Latite

Latite is defined at least two different ways. If crystals are visible enough to allow an identification by modal minerals (using the QAP diagram), latite is defined as a volcanic rock with almost no quartz and roughly equal amounts of alkali and plagioclase feldspars. If this procedure is too difficult, latite is also defined from chemical analysis using the TAS diagram. On that diagram, latite is a high-potassium trachyandesite, in which K₂O exceeds Na₂O minus 2. (A low-K trachyandesite is called benmoreite.)

This specimen is from Stanislaus Table Mountain, California (a well-known example of inverted topography), the locality where latite was originally defined, by F. L. Ransome in 1898, in U.S. Geological Survey Bulletin 89. He discussed the confusing variety of volcanic rocks that were neither basalt nor andesite but something intermediate, and proposed the name latite after the Latium district of Italy, where other volcanologists had long studied similar rocks. Ever since, latite has been a subject for professionals rather than amateurs. I always hear it pronounced "LAY-tite" with a long A, but from its origin it should be pronounced "LAT-tite" with a short A.

In the field, it is impossible to distinguish latite from basalt or andesite. This specimen has large crystals (phenocrysts) of plagioclase and smaller phenocrysts of pyroxene.

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