temperature reflects the ease with which calcium and aluminium can substitute for sodium and silicon in the plagioclase crystal structure. Although a calcium ion has a charge of +2, versus +1 for a sodium ion, the two ions have very nearly the same effective radius. The difference in charge is accommodated by the coupled substitution of aluminium (charge +3) for silicon (charge +4), both of which can occupy tetrahedral sites (surrounded by four oxygen ions). This contrasts with potassium, which has the same charge as sodium, but is a significantly larger ion. As a result of the size and charge difference between potassium and calcium, there is a very wide
Plagioclase of any composition shares many basic physical characteristics, while other characteristics vary smoothly with composition.
The
Plagioclase is usually white to greyish-white in color, with a slight tendency for more calcium-rich samples to be darker.
Plagioclase almost universally shows a characteristic
Plagioclase series members
The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%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.[19][20]
Name | % CaAl2Si2O8 |
% NaAlSi3O8 | Image |
---|---|---|---|
Anorthite | 90–100 | 10–0 | |
Bytownite | 70–90 | 30–10 | |
Labradorite | 50–70 | 50–30 | |
Andesine | 30–50 | 70–50 | |
Oligoclase | 10–30 | 90–70 | |
Albite | 0–10 | 100–90 |
The distinction between these minerals cannot easily be made
Endmembers
- calc-alkaline suites.[23]
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).
- basic rocks.[28]
- Labradorite is the characteristic feldspar of the more basic rock types such as gabbro or basalt.[28] Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal.[29] It is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase.[28] A variety of labradorite known as spectrolite is found in Finland.[30][31]
- Oligoclase is common in granite and monzonite.[28] The name oligoclase is derived from the Greek olígos ('small, slight') + klásis ('fracture'), in reference to the fact that its cleavage angle differs significantly from 90°. The term was first used by Breithaupt in 1826.[32] Sunstone is mainly oligoclase (sometimes albite) with flakes of hematite.[28]
Petrogenesis
Plagioclase is the primary aluminium-bearing mineral in
The
Crystallizing plagioclase is always richer in anorthite than the melt from which it crystallizes. This plagioclase effect causes the residual melt to be enriched in sodium and silicon and depleted in aluminium and calcium. However, the simultaneous crystallization of mafic minerals not containing aluminium can partially offset the depletion in aluminium.[37] In volcanic rock, the crystallized plagioclase incorporates most of the potassium in the melt as a trace element.[34]
New plagioclase crystals nucleate only with difficulty, and diffusion is very slow within the solid crystals.[35] As a result, as a magma cools, increasingly sodium-rich plagioclase is usually crystallized onto the rims of existing plagioclase crystals, which retain their more calcium-rich cores. This results in compositional zoning of plagioclase in igneous rocks.[28] In rare cases, plagioclase shows reverse zoning, with a more calcium-rich rim on a more sodium-rich core. Plagioclase also sometimes shows oscillatory zoning, with the zones fluctuating between sodium-rich and calcium-rich compositions, though this is usually superimposed on an overall normal zoning trend.[16]
Classification of igneous rocks
Plagioclase is very important for the classification of crystalline igneous rocks. Generally, the more silica is present in the rock, the fewer the mafic minerals, and the more sodium-rich the plagioclase. Alkali feldspar appears as the silica content becomes high.
Albite is an end member of both the alkali and plagioclase series. However, it is included in the alkali feldspar fraction of the rock in the QAPF classification.[40]
In metamorphic rocks
Plagioclase is also common in metamorphic rock.[41][28] Plagioclase tends to be albite in low-grade metamorphic rock, while oligoclase to andesine are more common in medium- to high-grade metamorphic rock. Metacarbonate rock sometimes contains fairly pure anorthite.[42]
In sedimentary rocks
Feldspar makes up between 10 and 20 percent of the framework grains in typical
At the Mohorovičić discontinuity
The
Exsolution
At very high temperatures, plagioclase forms a solid solution with potassium feldspar, but this becomes highly unstable on cooling. The plagioclase separates from the potassium feldspar, a process called
The solid solution between anorthite and albite remains stable to lower temperatures, but ultimately becomes unstable as the rock approaches ambient surface temperatures. The resulting exsolution results in very fine lamellar and other intergrowths, normally detected only by sophisticated means.
Uses
In addition to its importance to geologists in classifying igneous rocks, plagioclase finds practical use as construction aggregate, as dimension stone, and in powdered form as a filler in paint, plastics, and rubber. Sodium-rich plagioclase finds use in the manufacture of glass and ceramics.[47]
Anorthosite could someday be important as a source of aluminium.[47]
See also
References
- S2CID 235729616.
- ^ ISBN 0-471-80580-7
- ^ Plagioclase Mineral Data, WebMineral.com
- ^ "Plagioclase". Merriam-Webster.com Dictionary. Retrieved 2024-02-13.
- .
- ISBN 978-0-19-510691-6.
- ISBN 0-471-57452-X.
- ^ a b c d e f g h Klein & Hurlbut 1993, p. 542.
- ^ ISBN 978-0-19-965306-5.
- ^ ISBN 0-922152-34-9.
- ISBN 0-442-27624-9.
- ^ a b Nesse 2000, pp. 208–209.
- ^ a b c Nesse 2000, p. 216.
- ^ Sinkankas 1964, p. 457.
- ^ Klein & Hurlbut 1993, p. 541.
- ^ a b Nesse 2000, p. 215.
- ^ Sinkankas 1964, pp. 456–457.
- ^ "Minerals Colored by Metal Ions". minerals.gps.caltech.edu. Retrieved 2023-03-01.
- ^ a b Sinkankas 1964, p. 450.
- ^ Nesse 2000, p. 209.
- ^ Nesse 2000, p. 217-219.
- ^ "anorthite". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- ISBN 0-582-44210-9.)
{{cite book}}
: CS1 maint: multiple names: authors list (link - ^ "albite". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- ^ Jackson 1997, albite.
- ^ Klein & Hurlbut 1993, p. 568.
- ^ "bytownite". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- ^ a b c d e f g h i j Klein & Hurlbut 1993, p. 543.
- ^ a b Nesse 2000, p. 213.
- ISBN 0-7506-5856-8
- ISBN 1-4027-4016-6
- ^ "oligoclase". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- ^ McBirney, Anthony R. (1984). Igneous Petrology. Freeman, Cooper, and Company. p. 270.
- ^ a b McBirney 1984, p. 104.
- ^ a b McBirney 1984, p. 107.
- ^ McBirney 1984, pp. 318–320.
- ^ McBirney 1984, p. 396.
- S2CID 28548230.
- ^ "Rock Classification Scheme – Vol 1 – Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
- ^ ISBN 978-0-521-88006-0.
- ^ Nesse 2000, p. 219.
- ^ Nesse 2000, pp. 219–220.
- ISBN 0-13-154728-3.
- ISBN 978-1-4051-7783-2.
- ^ Philpotts & Ague 2009, p. 2.
- ^ McBirney 1984, p. 270.
- ^ a b Nesse 2000, p. 220.