Describing a rock texture characterized by crystals so fine and small they cannot be seen without magnification.
From Greek 'aphanes' (invisible) with the adjectival suffix '-itic' used in geology. The term became standard in petrology to classify rocks based on their crystal grain size.
Aphanitic texture is how geologists distinguish between rocks that cooled quickly (like lava that erupts from a volcano) and those that cooled slowly deep underground—quick cooling creates aphanitic texture, slow cooling creates visible crystals! This one word tells you the entire cooling history of a rock.
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