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Field CollectingPeridotite in the Franciscan complex

26th Mar 2024 21:08 UTCPaul Hernday

I'm located in coastal northern California and when I'm in the field, peridotite is always on my search list. I keep looking for the greenish color, but most of the green I see is serpentinite. I have read that peridotite weathers rapidly at the earth's surface, often turning dark brown because of oxidation of Fe, at least on the surfaces. And much peridotite has been altered to serpentinite. Any tips on how to identify weathered peridotite, or photos? Thanks.

26th Mar 2024 21:27 UTCPaul Hernday

Adding another question. In this geologic area, if one has a rock with weathered peridotite, does cracking the rock open sometimes reveal greenish, less weathered peridotite?

27th Mar 2024 01:16 UTCJohn Christian

I have never seen any unaltered peridotite in the Bay Area. Never seen any bottle green peridot. You can find altered peridotite SW of Mt. Diablo and on the beach in San Francisco west of the Golden Gate Bridge. The peridotite is greenish to brownish with altered pyroxenes (Bronzite), chromite, with veins of serpentine including chrysotile.

See page 19 for piece of peridotite.

https://pubs.usgs.gov/bul/2195/b2195.pdf

27th Mar 2024 03:11 UTCPaul Hernday

Thanks John. The mention of 'hob-nail' surface rang a bell, I have two stream-weathered rocks that are nearly black with a hint of green and probably serpentine related, and have hob-nail bumps of slightly different color. Both of them attract a magnet (on one rock, the magnet seems to be attracted to specific points in the rock). I may crack them open for more info. Thanks!

27th Mar 2024 03:18 UTCJohn Christian

Consider joining a local geology or mineralogy society. The Northern California Geological Society in Orinda has lots of great trips, many north of San Francisco and east in the Sierras.

27th Mar 2024 05:17 UTCHerwig Pelckmans

So the link mentioned above:

https://pubs.usgs.gov/bul/2195/b2195.pdf

takes you to USGS Bull. 2195 :

STOFFER 2002 Rocks and Geology in the San Francisco Bay Area

Surprisingly, the mindat library does not have that reference yet.

Would be nice if someone could add it (together with the link to the pdf-file).

27th Mar 2024 06:09 UTCJohn Christian

Thanks Herwig.

Classic paper on regional geology geared toward the general audience, but also appreciated by the more educated in geology.

27th Mar 2024 11:56 UTCUwe Kolitsch Manager

San Francisco Bay Area

Another possibly helpful ref. (in the Mindat library):

Geology and natural history of the San Francisco Bay area: A field-trip guidebook

Bulletin 2188

Edited by: Philip W. Stoffer and Leslie C. Gordon

https://doi.org/10.3133/b2188

27th Mar 2024 12:34 UTCFranz Bernhard Expert

Its a tricky thing. For example, in the Kraubath ultramafic complex, sepentinization ranges from 10-100 %. Specimens, that feel more "grainy" are more likely not so much serpentinized than smother specimens. But without polished surface, its still lot of guesswork.

Magnetit is not a good sign for unaltered material.

And yes, brownish weathered stuff may contain a fresh (unweathered) core, but this tells not much about the degree of serpentinization.

28th Mar 2024 21:31 UTCRalph S Bottrill 🌟 Manager

Yes it’s hard to pick the peridotite in ultramafic complexes are they are typically partly to completely serpentinised, so you really need a thin section to tell. Easier to find it in many basalts.

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