A rock forming mineral: Olivine

Everyone has heard of the very common mineral called quartz, most people have heard of the very common mineral called feldspar, but surprisingly few people have heard of the very common mineral called olivine. I speculate that this is for two reasons. one being that quartz is resistant to weathering and is very easy to find, feldspar often occurs in big crystals and is also somewhat resistant to weathering, whereas olivine quickly breaks down into clay and occurs in mafic (quartz poor) rocks. the second being that it is often only obvious as large crystals in some basaltic rock.

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

Olivine is crystalised in volcanic rocks at high temperatures. This means that as a mafic (basalt like) magma chamber cools the first mineral to form into crystals is olivine (see figure opposite). This indirectly means that if you see olivine crystals in the field it is usually because the rock was a lava that was erupted relatively rapidly to the surface from deep in the earths crust or upper mantle. But, sometimes you can come across rocks that are almost entirely made from olivine. These rocks are called dunite. It is formed at the boundary between the crust and the mantle and has crystalised there. It is thought that it has been bought to the surface through the action of plate tectonics where sometimes large chunks of oceanic crust can be scraped onto a continental plate as the process of subduction takes place. This is called an ophiolite sequence.

A metamorphic source of olivine is through the contact metamorphism of dolomite limestones.

Particulars:

Chemical Formula: (Mg,Fe)2SiO4
Fracture:Conchoidal
Hardness (Moh): 6.5-7
Specific Gravity:
Colour: Olive Green (Forsterite) to Browny-Green (Fayalite)
Luster: Vitreous (glassy)
Crystallography: Orthorhombic
Gem: Peridot
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.

References/Bibliography:

*Klein, K. Hurlbut, K. Manual of Mineralogy (After Dana, J.D.). Wiley 21st Ed.
*Encyclopedia of Earth: www.eoearth.org