Monday, January 12, 2009

Monday Mineral: Sunstone

Andrew Alden has up a piece on state minerals; Oregon doesn't have a state mineral, but we do have a state gem- which is a mineral. Gems are not necessarily minerals; two that come to mind are amber, which is solidified pitch from trees, and opal, which isn't crystalline. But Oregon's state gem is sunstone, which is a transparent form of plagioclase feldspar (labradorite, for us nerds).
I don't remember exactly when I collected these; the fun thing in this picture is the old apothecary jar I found to keep them in.Here are a few of them scattered out on a table. I didn't think to include something for scale, but I'd estimate the longest dimensions on these to be a bit less than a centimeter. I snapped these pictures as I was getting ready to leave for my coffee shop this morning, and (again) wasn't really thinking about trying to capture notable or interesting features. However in the following crop from the upper right area of the above picture, you can see the typical polysynthetic twinning on one of the crystals. The piece in the upper right, though a little out-of-focus, shows some of the play of light that makes better specimans of this mineral desirable. These particular pieces have no real value; they're small, the color is unremarkable, and the play of light (schiller)is not particularly striking. If memory serves, the value of high-quality cut gems can pass $2000 per carat. I'm not finding a citation for that, but there is an interesting piece at e-bay discussing the factors that might make a particular gem more or less valuable, along with some beautiful pictures of cut and polished gems. The source that I remember is Orr, Orr and Baldwin's Geology of Oregon. I'll double check that figure, but it has remained unchanged since the earliest editions of that book; I suspect it's actually significantly higher now. (Followup: wrong on two counts. First, the figure is $1000 per carat, not 2000. Second, the edition of Geology of Oregon I have at home is authored by Orr and Orr; though based heavily on the previous editions authored by Baldwin, he was not involved with the edition I have. I still suspect that the price has risen with inflation.)
While there are a number of mining claims, the most productive portion of the area, about 4 square miles, has been set aside by the BLM as unclaimable, and is open, free, to the public (though mechanized digging is prohibited: hand tools only).
The above photo was lifted from Google Earth; is is somewhat misplaced to the east. If you go to 42.722960, -119.861271 (decimal degrees) in Google Earth, you can zoom in close enough to see the sign. I'm always surprised by the resolution of coverage in GE- it varys from place to place, but I'm still generally pleasantly surprised. One of the fun things about camping at the sunstone area is how terribly remote and removed you feel. As it gets dark the coyotes sing to each other across the low hills surrounding this basin. Often there are no other people there at all, and the nearest community, Plush, is about 25 road miles away. The population of that metropolis is 82. Below is a GE view toward Hart Mountain to the southeast from the sunstone area.
This is quite true-to-life; at least it's quite true to my memories. Hart Mountain is a fault block in the northern Basin and Range; it is composed of Steens Mountain Basalt. SMB is more or less contemporaneous with the Columbia River Basalt. As I remember, its composition is also quite similar to CRB. Its source (Steens Mountain area) is 200-300 miles south of the (much larger) CRB eruptive area and the span of eruption of these basalts was shorter than that of the CRB. The sunstones are simply large (and they can be enormous) phenocrysts of plagioclase that have survived weathering better than the surrounding ground mass of the basalt. My suspicion is that mechanical weathering and eolian erosion are the two major processes at work here; as you can see in the above picture the local relief is nearly non-existent. A couple of links of interest are the Dust Devil Mining website, which has some great photos of gem quality specimans, and the BLM website, which offers some basic information on the area.

I have been thinking about starting this series for a while, and I thank Andrew for providing the motivation to get going. Other posts may not be as elaborate as this one, but I think I have enough samples to keep this going for a while... and I can always go get more! I'm also considering a "Friday Fragment" post, which would be about interesting rocks I have known.


Callan Bentley said...

Nice -- I like this series. Looking forward to some more!

Silver Fox said...

Nice sunstones! I have a few scattered around my house, also. I might have to show a photo of them after awhile, but will be sure to link here if I do.