?With reference to the phase diagram for the olivine mineral family, shown below

Question

?With reference to the phase diagram for the olivine mineral family, shown below:

      a. If a liquid with a composition 60% forsterite, 40% fayalite is cooled, what would be the composition of the first solid to crystallize?

        b. If chemical equilibrium is maintained throughout the cooling process, what would be the composition of the last crystals to form, and the last drop of

liquid to crystallize?

  c. Olivine crystals in rocks are often not homogeneous, but instead are zoned in chemical composition. Qualitatively, describe the change you would expect   

             to see in a zoned crystal going from core to rim. Under what conditions does such zoning occur.

Explanation / Answer

Part A:

The first mineral to crystallize will be enriched in Fo composition, consisting of Fo80 Fa20 composition. The composition can be ascertained by drawing a horizontal line from liquidus to solidus curve. As it can be seen from the enclosed diagram, the melt is in liquid state above the liquidus curve and once it is cooled it crystallizes as identified by solidus.

Part B:

If chemical equilibrium is maintained the last crystal to crystallize will be of Faylite composition and the last drop will entirely composed of Fe2SiO4.

Part C:

The quantitative change in chemical composition can be described as change in composition from- Fo80Fa20 (Fosterite 80% and Fayalite 20%) to Fo100 (Fayalite).

The zoning is observed as a function of growth of first crystals of foresterite enriched composition of with continued drop in temperature and subsequent crystallization more faylite enriched crystallization occurs as rim around the initial crystals. This gives rise to zoning.

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