Vaca Muerta Olivine

Steve Arnold looked like he was passing appetizers. He was circulating with a tray of gem jars in Tucson. Each one held a cm size morsel of Vaca Muerta olivine. Mesosiderites aren’t known to be olivine rich so these were pretty cool. Yes, please, may I have two?

Vaca Muerta olivine appears to be from the mantel of the mesosiderite parent body, not from the impactor that disrupted it. That PB is very similar to the Howardite – Eucrite – Diogenite PB but there is now evidence that suggests they are not the same. You can Google for details but I suggest you go first to David Weir’s always informative site for an up to date overview. Start with a few words on mesosiderites
http://www.meteoritestudies.com/protected_append4.htm#stoiro
then go to his Vaca Muerta page.
http://www.meteoritestudies.com/protected_VACA.HTM

About two and a half grams each.

21 mm x 18 mm

Translucent at two millimeters thick.

One of the two thin sections shown below.

Olivine fragments of all sizes fill cracks between large clasts. The literature on Vaca Muerta describes cracks with fragments in a sulfide rich matrix. That is not the case here. Field of view is 3 millimeters wide. Cross polarized light.

FOV = 3mm. XPL

FOV = 3mm. XPL

Here the crossed polarizing filters are positioned so the large tan clasts are near optical extinction. The mottled shading is indicative of undulatory extinction. As the filters are rotated relative to the sample portions of the same crystal go to extinction (go dark) at different times. This is because shock has deformed the crystal lattice.

Undulatory extinction. We move through extinction in 7 steps. The outside of this fragment darkens before the center. Total filter rotation over the 7 steps is about 35 degrees. If the crystal was without shock it would darken uniformly.

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About the Author

John Kashuba
John is a natural history enthusiast living in Oregon.
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