Anorthosite
Plagioclase feldspar-rich intrusive rock
An intrusive igneous rock made almost entirely of plagioclase feldspar, famous as the rock of the lunar highlands.
- Mohs hardness
- 6-6.5 (plagioclase)
- Color
- White, grey, bluish to dark grey
- Type
- igneous
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Overview
Anorthosite is a coarse-grained intrusive igneous rock composed of 90% or more plagioclase feldspar, with only minor pyroxene, olivine, or other dark minerals. It is unusual because most igneous rocks contain a broader mix of minerals.
It ranges from white and pale grey to dark grey, and some varieties (rich in labradorite) show beautiful blue iridescence. Anorthosite is best known beyond Earth: the bright lunar highlands are largely anorthosite, making it literally the rock of the Moon's ancient crust.
On Earth it forms massive ancient bodies and is the source rock of much commercial labradorite.
Formation & geology
Anorthosite forms when plagioclase crystals accumulate in large magma chambers, often by floating or settling to concentrate as a nearly monomineralic cumulate. Two main types exist: massif (Proterozoic) anorthosites that form enormous ancient intrusions, and layered anorthosites within mafic complexes.
Many terrestrial anorthosites are about 1 to 1.7 billion years old. On the Moon, anorthosite formed early in lunar history when light plagioclase floated to the top of a global magma ocean. Notable Earth localities include the Adirondacks of New York, Quebec's Grenville Province, and Norway.
How to identify it
Look for a light grey to white, coarse-grained rock dominated almost entirely by blocky plagioclase feldspar with few dark minerals. Some specimens flash blue, green, or gold (labradorescence) when tilted.
The overwhelming dominance of plagioclase and scarcity of quartz and dark minerals is the key clue. Feldspar cleavage surfaces, hardness near 6, and white streak help confirm it.
Look-alikes include gabbro (much more pyroxene, darker overall) and larvikite (a feldspar rock but alkali-feldspar rich syenite rather than plagioclase). Iridescent anorthosite can resemble massive labradorite.
Uses & significance
Anorthosite is quarried as dimension and decorative stone, and its labradorite-rich varieties supply much of the iridescent labradorite used in jewelry and ornaments. Some anorthosites host ilmenite and titanium ore deposits, and aluminum-rich plagioclase has been studied as an alternative aluminum source.
Scientifically, anorthosite is hugely significant: lunar highland anorthosite recovered by the Apollo missions revealed that the early Moon had a magma ocean. On Earth it records ancient crust-forming events. Iridescent specimens are valued by collectors.
Frequently asked questions
Is the Moon made of anorthosite?
The bright lunar highlands are largely anorthosite, formed when light plagioclase floated to the top of an early lunar magma ocean.
What is anorthosite made of?
It is made of 90 percent or more plagioclase feldspar, with only small amounts of pyroxene, olivine, or other dark minerals.
Does anorthosite contain labradorite?
Yes. Many anorthosites are rich in labradorite, and they are a major commercial source of the iridescent gem feldspar.
How is anorthosite different from gabbro?
Both are intrusive and feldspar-bearing, but gabbro contains abundant dark pyroxene, while anorthosite is almost entirely pale plagioclase.
Anorthosite guides
In-depth guides for identifying, valuing, and understanding Anorthosite.
Anorthosite identified by the community
Real specimens identified with Rock Identifier.
