I couldn’t help myself

This morning one of my friends posted as her facebook status update the comment "Tell me something I don't know..." "Without mucus, your stomach would digest itself." "Ok, tell me something ELSE I don't know. Something less... disgusting...".

Since she isn’t a geologist, I couldn’t resist typing up the following paragraph to share with her, as something she (and, likely, most of her other on-line friends) probably didn’t already know.

The presence of even a small amount Mn lowers the temperature at which garnet first starts to crystallize in a metapelitic rock; Mn is preferentially incorporated into garnet as compared to the other minerals. It substitutes into the same position in the garnet crystal structure as Fe, Ca and Mg (all of which are usually far more common). As a result the earliest garnet grown in a metamorphic rock is usually the highest in Mn-concentration, and as the crystal grows and depletes the reservoir of Mn its composition changes, gradually incorporating less and less Mn and more and more Fe into its crystal structure. The analysis of a typical crystal of garnet in such rocks will usually show a bell-shaped curve for Mn—decreasing in quantity towards the edges of the grain, while Fe increases. (Ca and Mg are also usually zoned, but they tend to respond more to changes in pressure to dictate which has the greater concentration.)