xenolith - teaching hand specimen of an olivine xenolith in vesicular basalt
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A xenolith is a piece of country rock introduced into a magma during eruption and emplacement. These xenoliths may represent fragments torn at depth from a peridotite rock mass through which the basalt magma was forced. Their granular texture resulted from multiple crystallization nuclei. The vesicular basalt that contains these xenoliths erupted in the late Tertiary or Quaternary and caps Peridot Mesa on the San Carlos Apache Reservation east of Globe, Arizona.
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. When the A is CaMg, the mineral is Monticellite. When A is CaFe, the mineral is Kirschsteinite, and when it is Mn2, Tephroite is formed.
Since detailed testing is required to determine the mineral, the name olivine is used for convenience. A gem variety is called peridot. The darker grains are higher in iron content.
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