Olivine Occurrence and Compositon Olivine is typically found in mafic to ultramafic igneous rocks and in their metamorphic equivalents. Less commonly it is found in marbles and a few other metamorphic rock types. Olivine has general formula (Mg,Fe)2SiO4. Most natural olivines contain only minor other components. Identification Olivine is in many ways similar to clinopyroxene. The keys to identifying olivine are its high birefringence, lack of cleavage (but often having fractures), and alteration. Important properties:  ·Color - Usually colorless or very pale yellow.  ·Interference colors - Interference colors may range up to strong third order.  ·Cleavage and fracture - Olivine has one very poor cleavage not seen in thin section; instead grains typically fracture in an irregular manner.  ·Alteration - Olivine easily alters, producing deep red or brown iddingsite, scalely serpentine or any of a numer of other products. Alteration often occurs along cracks and grain peripheries.  ·Form - May forrm euhedral phenocrysts in volcanic rocks. In most metamorphic and igneous rocks olivine is generally in subhedral grains, sometimes aggregated. Similar minerals:  ·Clinopyroxene has higher birefringence and non-parallel extinction.  ·Andalusite is very similar but does not occur in the same kinds of rocks.

Olivine in an Olivine Websterite Xenolith from the Cima Volcanic Field, California These photos show one large grain of olivine (center with a major fracture) surrounded by clinopyroxene and orthopyroxene. The two pyroxenes cannot be reliably distinguished in these views. Note there is cleavage present in the pyroxenes while the olivine is cleavage free. The field of view is 4.5 mm.

Olivine in a Xenolith from the Cima Volcanic Field, California This veiw shows a large grain of olivine surrounded by fine grained magnetite, clinopyroxene and minor plagioclase. The fine grained material is hard to identify in this view. The field of view is 4.5 mm.

Olivine, Augite, and Plagioclase in a Porphyry from Near Fish Lake, Oregon This view shows three kinds of phenocrysts in a groundmass of mostly plagioclase, magnetite (opaque) and clinopyroxene (augite). The phenocrysts are lathes of plagioclase, several more equant grains of olivine, and two grains of augite. In PP light all three are nearly clear, but the augite (hard to pick out in PP) has a slightly greenish tint. In XP light, the olivine can be seen to have 2nd order pastel interference colors that vary within individual grains. The largest olivine grain, a quarter of the way down from the top center of the photo) shows red, violet, blue and green interference colors. The two largest augite grains are next to it - one up and to the right and one below and slightly left. Both augite grains show somewhat anomalous yelllow-brown interference colors. The field of view is about 2.5 mm across.