But firstly olivine is made from similar components as most of the other common minerals. In particular it is comprised of silica with either/or some magnesium (Mg2SiO4), known as forsterite or iron (Fe2SiO4), known as fayalite. Its chemical formula is often given as ((Mg,Fe)2SiO4) because the magnesium or iron can substitute for each other and are usually present together. Because of the nature of the chemical bonds between the magnesium, iron and the silica group the mineral weathers quite rapidly (geologically speaking). Forsterite (mg rich) tends to be an olive green colour and because of the iron content fayalite is more browny-green.
|Bowens Reaction Series from Encyclopedia of Earth|
A metamorphic source of olivine is through the contact metamorphism of dolomite limestones.
Chemical Formula: (Mg,Fe)2SiO4
Hardness (Moh): 6.5-7
Colour: Olive Green (Forsterite) to Browny-Green (Fayalite)
Luster: Vitreous (glassy)
Common accompanying Minerals: Not found with free quartz crystals. reguarly found with feldspar, pyroxene, augite
More information on olivine can be found one the Mineralogy Database.
Just a quick note on dunite and ophiolite sequences, this rock type is named after Dun Mountain in the northern part of the South Island of New Zealand. Dun Mountain is almost exclusively made from dunite and is part of a geological feature known as an ophiolite sequence which stretches along and off the Alpine Fault in New Zealand. Another ophiolite sequence is present in New Caledonia. Closer to home, the Peel Fault which runs along the western side of the New England Tablelands past Tamworth eventually to somewhere near Port Macquarie, also resembles an ophiolite sequence. I will discuss the Port Macquarie part of the Peel Fault at some time in the near future.
*Klein, K. Hurlbut, K. Manual of Mineralogy (After Dana, J.D.). Wiley 21st Ed.
*Encyclopedia of Earth: www.eoearth.org