Meteorite Minerals


For the purposes of this tutorial, we are mostly concerned with meteorite minerals. A mineral is a naturally occurring chemical compound that has characteristic crystal structure. Quartz, SiO2, is a mineral, whereas glass is not because it lacks crystalline structure. Minerals are the building blocks of rocks. Most of the classification schemes for meteorites are based on their mineral content, which also reflects their bulk chemical composition. The type of minerals that are present, their abundance, size, and shape tell us a lot about the composition of the parent magma, in addition to the temperature and depth of crystallization, (e.g., surface lava flows vs. deep plutonic rocks), and how fast the minerals crystallized (hours to weeks).

Ringwoodite, a blue high-pressure denser form of olivine, in the Taiban chondrite. Source: http://www.psrd.hawaii.edu/April04/asteroidHeating.html

Before 1962, about 40 minerals were recognized in meteorites with 5 unique to meteorites. In 1972, an additional 47 minerals were recorded, 20 of which are unique. Today, nearly 300 minerals are found in meteorites with about 40 found exclusively in meteorites. The common meteoritic minerals are listed below in Tables 1-6. Of the minerals listed below, the most diagnostic minerals for meteorites are the Ni-Fe minerals: kamacite and taenite. These minerals contain nickel (Ni) and iron (Fe) in their native state, i.e., not combined with oxygen, and are rarely found in terrestrial rocks. In addition, they comprise most of the minerals in "iron meteorites." They impart a magnetic quality to meteorites, which is the first test used when a suspected meteorite is being examined. However, magnetite (Fe3O4) or "lodestone" is a well-known magnetic mineral and is very common in terrestrial rocks as well as in some meteorites.

Follow-on examination involves the study of constituent minerals. Scientists, some students, and meteorite collectors may have access to a petrographic microscope and may also be able to recognize diagnostic meteorite minerals in hand specimens. Armed with this expertise, minerals in a meteorite are studied for their inherent characteristics. Although there are other unique minerals in meteorites, they are very often too rare or small to be useful. The trained person can make use of the common minerals to classify and define a meteorite's history.

The most common minerals are pyroxenes followed by plagioclase feldspar, and olivine. These minerals are those that typically crystallize from a molten magma whether on Earth or in a meteorite parent body. Pyroxenes are mostly composed of the oxides FeO, CaO and MgO structurally arranged in various proportions with SiO4 tetrahedra to form continuous unit cell chains. Other elemental/oxide enrichments found in pyroxenes include MnO, TiO2, Al2O3, and Cr2O3. The enstatite pyroxene family is dominated by MgO- to FeO-rich members that are low in CaO; these are collectively referred to as orthopyroxenes because of their orthorhombic crystalline structure. Diopside-augite-pigeonite pyroxenes are richer in CaO and have a different crystalline structure; these are termed clinopyroxenes because of their monoclinic crystalline structure. Feldspars are divided into plagioclase, enriched in oxides of CaO and Na2O in a framework structure of AlO4 and SiO4 tetrahedra, and alkali feldspars that consist of various proportions of K2O and Na2O in a similar structure. Olivine contains FeO and MgO, but is structurally different from other minerals in its arrangement of SiO4. Other minerals listed below may be present in the meteorite classes, but in lesser amounts. Although no less significant, the presence or absence of minor minerals can say a lot about the host rock's origin and environment of formation.

Table 1. Silicates

MineralCompositionComments
Olivine Group
olivine group(Mg,Fe)2SiO4Color: buff, brown, to honey yellow; darker with higher Fe
forsteriteMg2SiO4Color: colorless to green
fayaliteFe2SiO4Color: very dark
Pyroxene Group
enstatiteMgSiO3Color: colorless to gray
orthopyroxene(Mg,Fe)SiO3Color: gray to buff
pigeonite(Mg,Fe,Ca)SiO3Color: gray, buff, brown, greenish yellow
clinopyroxene(Ca,Mg,Fe)SiO3Color: colorless. yellow. greenish, buff
diopsideCaMgSi2O6Color: white, pale to dark green (depends on Cr content)
fassaite-Diopside with large amounts of Al and Ti; color: green to purple
augiteMg(Ca,Fe,Al,Ti)Si2O6Color: green, pale brown, purplish brown, brown
hedenbergiteCaFeSi2O6Color: brownish green, brown, black
Feldspar Group
orthoclaseKAlSi3O8Color: colorless, white, gray, yellowish
albiteNaAlSi3O8Color: white
anorthiteCaAl2Si2O8Color: colorless to white to gray
Silica Group
Silica (tridymite, cristobalite)SiO2Rare: occurs in many meteorite classes; color: clear to white
Melilite Group
akermaniteCa2MgSi2O7Occurs mostly in refractory Ca,-Al,-Ti-rich inclusions
gehleniteCa2Al2SiO7
Other
serpentine(Mg,Fe)6Si4O10(OH)8Water-bearing, alteration mineral of olivine, pyroxene, etc. Found mostly in carbonaceous chondrites.

Table 2. Oxides

Mineral Composition Comments
ChromiteFeCr2O4Opaque; dull gray color. May contain large amounts of Ti and/or Al
IlmeniteFeTiO3Opaque; red brown
MagnetiteFe3O4Opaque; black and magnetic
SpinelMgAl2O4Opaque; variable color. May contain large amounts of Fe and/or Ti
Hercynite FeAl2O4 Opaque; variable color.
HiboniteCaAl12O19Occurs mostly in refractory Ca,-Al,-Ti-rich inclusions

Table 3. Carbonates

Mineral Composition Comments
CalciteCaCO3Colors of carbonates range from clear to yellow to brown. Occur mostly in carbonaceous chondrites from secondary alteration and in the celebrated Martian meteorite, ALHA 84001 where they may have formed from biotic activity.
DolomiteCaMg(CO3)2
Magnesite(Mg,Fe)CO3
SideriteFeCO3

Table 4. Phosphides & Phosphates

Mineral Composition Comments
Schreibersite(Fe,Ni)3PCommon in iron and stony iron meteorites. Opaque; shiny yellow in color.
Cl-ApatiteCa5(PO4)3ClColors of phosphates range from clear, yellow, green, to purple.
MerrilliteCa9NaMg(PO4)7
WhitlockiteCa3(PO4)2

Table 5. Sulfides

Mineral Composition Comments
Oldhamite CaS Highly reduced enstatite chondrites
Pentlandite(Fe,Ni)9S8These sulfides are common in nearly all meteorites. Color: yellowish to brass.
PyrrhotiteFe1-xS
TroiliteFeS

Table 6. Native Elements & Metals

Mineral Composition Comments
Diamond, LonsdaleiteCCommon as nanodiamonds (< 1 micrometer in size) in carbonaceous chondrites.
GraphiteCCommon in many meteorites, but in small amounts; black and microscopic.
Kamaciteγ-(Ni,Fe)Nickel-iron metal is common in nearly all meteorites and is the main distinguishing characteristic of meteorites verses terrestrial rocks.
Taeniteα-(Ni,Fe)

Table 7. Carbides

Mineral Composition Comments
Moissanite (carborundum) SiC Occurs in carbonaceous chondrites.
Cohenite(Fe,Ni)3COccurs in iron metorites.