Olivine

Structure

Olivine is a common green or brown rock forming minerals which consists of a solid-solution series between Forsterite (Fo) and Fayalite (Fa).

It is an orthorhombic orthosilicate with isolated SiO4 tetrahedra linked by divalent Mg or Fe ions. The oxygen atoms lie in sheets parallel to the (100) plane and the points of the tetrahedra point in alternate directions.

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Chemistry (Mg,Fe)2SiO4

Olivine is forms what is called an isomorphous solid solution series that ranges between two end members:

Forsterite Mg2SiO4 to Fayalite Fe2SiO4.

As an aside: The Fe rich members (Fayalite) have Mn and Ca entering the crystal whereas the Mg rich members (Forsterite) have Ni and Cr entering the lattice.

Solid Solution Series

In nature, a complete series from Mg to Fe olivine can be found. However, some problems arise in natural, almost pure Fayalite as Mn or Ca (divalent) can substitute for the Fe up to 1%.

It is also important to note that ferric (Fe3+) iron does not substitute into olivine. Any ferric iron present is contained in Spinel group (MgAl2O4) inclusions usually in the Mg (forsterite) end member.

Alterations

Olivines are not stable at surface temperatures because of their high temperature of formation. They are therefore are susceptible to low grade metamorphism and weathering.

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Alterations cont.

3Mg2SiO4 + 4H2O + SiO2 2Mg3Si2O5(OH)4

(olivine) (serpentine)

2Mg3Si2O5(OH)4 + 3CO2 Mg3Si4O10(OH)2 + 3MgCO3 +3H2O(serpentine) (talc) (magnesite)

These sort of minerals are very common in ultramafic and basic plutonic rocks (ie basic lavas).

Iddingsite MgO·Fe2O3·3SiO2·4H2O is a brown, pleochroic mineral with strong birefringence found in basalts and basalt porphyries as an alteration product of olivine (pseudomorph).Optical and Physical Properties

Refractive Index and 2V angle change linearly with composition from Fo to Fa.

Forsterite has 2V=85° to 90°, Opt. (+)

Fayalite has 2V=47° to 54°, Opt. (-)

Intermediate compositions have 2V=70° to 90°, Opt. (+) or (-)

Cleavage is virtually absent in basic olivines and rare in Fe varieties although sometimes present on some surfaces.

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Distinguishing Features

•large 2Vangle•high birefringence•lack of cleavage, fractures present•extinction parallel to crystal outline•shape•alteration•lack of colour•zoning is rare in plutonic rocks but not uncommon in more basic rocks.

Phase Relationships

The Forsterite - Fayalite phase diagram is a classic that represents one of the simplist systems.

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Phase Relationships cont.

This is an equilibrium diagram plotting Temperature vs composition. Results are based on experimental studies of synthetic olivine mixtures.

Simple binary system

Forsterite has melting point of 1890 °C

Fayalite has melting point of 1205 °C

Therefore, the first olivine crystals to form should be Mg rich Forsterite at ~1889 °C and, as a consequence, the remaining liquid would become progressively more Fe rich.

Look at the phase diagram:

The two intercepts on the temperature axis are at 1890 ° and 1205 °C, Fo=100% and Fa=100% respectively.

The upper curve is the LIQUIDUS and describes the temperature range where the onset of crystallization occurs.

The lower curve is the SOLIDUS and describes the onset of a solid.

Between the curves both liquid and solid material exists.

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Phase Relationships cont.

Example:

A melt of composition Fo50, Fa50 exists at temperature T1 (X on diagram).

Cool the melt, at T2 the line for Fo50, Fa50 hits the LIQUIDUS (point Y). At this point crystals of Fo80 begin to form from the liquid. The remaining liquid becomes more Fe or Fayalite rich.

As cooling continues, the liquid becomes progressively more Fe rich, therefore the crystals forming also become more Fe rich.

At temperature T3 the last of the liquid is used up. A liquid of composition Fo50, Fa50 has to completely solidify by the time the temperature drops to T3 (the SOLIDUS).

Under equilibrium conditions, the crystals are constantly altered and converted to a composition approaching Fo50 from a Fo80 start. Equilibrium implies slow cooling and constant diffusion of Fe ions into the crystals. HOWEVER, it is very common for DISEQUILIBRIUM to occur.

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Phase Relationships cont.

Disequilibrium can occur by:

(a) Removal of crystals.

Crystals precipitate or settle out of the melt and sink due to higher density.

As crystals are removed, the remaining liquid becomes more Fe rich Fo40 Fo30.

This in turn extends the range of crystallization to lower temperatures.

The final crystals in our example would be a Fa composition of about 80%.

Result: Cummulative Rocks - many basaltic piles.

(b) Do not allow Diffusion

If crystals are layered and cooled fast enough, we get the first crystals layered off or zoned by further layers. This has the same effect as removal of crystals.

Zoned Crystal

Mg core, Fe rims

Pure olivine melts do not exist in nature, however, the diagram is useful to explain what we see.

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Olivine Paragenesis

What type of rock is olivine found in?

(i) Fo-rich olivine (range Fo80-90) is one of the first phases to crystallize from basaltic melts. Typical of ocean floor or Hawaii. Pure Fo is unknown in igneous rocks. Get a range from Fo (Mg) rich to Fa (Fe) rich.

(ii) Generally more Fe rich olivine in basaltic lavas, dykes and sills (usually as phenocrysts). Why? Magmatic differentiation. Deeper in the crust you get Mg rich olivine in gabbros and ultramafic mono-mineralic rocks like dunite. These form in magma chambers (often as bands of euhedral crystals in gabbro)

(iii) If a basalt is crystallizing plagioclase as well, you get Troctolites (plag + olivine rock). What type of plag? Ca-rich.

(iv) Olivine nodules occur in certain types of lavas (Mona Kea) and shallow intrusives. The olivine composition is nearly always Forsteritic. They are moved out of the chamber by volatiles and are often vesicular. There are two forms:

1) Nodules which are cumulates, ie. Bits of crystals (mush) which collect in the magma chamber (often associated with plag and clinopyroxene

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Olivine Paragenesis cont.

2)Mantle - pieces directly derived from mantle lumps. These also contain Mg-rich orthopyroxenes and spinel.

Nodules of Opx - Cpx - Ol - Spinel Mantle

Nodule of Plag - Ol - Cpx Cumulates

(v) Fayalite Olivines (unusual) are found in silicic rocks when Na>K ie Na (H2O) rich rocks.

(vi) Large olivine rich ultramafic bodies. These are regionally metamorphosed (may be) mantle which show signs of high stress, shearing and serpentinization. These rocks are thought to have been brought to the surface from depth.

(vii) Forsterite in metamorphic impure limestones

2CaMg(CO3)2 + SiO2 Mg2SiO4 + 2CaCO3 +2CO2

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