Anorthosite Rock
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
Age of the formation (According to the age of the formation of rock, they are categorized as Archean anorthosite and Proteroic anorthosite.)
Occurrence and association (According to the occurrence, they are categorized as Lunar, Layered and Massif anorthosite.)
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.
FAQs on Anorthosite
1. What is anorthosite?
Anorthosite is a phaneritic, intrusive igneous rock primarily composed of plagioclase feldspar (90–100%), with a minimal mafic component (less than 10%). As a plutonic rock, it crystallises slowly beneath the Earth's surface. Its appearance is typically white, greyish, or sometimes bluish, and it is known for its coarse-grained texture.
2. What are the main types of anorthosite?
Geologists classify anorthosite into several main types based on their age and geological setting. The primary types include:
Proterozoic (massif-type) anorthosites: These are the most abundant type on Earth, forming large, mountain-sized bodies.
Archean anorthosites: These are among the Earth's oldest rocks, notable for their very large plagioclase megacrysts.
Anorthosites of layered mafic complexes: These are found as distinct layers within larger intrusions of mafic rocks like gabbro.
Extraterrestrial anorthosites: Famously, these form the light-coloured, ancient highlands of the Moon.
3. What are the primary uses of anorthosite rock?
Anorthosite has several important industrial applications. It is mined as a source of aluminium and is used in the manufacturing of glass, ceramics, and cement. High-purity anorthosite is also polished and sold as a dimension stone for building facades and countertops, often marketed under trade names like 'Blue Granite' or 'Black Granite'.
4. Where is anorthosite found in India?
In India, significant anorthosite deposits are primarily located in the Eastern Ghats mobile belt. Major occurrences are found in the states of Odisha (near the Chilka Lake complex), Andhra Pradesh, and Tamil Nadu. These formations are crucial for studying the Proterozoic geological history of the Indian subcontinent.
5. Why is anorthosite so important for understanding the Moon's geology?
Anorthosite is vital for lunar science because it constitutes the bulk of the ancient lunar highlands—the bright, heavily cratered areas visible from Earth. Its widespread presence strongly supports the Lunar Magma Ocean hypothesis. This theory posits that the early Moon was covered by a global ocean of molten rock. As it cooled, the lighter anorthositic plagioclase crystals floated to the top, forming the Moon's primordial crust.
6. What is the "anorthosite problem" in geology?
The "anorthosite problem" refers to a long-standing geological puzzle concerning the origin of massive, mountain-sized anorthosite bodies. The core issue is the nature of the parental magma from which they formed. Since plagioclase is less dense than most magmas, it should float. However, the enormous volume of these anorthosite massifs makes it difficult to account for the equally vast quantities of denser, mafic minerals that should have crystallised and settled out from the same magma.
7. How do geologists distinguish Proterozoic anorthosite from Archean anorthosite?
The key distinction lies in their geological context and texture. Proterozoic anorthosites typically occur as huge, independent intrusions (massifs) and are often associated with other plutonic rocks like mangerite and charnockite. In contrast, Archean anorthosites are characterised by very large, equidimensional megacrysts of calcic plagioclase (up to 30 cm) embedded in a finer mafic groundmass and are typically found within ancient greenstone belts.
8. What gives some anorthosites, like labradorite, their iridescent play of colour?
The stunning play of colour, known as labradorescence, is an optical effect, not a pigment. It is caused by light interference within the rock's microscopic, alternating layers (lamellae) of different feldspar compositions. This internal structure, called the Bøggild intergrowth, diffracts light, splitting it into its component colours and creating the shimmering flashes of blue, green, and gold for which the gemstone labradorite is famous.
