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:
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.
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:
Hope you enjoyed my musings on my favorite rock.