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My state’s official rock is a mineral!

I live in California.  Our official State Rock is serpentine.  The only problem with that is that serpentine is a mineral, not a rock.   Rock consisting mostly of serpentine is called serpentinite.  AAAAGGGGHHHH!  (Jumps up and down in frustration.)  Admittedly, where serpentinite occurs, it’s really, really, full of serpentine.  But still…mutter grumble grumble mutter.

So I’ll talk a bit about serpentine.  It starts out life as olivine, a mantle rock.  Another name for gemmy olivine is peridot, a lovely green stone you may have seen in jewelry.   Olivine is the base rock in the undersea spreading zones where mantle rock is erupted to generate new earth-surface material.  But seawater eventually penetrates the base rock, and then Stuff Happens.  To understand, I have to introduce a little chemistry.  Don’t let your eyes glaze over; it’s simple, really.  Olivine is actually a chemical series: (Mg,Fe)2SiO4.  This means that Mg (magnesium) and Fe (iron) are interchangeable in the chemical composition of the stuff. Where there’s a lot of magnesium, the mineral is called forsterite; where there’s a lot of iron, the mineral is called fayalite.  Forsterite is green, while fayalite is dark brown, the color being dictated by how much magnesium versus iron is in the rock.  But forsterite, fayalite, or somewhere inbetween,  olivine is a dry silicate.  In the presence of seawater, it begins to turn into serpentine, (Mg, Fe)3Si2O5(OH)4  Again, magnesium and iron readily swap out for one another.

Those (OH) molecules make serpentine into a wet silicate.  That means it can survive  a very long time before being scooped up somehow onto the  dry land, most often in a subduction zone where some of the rock gets scraped off the subducting seafloor, and ends up on the land where we humans can study it.  The conversion takes time, though, which is why we find and mine peridot (gemmy forsterite) on the land.  (Beader Karen adds: that’s why peridot beads are so $%#@& expensive!)

I have a lovely sample of serpentinite.  I can’t find the damned thing; this house is so full of rocks it’s bulging at the seams.  However, Blogger Garry Hayes (http://geotripper.blogspot.com/) has an excellent specimen pic he’s allowed me to use:

Serpentinite

Serpentinite

This is better than my sample, which is mostly plain green; this is a mix of mineral variations with lots of different magnesium/iron induced color. Thank you, Garry!

A good field geologist can often spot serpentinite even without an exposed outcrop.  Most plants don’t like serpentiniferous soils.  There’s a place in the Santa Teresa Hills in the eastern part of my Santa Clara Valley where you can be wandering along a park trail, and all of a sudden the plants around you change.  I’m not a plant expert; just about everything I know about plants comes from a gardening manual.  But even I had  no trouble noticing the distinction.  The shrubs living in the serpentiniferous soil were definitely different from the shrubs living in other soil.

Now, a few readers are going to ask, “but isn’t serpentinite asbestos?  Well, serpentine is, sometimes.  Like some other minerals, it can form in long thin, fragile threads; in that state it’s described as asbestiform, and it that form it has a different mineral name, chrysotile.  Chrysotile crumbles easily and you really, really don’t want to breathe it.  Supposedly it isn’t as toxic as other asbestiform minerals, but I don’t recommend taking chances.  However, most of the serpentinite you see commercially is not asbestiform, and it doesn’t pose any sort of health hazard.

Serpentinite is a popular material in the bead trade, where it goes by various names.  Some are fairly honest, like “serpentine”; others are outright dishonest, like “new jade”.  Real jade is one of two minerals, nephrite or jadeite, both of which are much rarer, stronger, and far more resistant to wear than serpentinite.  Buyer beware!  Also, for some unfathomable reason, serpentinite is sometimes sold dyed red, orange, purple…  why take something so pretty and dye it?

Here’s a random sampling of some serpentinite beads I bought awhile back.

Serpentinite beads

Serpentinite beads

The little (6 mm) spotted round beads are from Russia; the others are from China. You can see the variation in colors, mostly produced by variations in the magnesium vs iron content of particular places in the beads. Some of the color differences come from other minerals, too; whatever the seller says, this is serpentinite.

Here’s a necklace I made for a friend,  with various serpentinite beads and peach moonstone (a gemmy version of a feldspar), along with sterling silver:

Serpentinite and Moonstone

Serpentinite and Moonstone

Hope you enjoyed my musings on my favorite rock.

Karen

Comments

  1. says

    Every time I see the word “chrysotile”, my brain assumes it’s a misspelling of “chrysolite”. And Wikipedia tells me that chrysolite is another name for olivine, which is unnecessarily confusing!

    That said, I like serpentine/serpentinite. Not only is it pretty, there’s also some exciting botany going on on serpentine soils. It’s pretty mind-opening to think that these serpentine-adapted plants are dealing with the aftereffects of some long-passed tectonic activity.

  2. redpanda says

    As a resident of Southern California, what would you recommend as a good field guide for someone hoping to acquire some practical rock-reading skills while exploring the less civilized areas nearby? I have a basic background in just about every major field of natural science except geology, so I’m not afraid of technical details as I figure I can Google anything over my head to fill in the gaps. There are lots of field guides out there, so I’m hoping you or someone you know has some experience with a few that I can make use of.

    • Karen Locke says

      I don’t know of any pubs local to SoCal, but I would start with Deborah Harden’s “California Geology”. It’s used as a textbook, so it’s overly expensive, but you might look for it used. While Debbie lives and taught in Northern California (she retired from teaching a few years ago), the book certainly doesn’t stint Southern California; it’s by and large where San José State goes for it’s field trips, and where Debbie’s husband Andrei Sarna-Wojcicki has done some major work for the USGS.

      As far as rock-reading skills go, it’s almost impossible to learn them from a book. Rocks are very much a 3-D experience. If you can swing it, take a community college introductory geology course with a lab. If they offer the lab separately, just take the lab. Handling rocks is the quickest, surest way to learn rock language.

      • redpanda says

        Thanks for the suggestion! Unfortunately my schedule is so unpredictable that there’s no way I’m going to be able to take a real class for the next several years, so I’ll just have to make do with books and friends until then. When you say impossible to learn from a book, are you talking about IDing rocks or are you talking about IDing the signatures of geological events in rocks (or both)?

        • Karen Locke says

          I was just thinking about IDing rocks. It’s almost impossible to do from pictures unless you’ve already handled a bunch of rocks, learned what their textures are, see how different one face is from another.

    • Karen Locke says

      The Skeptic Society in SoCal has one space left on a field trip next weekend. Don Prothero is leading it. Field trips are an excellent way to get up close to rocks in the field! Go to http://www.skeptic.com to register.

  3. Alverant says

    Hey, could be worse. Oklahoma has the watermelon as the state vegetable! At least California made an understandable mistake. But having a berry be a state vegetable is an act of ignorance.

  4. Hairy Chris, blah blah blah etc says

    Sorry, this might be a stupid question, but as a Brit… you have an official rock?? WTF???

    • IslandBrewer says

      Each state among our glorious and united 50 often names an official state flower (eg., California golden poppy), bird (California quail), fossil (Smilodon fatalis), butterfly. dessert, insect, dance, etc. Three states have official firearms! (Arizona has the Colt Single Action Army Revolver).

      The “official” designation doesn’t really mean anything or carry any particular weight, however. But, when I was a kid, we always used to tell each other that it was illegal to pick the poppies because they were the state flower (it’s not illegal).

      Also, the San Joaquin Sandy Loam is our state soil.

    • Karen Locke says

      The state mineral is gold, and the state gem is benitoite. My personal opinion is that voting on these things keeps our state legislature busy when they’re too deadlocked to get any REAL work done, but then I’m a bit of a cynic.