Teaching hand specimen of a peridotite xenolith in vesicular phonotephrite from the San Carlos Volcanic Field, Arizona 

A xenolith is a piece of country rock introduced into a magma during an eruption or emplacement of a batholith. These xenoliths represent fragments torn at depth from a peridotite rock mass through which the phonotephrite magma was forced. Their granular texture resulted from multiple crystallization nuclei. The vesicular phonotephrite that contains these xenoliths erupted in the Middle Pleistocene ~580,000 years ago and caps Peridot Mesa on the San Carlos Apache Reservation east of Globe, Arizona.

In these xenoliths, the pale lime green is forsterite olivine, the black grains are spinel and the uncommon bright emerald green grains are chromian diopside pyroxene. Olivine is the name given to a group of minerals with the general composition of A₂SiO₄. The A is usually Fe or Mg, but it can be Ca, Mn or Ni. A complete solid-solution series exists, with the composition of most olivine somewhere between Forsterite (Mg₂SiO₄) and Fayalite (Fe₂SiO₄), so the formula for olivine is often given as (Mg,Fe)₂SiO₄. Since detailed testing is required to determine the specific olivine type, the name olivine is normally used for convenience. A gem variety is called peridot. 

The host rock is vesicular phonotephrite (phonolitic tephrite). If your students see this as vesicular basalt, that’s acceptable, since the exact composition cannot be determined in the field and requires the use of a petrographic microscope.

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it - a black mechanical pencil, a silver and black pen, or one of those plus some number of coins, or you can let us make the selection.

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