Fayalite content (Fa)
Fayalite content (Fa) is the proportion of iron-rich fayalite in a meteorite's olivine, given in mole per cent, and is a key number for classifying chondrites.
Olivine is a solid solution between forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Its composition is expressed as Fa, the molar percentage of fayalite: Fa = 100 × Fe / (Fe + Mg), counting atoms. Fa0 is pure forsterite and Fa100 pure fayalite. Low-calcium pyroxene is described in a parallel way with Fs, the ferrosilite content.
In equilibrated ordinary chondrites, Fa is the quickest route to the chemical group, because the groups differ in how oxidised they are. Typical ranges are roughly Fa16–20 for H chondrites, Fa23–26 for L and Fa27–32 for LL, with Fs ranges following the same order. Because olivine in types 4–6 is homogeneous, a set of analyses gives a tight mean that can be compared directly with these ranges. In unequilibrated type 3 chondrites Fa varies widely from grain to grain, so its spread, expressed as a percent mean deviation, becomes a measure of how unequilibrated the rock is. R chondrites have notably iron-rich olivine, and pallasite groups are separated partly by olivine Fa as well.
Fa is measured with an electron microprobe on polished sections, and reported values usually give the mean and standard deviation of a set of analyses.
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