Day #117 Deskcrop: Dunite with Cumulate Chromite

Another of my well-loved teaching samples comes from the Del Puerto Ophiolite in central California (though my recollection of the specific locality is somewhat vague). The sample is dunite, an ultramafic rock made of >90% olivine, with layers of cumulate chromite. This is a great sample for discussing crystal settling in magma chambers and is one of my favorite thin sections for introducing students to the mineral olivine, in all of its technicolor glory. In addition to the hand sample shot below, there are also standard photomicrographs of the sample in plane and cross-polarized light and MicroGigaPans of the same two lighting schemes. None of these is going to replace the experience of actually viewing the sample in a petrographic microscope, where one can turn the sample to see different optical properties, but with a bit of work and a little more technological innovation I think there’s the possibility to make a web-deliverable experience that’s a reasonable approximation.

Dunite with Cumulate Chromite

Photomicrograph in Plane Light

Photomicrograph in X-Polarized Light

Dunite in Plane-Polarized Light
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Dunite in Cross-Polarized Light
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