(a) Two minerals that crystallize from common magmas are olivine ...

[Kthamas]

(a) Two minerals that crystallize from common magmas are olivine (Mg2SiO4) and quartz (SiO2).
 
  If a magma liquid is getting enriched in Si and depleted in Mg as it cools and starts to crystallize, which of these minerals crystallizes first? Hint: use the mineral formulas to help explain your answer.
   a) Olivine crystallizes first
   b) Quartz crystallizes first
   (b) Explain your reasoning behind the answer you selected:

Question 2

Compare the two diagrams. Rock X and rock Y are located at exactly the same depth and experience the same temperature in both diagrams. Which rock is melting?
 
  What will be an ideal response?

[kxciann]

Answer to Question 1

a) A
b) Because the silica content of the magma is increasing and the magnesium content is decreasing, the crystallizing mineral must contain more magnesium than the magma does. Quartz contains only silica, so if it crystallized, the silica content of the remaining liquid would decrease and the magnesium content would increase. Only removal of olivine, which contains magnesium, can cause the magnesium content of the remaining liquid to decrease.

Answer to Question 2

Rock Y is melting because the temperature and pressure conditions that this rock is experiencing plots in the part of the graph where crystals and liquid both exist. Rock X is in the solid-rock part of the diagram.

[Kthamas]

Excellent

[tkempin]

Thanks for the timely response, appreciate it