Anorthosite is a plutonic igneous rock (formed at high temperature and pressure deeply underground) and is characterized by its mostly calcium-rich plagioclase feldspar composition (90 to 100%), with a minimal mafic (iron and magnesium) component. The plagioclase feldspar is a gradational solid-solution series, and its composition ranges from anorthite (calcium rich) to albite (sodium rich), with each member forming at a particular temperature and pressure. Early mineralogical authorities (e.g., Dana) considered that each part of this series represents individual minerals. The consensus today is that only the end members (anorthite and albite) are distinct minerals.
If the preceding paragraph makes little or no sense to you, please see my Aug. 21, 2020 blog post entitled Bowen’s Reaction Series. In that post, you will see my poster on the two mineralogical sequences of several minerals. On the upper right-hand side of this poster is the sequence beginning with the mineral anorthite, which is the calcium-rich end member of the plagioclase solid-solution series. The sample of anorthite (35 mm wide) used on my poster is shown here.
The mineral anorthite can be the main component of the rock anorthosite (a plagioclase feldspar). Although anorthosite can be essentially mono-mineralic, its plagioclase-feldspar composition can be different from region to region. Two hand specimens of anorthosite from the San Gabriel Mountains, northern Los Angeles
County are shown below.
I do not know the exact composition of this first hand specimen (119 mm wide), but its bluish-gray color is indicative of it being more toward a high percentage of calcium.
This second hand specimen (135 mm high) is a bimineralic rock. It has a similar grayish appearance indicative of a high-calcium content as present in the previous hand specimen but also contains a significant content of mafic (dark) minerals. This second specimen is more appropriately referred to as an anorthosite-gabbro rock. Some geologists might call it by its equivalent name of norite.
Anorthosite is not that common on Earth but can be locally extensive mainly in batholiths (large bodies, many kilometers across). Every continent has a fairly large area consisting of Precambrian-age exposures of this rock, mainly near the ancestral cores (shields) of the continents. Examples are in the Bushveld Complex in Africa, the Grenville Province of eastern Canada, the Lake Superior region in Minnesota, as well as in Nambia/Angola, India, Australia, eastern Europe, etc.). The anorthosite found in southern California is unusual because 1) it occurs near the edge of a continent occurrence and 2) its small extent.
Anorthosite has been found also on the Moon, in certain rare varieties of chondritic meteorites, in the comet Wild 2, and has been detected on Mars. In these occurrences, as well as on Earth, anorthosite represents apparently a primordial phase of planetary and other astronomical rocky material.
As evident in the image above (source Wikipedia.org), the Moon’s crust consists and two sharply contrasting materials. There is the mostly bright material, which is mostly anorthosite. It is confined to the lunar highlands. There is also the dark material, which is basalt (lava). It is confined to the maria regions. After the Moon formed about 4.5 billion years ago, it had a huge magma ocean over a solid interior. As the magma coolled, iron and magnesium silicates crystallized and sank to the bottom. Plagioclase-feldspar crystallized and floated up to form the anorthostie lunar crust. Later, about 4 billion years ago, magma rose and infiltrated the lunar crust, where reactions occurred and basalt was formed. Afterward, the surfaced was intensely bombarded by meteors, thus causing the cratered surface of the lunar highlands. When bombardment ceased, lava flowed from the inside of the moon through volcanoes and cracks in the crust. This lava filled the mare and cooled to form basalts (3.7 to 2.6 billion years old). Since then, some more meteors have hit the surface. The present-day regolith (soil) consists of fine dust.
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