What is the chemical formula of olivine?	(Mg,Fe)2SiO4.  Olivine forms a solid solution series between two end members - pure Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite). Return to question
Describe the crystal structure of olivine.	Olivine consists of isolated silicate tetrahedra (another way to put this is that olivine has a very low degree of polymerization [EM, pp. 45-48; SP, p. 13]). Return to question
At what temperature does olivine crystallize?	Olivine has a high crystallization temperature.  According to EM Plate 6.5 it crystallizes at temperatures in the range c.1150-c.1320°C.  This is a higher range of crystallization temperatures than that of any other common mineral. Return to question
What evidence have you seen to suggest that olivine is unstable at low temperatures and pressures?	Olivine crystals are common in several of the thin sections you have examined using your petrological microscope.  These crystals are frequently cracked.  The cracks are locations where the the olivine crystal has altered to iron oxide and other new minerals [EM p. 48].   Return to question
Olivine is a very hard mineral (6-7 on Mohs' scale) - almost as hard as quartz, with which is shares its 3-D bonding [EM Sections 4.3.1 and 4.6.1].  Why, in spite of this, is olivine so unstable at low temperatures and pressures while quartz is so stable?	The isolated silicate tetrahedra of olivine are not bonded to each other (the 3-D bonding refers to the bonds between the oxygen ions in each tetrahedron, in both olivine and quartz).  This makes it easy for the tetrahedra to become disarticulated, so that olivine is much more soluble than quartz.  Imperfections in the crystal lattice will admit oxygen, which readily reacts with the iron ions in the octahedral interstices [EM Figure 4.4] to form the iron oxides in the cracks, as noted in point 4 above. Return to question
In what rocks is olivine typically found?	Olivine is a major constituent mineral of the ultramafic rock peridotite.  It is also an important mineral in mafic rocks, including gabbro, dolerite and basalt [EM Plate 6.12]. Return to question
Why would you predict olivine to be a major constituent of these rocks?	Gabbro and basalt crystallize from primary magmas formed by partial melting of mantle peridotite.  These magmas are rich in iron and magnesium and relatively poor in silica, as reflecting the parent rock.  They also form at high temperatures.  As these basaltic magmas cool, olivine is typically the first mineral to crystallize. Return to question
Why is olivine not a constituent of intermediate or felsic igneous rocks?	Rocks of intermediate composition such as andesites and diorites are formed by the partial melting of mafic rocks (or the partial melting of other intemediate rocks).  Each stage of partial melting reduces the iron/magnesium content of the resulting magma and increases the relative silica content.  By the time partial melting yields a magma of intermediate composition there is not sufficient iron and magnesium in the melt to form olivine.  Also, the temperatures at which partial melting forms intermediate magmas are typically too low for olivine to melt, so any olivine in the parent rock is left behind.  What applies to intermediate rocks applies with even more force to felsic rocks:  in granitic magmas, biotite is usually the only ferro-magnesian mineral able to form. Return to question
What part of the plate tectonic cycle produces rocks rich in olivine?	Partial melting of mantle peridotite at constructive plate margins, such as the Mid-Atlantic Ridge. Return to question
Where in the Earth's crust is olivine predominantly found?	In oceanic crust, which is mafic in composition.  Oceanic crust is divided into three layers [IP Figure 7.1] consisting of basalt, dolerite dykes and gabbro respectively.  These differ in grain size but have the same mineralogical composition.  All are rich in olivine. Return to question
Oceanic crust stands uniformly at a lower level than continental crust.  Can you relate this fact to the chemical composition of olivine?	Olivine is rich in Fe2+ and Mg2+ ions, which nestle in the octahedral interstices of the olivine crystal structure [EM Figure 4.4].  These elements make olivine very dense.  Pyroxene is also rich in iron and magnesium.  The prevalence of these ferro-magnesian minerals in gabbro gives oceanic crust a greater density than continental crust, which is predominantly of intermediate composition.  This greater density accounts for the fact that oceanic crust rides lower on the aesthenosphere. The greater density of oceanic crust also accounts for the fact that it can subduct back into the mantle, whereas continental crust does not, for the most part, return to the mantle.  Hence the chemical composition of olivine is implicated in the operation of the plate tectonic cycle on the Earth.  The behaviour of olivine during repeated partial melting, in which it is distilled out of the intermediate rocks that form continental crust, also means that the chemical composition and crystal structure of olivine are implicated in the formation of the continents! Return to question
Where would you look for olivine in rocks on the continents?	Mainly in flood basalt provinces [IP Section 7.3], where hot spots have caused the eruption onto the continents of basaltic magmas generated in the mantle.  A good example is the Columbia Flood Basalt Province in Oregon and Washington, USA, where thousands of cubic kilometres of basalt were erupted onto the North American continent during the Miocene epoch, about 15 Ma. Return to question
How well does olivine stand up to weathering and transport when exposed in rocks at the Earth's surface?	Very poorly.  Olivine is very vulnerable to chemical weathering.  The isolated tetrahedra of the olivine crystal structure allow olivine to be dissolved readily by acidic groundwaters and rainwater.  Dissolution of the silicate tetrahedra releases the Fe2+ and Mg2+ ions held in the crystal lattice, and these too are carried away in solution.  Cracking in olivine crystals increases the surface area exposed to chemical weathering, and allows pore waters to penetrate the crystal and speed up the process of dissolution. Return to question
Mafic rocks can be found on the continents.  So why is olivine practically non-existent in sedimentary rocks formed by the weathering of gabbro and basalt on land?	When mafic rocks such as basalt are weathered, the olivine they contain usually does not survive long enough in transport to be deposited elsewhere in a sediment.  The constituents of olivine such as silica and iron have to go somewhere, of course.  The iron, for example, will be oxidized when released into surface environments, and may be deposited as insoluble ferric oxide, which may be observed as iron staining on the surfaces of other rocks. Return to question

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