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Anorthosite

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Anorthosite Rock

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Anorthosite is a coarse-grained, light-coloured containing rich elements of plutonic rock. It is mainly composed of plagioclase (generally bytownite or labradorite) also sometimes with small amounts of pyroxene. Mineral elements like amphibole, ilmenite, magnetite, olivine, pyroxene and spinel are the mafic minerals also often present. Anorthosite is a phaneritic type of intrusive igneous rock featured by a predominance of plagioclase feldspar (90-100%) and a nominal mafic component (0-10%).


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Anorthosite Chemical Formula               

The chemical formula of a real anorthite mineral is CaAl2Si2O8.


Anorthosite Uses

Anorthosite is quite significant and commonly used in academic and economic fields.


Anorthosite Mineralogy

Anorthosite mineral composition is as follows:

  • Anorthosite mainly contains plagioclase with 90-100% component in the andesine-labradorite range.

  • Mafic minerals never cross beyond 10% of the rock.

  • Some of the plagioclase is overly dense and flourished by later crystallization of ilmenite magnetite, plagioclase or pyroxene forming massive poikilitic grains.


Anorthosite Texture

Anorthosite is coarse-grained, leucocratic and hypidiomorphic. Some crystals are in the form of euhedral, while some have imperfectly developed faces and others contain no crystal form at all.


Classification of Anorthosite

  1. Age of the formation (According to the age of the formation of rock, they are categorized as Archean anorthosite and Proteroic anorthosite.)

  2. Occurrence and association (According to the occurrence, they are categorized as Lunar, Layered and Massif anorthosite.)

  3. IUGS (International Union of Geological Society)


Archean Anorthosite

They are deposed during the Archean era and are reflected by the high composition of An 85-100 calcic plagioclase. Chemically, calcic anorthosites layers are high in Al2O3 and are typically surrounded by a fine-grained mafic.


Proterozoic Anorthosite

They occur during the Proterozoic era creating stocks of batholith size plutons. Apparently, they are restrained to high-grade metamorphism terrains of the Precambrian age.


Lunar Anorthosite

Likely to form with the primitive lunar crust by crystallization of An-rich plagioclase, a lunar anorthosite has highly calcic plagioclase with lesser olivine, orthopyroxene, and spinel (Mg-Al). They are fine-grained, light-coloured, and quite close to 100% Ca-content.


Anorthosite Moon Rock

From the Earth's surface, the lunar anorthosite is seen as the light-coloured, highly reflective element of the Moon's surface referred to as the lunar highlands. These are actually the Moon's oldest rocks—over 4 billion years old—and enveloped the young Moon's whole surface before its crust was pummeled and fragmented by comets and asteroids.

When we talk about when a type of rock is anorthosite moon then it is a flat, dark commonly circular regions referred to as lunar maria (singular form: mare) are made up of the rock basalt. This basalt specimen was gathered near the rim of Hadley Rille. The fine-grained crystallinity and huge holes imply that this rock crystallized in the neighbourhood of the top of a molten lava flow.


Anorthosite Adirondacks

Anorthosite, the designated grey rock well acquainted to hikers and climbers in the Adirondacks is an antiquated type of granite that occurred 15 miles underneath the surface over a billion years ago. Forced to the surface by recent mountain constructing activity, its deep cracks brought into being the valleys and deep lakes of the area.


Anorthosite Thin Section  

This rock is a type of highland anorthosite, supposedly a cumulate that occurred as a component of the early lunar crust. This rock consists of plagioclase (~98%) of An95 to An97 and is truly a lithified anorthosite breccia or cataclasite.


Quick Facts

  • The term anorthosite is derived from French ‘anorthose’ (referring to french—plagioclase) was coined by Sterry Hunt.

  • Though Anorthite is typically rare on the Earth, it is abundant on the Moon.

  • Anorthosite minerals are not specifically abundant on Earth besides a few places such as the Grenville Province of the eastern Canadian Shield.

  • As a rock type, anorthosites have occurred over the entire range of geological time, and are expectedly still forming today.

  • Anorthosite formations are very diverse, and when their distinctive characteristics are used to classify them, it becomes evident that some types show very clear temporal restrictions.

  • Lunar anorthosite plays a significant part in the investigation of Venus, Mars and meteorites. They also possess the quality of a gem rock.

FAQ (Frequently Asked Questions)

1. What are the Various Types of Anorthosites Rock?


Answer: Anorthosites are typically classified into six basic types i.e.:

  1. Archean megacrystic anorthosites

  2. Anorthosites of oceanic settings

  3. Anorthosites of layered mafic complexes

  4. Anorthosite inclusions in other rock types

  5. Proterozoic (massif-type) anorthosites

  6. Extraterrestrial anorthosites.

2. How Do We Identify the Type of Anorthosite?

Answer: Below is a quick characteristic distinction of the different types based on anorthosite composition and anorthosite gneiss. For example, let’s consider the three of six anorthosite.


1. Archean Megacrystic Anorthosites

Archean anorthositic rocks can be seen as a minimal component of many were preserved, their principal textures are distinctive, being distinguished and identified by equidimensional megacrysts (up to 30 cm diameter) of calcic (generally > An80) plagioclase in a mafic groundmass. A genetic association between this kind of anorthosite and the mafic volcanic rocks of greenstone belts is suggested by formations of basaltic flows, dikes and sills which contain similar megacrysts of calcic plagioclase.


2. Anorthosites of Oceanic Settings

Small amounts of anorthosite are recovered by digging programs in both mid-oceanic ridge and fracture-zone, particularly the Atlantic and Indian oceans and the Caribbean Sea. Ophiolitic anorthosites mainly occur as sharply bounded layers similar to those in Layered mafic intrusions. The ages of occurence of anorthosite-carrying ophiolite complexes range from late Cambrian to early Eocene.


3. Anorthosites of Layered Mafic Complexes

Layered mafic intrusions of rock range in age from Archean to Tertiary, and often has anorthosites in different proportions. Commonly, anorthosites form at the tops of several meters up to 100 meters or more of modally graded layers. Grain size (essentially <1 cm) is identifiably smaller than the massif-type or Archean anorthosites The textures differ from orthocumulates having tabular crysts of plagioclase to accumulates with equant polygonal grains.