Olivine

It is the most abundant mineral in the Earth's upper mantle. Chemically, it is a Nesosilicate (isolated tetrahedra), meaning it is composed of individual silica tetrahedra $(\mathrm{SiO}{\phantom{A}}_4){\phantom{A}}^{4-}$ held together by divalent metal cations (usually $\mathrm{Mg}{\phantom{A}}^{2+}$ or $\mathrm{Fe}{\phantom{A}}^{2+}$ -> $(\mathrm{Mg},\mathrm{Fe}){\phantom{A}}_2\mathrm{SiO}{\phantom{A}}_4$). So the general formula is:

with $\mathrm{A}$ can be $\mathrm{Pb},\mathrm{Ca},\mathrm{Co},\mathrm{Fe},\mathrm{Mg},\mathrm{Mn},\mathrm{Ni}$.

The Solid Solution Series

Olivine is not a single mineral is a series of Minerals between two (or three) end-members.

The End-Members

Forsterite ($\mathrm{Fo}$):

• Formula: $\mathrm{Mg}{\phantom{A}}_2\mathrm{SiO}{\phantom{A}}_4$
• Color: Bright green / white
• Melting Point: 1890 °C
• Occurrence: Common in the Mantle and primitive meteorites.

Fayalite ($\mathrm{Fa}$):

• Formula: $\mathrm{Fe}{\phantom{A}}_2\mathrm{SiO}{\phantom{A}}_4$
• Color: Brown / Black
• Melting Point: 1205 °C
• Occurrence: Rare in the mantle, found in evolved iron-rich rocks or sediments.

There can also be a Manganese-rich end-member
Tephorite:

• Formula: $\mathrm{Mn}{\phantom{A}}_2\mathrm{SiO}{\phantom{A}}_4$
• Color: grayish-olive, can also be reddish-brown
• Occurrence: Rare and not found in normal Basalts or the upper mantle.

Notation

Often olivine gets described by its Forsterite Number ($\mathrm{Fo}$#):

• Formula: $\mathrm{Fo}$# = $\frac{\mathrm{Mg}}{\mathrm{Mg}+\mathrm{Fe}}\cdot 100$
• Mantle Olivine: Typical $\mathrm{Fo}{\phantom{A}}_{90}$ (90% Magnesium, 10% Iron)
• Crustal (Basalt) Olivine: Typically $\mathrm{Fo}{\phantom{A}}_{80}$ - $\mathrm{Fo}{\phantom{A}}_{50}$ (More iron-rich as magma cools)

Geochemical significance

In Bowen's Reaction Series, Olivine is the very first mineral to form from cooling basaltic magma. This leads to olivine crystals sinking to the bottom of magma chambers, forming rocks made almost entirely of olivine.

Leading to Mantle Discontinuities, Olivine cannot survive immense pressures in the transition zones:

• 410 km: Olivine -> Wadsleyite
• 520 km: Wadsleyite -> Ringwoodite

Question and Answers

What are the two end-members of the Olivine solid solution series?::Forsterite (Mg-rich) and Fayalite (Fe-rich).

What is the chemical formula for Forsterite?::${\text{Mg}}_2{\text{SiO}}_4$.

What is the chemical formula for Fayalite?::${\text{Fe}}_2{\text{SiO}}_4$.

What is the typical composition of Olivine found in the Earth's upper mantle (in Fo#)?::${\text{Fo}}_{90}$ (90% Forsterite).

In Bowen's Reaction Series, which mineral is the first to crystallize from a primitive basaltic magma?::Olivine.

What happens to Olivine crystals in a magma chamber due to their high density?
?
They sink to the bottom, forming "Cumulate" rocks (like Dunite).

Describe "Normal Zoning" in an Olivine crystal.::The core is Magnesium-rich (formed first, high T), and the rim is Iron-rich (formed last, lower T).