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Can you determine the order of crystallization of minerals in an igneous rock without using Bowen's reaction series?

- Jenny, Tampa, FL


Great question.

I'd start by looking at the shapes of the different minerals in the rock, working under the assumption that the earliest minerals to crystallize would have grown unimpeded and therefore exhibit the more perfect crystalline shapes (called "euhedral" because they have "all" of their crystalline "faces"), while the mineral that "fills in whatever space is left at the end" probably came last (called "anhedral" - no faces).

A good example would be in granite. The dark mineral (usually biotite or hornblende) will - according to Bowens - crystallize first (and sure enough usually has well-formed crystalline shapes). The feldspar will be less perfect (called "subhedral" because it sometimes ran into some of the dark mineral and only has some of its faces). And the quartz? Well, it falls at the bottom of Bowens and so we usually find it in anhedral grains, filling in whatever space was left.

Hope this helps.


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