Questions about Olivine?

Question

1) As Ovivine consists of a mixture of Fayalite and Forsterite are these chemically bonded to each other? If so would it be Mg to Si02 and Fe to Si02 as ionic bonds? Importantly, are the Mg and Fe bonded to each other which I dont think is the case?
2) A text states that Olivine consists of either or both of the above minerals (as end members) or an intermediate composition? What do they mean by intermediate composition?

Answer 1

OLIVINE
Chemical Formula: (Mg,Fe)2SiO4
Empirical Formula: Mg1.6Fe2+0.4(SiO4)
Environment: Basic and ultra basic igneous rocks. Goup species name.
Name Origin: Named after the green color.
Synonym: Chrysolite - light yellowish green Peridot

FORSTERITE
Chemical Formula: Mg2SiO4
Empirical Formula: Mg2(SiO4)
Environment: Ultramafic igneous rocks or dolomitic marbles.
Name Origin: Named for Johann Forster, German naturalist.
Synonym: Chrysolite
ICSD 9334
Olivine
PDF 34-189
Peridot

FAYALITE
General Fayalite Information
Chemical Formula: Fe++2SiO4
Empirical Formula: Fe2+2(SiO4)
Environment: Ultra mafic silica-poor igneous rocks.
Locality: Fayal Island, Azores Link to MinDat.org Location Data.
Name Origin: Named for the locality
Synonym: Hortonolite - Mn,Mg
ICSD 26375
Knebelite(Mn)
PDF 31-633

Answer 2

Olivine is a solid-solution, meaning that there can be anywhere from pure magnesium olivine (foresterite) to pure iron olivine (fayallite), with most olivine consisting of both minerals freely substituting for each other. Olivine consists of individual silicon tetrahedrons (one silicon atom surrounded by four oxygen atoms), with iron or magnesium (or calcium) filling up the spaces between silicon tetrahedra. Iron and magnesium are not bonded together. Intermediate composition describes the vast majority of olivine, since it is rare that pure foresterite or pure fayallite would be seen. Typically, foresterite forms first, and as the mineral grows it is zoned in increasingly iron-rich components. The outside of an olivine crystal might be pure fayallite.