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Andrew Alden

Cryst, Blast or Clast?

By March 26, 2013

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One of the hardest things in geology—no, not gneiss—is the terminology. If you find a big mineral grain, or just a big thing, inside another rock, what do you call it? The experts have words for each possibility: phenocryst, porphyroblast or megaclast. If you've seen these terms in the literature and despair of telling them apart, here's a key to their identities: look at the suffixes. The -crysts, -blasts and -clasts almost always correspond to igneous, metamorphic and sedimentary rocks. Read the gritty details in this article.

Comments

March 26, 2013 at 4:00 pm
(1) Carlos Rosière says:

In my opinion large crystals of hydrothermal origin should be included in the “cryst” classification, namely as megacryst (they are not “blasts” nor “pheno” although may be occasionally poikilitic). You did not bring this matter into discussion but is nevertheless an important one. What do you think?

March 26, 2013 at 6:20 pm
(2) Geology Guide says:

I think that you’re right, if we need to have a word for them at all.

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