Forsterite in the context of "Space group"

An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of some chemical elements, rather than as substances with an exact chemical formula. There may be two or more endmembers in a group or series of minerals.

For example, forsterite (Mg
₂SiO
₄) and fayalite (Fe
₂SiO
₄) are the two end-members of the olivine solid-solution series, varying in Mg
and Fe
in their chemical composition. So, the chemical formula of olivine can be better expressed as Mg_(2−x)Fe_xSiO₄ or Mg_xFe_(2−x)SiO₄.

Circumstellar dust is cosmic dust around a star. It can be in the form of a spherical shell or a disc, e.g. an accretion disk. Circumstellar dust can be responsible for significant extinction and is usually the source of an infrared excess for stars that have it. For some evolved stars on the asymptotic giant branch, the dust can be composed of silicate emissions. According to a study, it is still uncertain whether the dust is a result of crystalline silicate or polycyclic aromatic hydrocarbon. However, recent observations revealed that Vega-type stars display broad silicate emission. It is suggested that the circumstellar dust components can depend on the evolutionary stage of a star and is related to the changes in its physical conditions.

The study of the composition of this dust is dubbed astrominerology. The circumstellar dust around aging stars has been observed to include, "almost pure crystalline Mg-rich silicates (forsterite and clinoenstatite), amorphous silicates, diopside, spinel, oxides (corundum and Fe0.9Mg0.1O), and also carbon-rich solids such as: (hydrogenated) amorphous carbons, aromatic hydrocarbons and silicon carbide."

While chemically pure materials have a single melting point, chemical mixtures often partially melt at the temperature known as the solidus (T_S or T_sol), and fully melt at the higher liquidus temperature (T_L or T_liq). The solidus is always less than or equal to the liquidus, but they need not coincide. If a gap exists between the solidus and liquidus it is called the freezing range, and within that gap, the substance consists of a mixture of solid and liquid phases (like a slurry). Such is the case, for example, with the olivine (forsterite-fayalite) system, which is common in Earth's mantle.

The mineral olivine (/ˈɒl.ɪˌviːn/) is a magnesium iron silicate with the chemical formula (Mg,Fe)₂SiO₄. It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface. Olivine has many uses, such as the gemstone peridot (or chrysolite), as well as industrial applications like metalworking processes.

The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: Mg
₂SiO
₄) and fayalite (Fe-endmember: Fe
₂SiO
₄). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and/or fayalite (Fa) (e.g., Fo₇₀Fa₃₀, or just Fo₇₀ with Fa₃₀ implied). Forsterite's melting temperature is unusually high at atmospheric pressure, almost 1,900 °C (3,450 °F), while fayalite's is much lower – about 1,200 °C (2,190 °F). Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen (O), silicon (Si), magnesium (Mg) and iron (Fe). Manganese (Mn) and nickel (Ni) commonly are the additional elements present in highest concentrations.

Fayalite (Fe
₂SiO
₄, commonly abbreviated to Fa) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a = 4.82 Å, b = 10.48 Å and c = 6.09 Å.

Fayalite forms solid solution series with the magnesium olivine endmember forsterite (Mg₂SiO₄) and also with the manganese rich olivine endmember tephroite (Mn₂SiO₄).