Why doesn't Olivine go into extinction through crossed polars?

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Offline endgamex

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Hello there, I've just been looking at a thin section of peridotite through crossed polars and in my workbook it says 'Extinction (to cleavage) n/a'.  So I studied the olivine a bit more and granted it just seems to change interference colours not actually go into extinction.  I can't figure out why this is though.  I thought it might be because it has no cleavage but neither does quartz and that has a wavy extinction. 

Thanks for any replies
Sarah