Abrasive Sunday Sand

 
All in all, it’s been an abrasive week. No, I’m not referring to my day job, but rather an emerging theme that started with my last post on the stories of three indefatigable women and a mystery Gobi sand. So far, there have been two very interesting responses to my question of what the mineral content of this “special sand” might be that made it so prized for polishing jade (and I’m also following up on other suggestions): Howard Allen helpfully defines some constraining criteria in terms of hardness and density (we must be dealing with something heavy enough to form a placer deposit, and hard enough – but not too hard – to polish a tough material like jade). Richard Bready – knowledgeable and stimulating as always – points to the work of a researcher at The British Museum whom I should contact, and sources that mention corundum, quartz, and garnet.

Earlier in the week, I heard from Siim Sepp, a graduate student of geology at the University of Tartu in Estonia, who alerted me to his areno-blog, Sandatlas, which he has been expanding into a wide-ranging and informative source for a variety of geo-topics. Browsing around Siim’s site, I was struck by one of the images in his gallery, above. This stunning sand is from the area of the Emerald Creek garnet district in the panhandle of Idaho, where garnets from sand- to golf ball-size occur in placer deposits that, organised by the Forest Service, can be visited by the public for a “mine your own garnets” day out. Garnets have been used since antiquity as both gemstones and abrasives, and the latter continues to be a widespread application today; Emerald Creek, in addition to being a Forest Service recreational area, is still one of the leading sources of industrial garnets in North America, producing a range of product sizes for everything from serious sand-blasting to fine polishing via water-jet cutting.

There are quite a few different “species” of the mineral group collectively referred to as garnet; they are all silicate minerals (strictly, nesosilicates or orthosilicates), but their composition (and colour) varies, with different sites within the crystal structure being occupied by iron, magnesium, calcium, aluminium, and chromium. Nevertheless, they all belong to a single group because they all have the same crystal form – the crystals are isometric, their internal structural symmetry being that of a cube. In garnets, these highly symmetrical internal building blocks most commonly organise themselves into the overall shape of a dodecahedron, but other related shapes can also form. The combination of colour and symmetry produces some truly stunning crystals, such as this one from Emerald Creek, the big one being 2.5 centimetres across (image from Exceptional Minerals):

 
One particular form, the star garnet, is the State Gem of Idaho; these extremely rare specimens are examples of asterism, the appearance of a luminous star shape as a result of tiny fibrous crystals of the mineral rutile (titanium dioxide) being included in the structure:

Garnets have a torrid geological history, born at high temperatures and pressures deep in the earth’s crustal kitchen, and, as a result, showing up at the surface in metamorphic, or, less commonly, igneous rocks. The garnets of the Emerald Creek district originate from sediments deposited around 1.5 billion years ago, buried, cooked, and rudely ejected back to the surface; weathering has liberated the tough garnet crystals to be concentrated in the stream placer deposits, but the original hard rock has also been extensively mined for not only garnets but a wide variety of minerals and metals.

So, not a solution yet to the Gobi mystery, but, for me at least, an interesting detour from the daily grind.

Comments

[Richard Bready]

The daily grind!

[F]

Interesting, as ever. Strange you should mention Sandatlas, as I had just recently run into that site via a link somewhere or other. But here's a link I found at En Tequila Es Verdad, and I thought of you:
http://galileospendulum.org/2011/11/15/tsunamis-of-sand-in-the-sahara/

Of course, this is the same blog that gave me the Sandatlas link. I should have known.
http://www.sandatlas.org/2011/11/brain-games-with-sand-grains/

http://freethoughtblogs.com/entequilaesverdad/2011/11/21/los-links-1118/
(Under Science, naturally.)

[F]

Oh, and gigapans of the dunes at White Sands:
http://ron.outcrop.org/blog/?p=1419

[elliptical reviews]

No offense, but if there's a facebook like button, it'll be much easier for me to share.

[Sandglass]

There is a facebook button - right next to the word "share."