Diaspore

Day 296: From the time I was a very young kid, I enjoyed collecting rocks. My interest was piqued further when one day on my way to school, I picked up an egg-shaped lump and threw it against a handy boulder where it broke apart to reveal crystals and a radiating round white mass similar to the green ones shown here. At 8 or 9 years old, I was convinced I'd found a fossil mushroom and, to this day, I'm not sure what it really was, but it occupied a place of honour on my dresser alongside a geode, a 3-inch quartz crystal and a 1-inch diameter garnet as well as other less intriguing specimens. As an adult, I kept a mineral collection in various display cases, and I dabbled in cutting cabochons from agate and other semi-precious stones. During a dayhike, I happened to find this conglomerate lump in an unlikely location and rather than forget to pick it up on the way back, I tucked it into my pack and carted it around all day. On arriving home, I tried to identify the green mineral but was unable to determine much about it other than its hardness (near 7 on the Mohs scale). Years later, I asked a visiting geologist for his expert opinion. He was reluctant to commit without testing, so I allowed him to chip off a bit to take back to his lab. A few weeks later, the diagnosis came in: diaspore, an aluminum hydroxide oxide which occurs only very rarely in gemstone quality (obviously, this specimen is not gem grade). I was very grateful to finally have a label to put on it, and it remains one of the best finds of my rockhounding career.

A Mystery Solved

Day 334: The human being is by nature a collector, transporting objects from one location to another, there to confound archaeologists ten thousand years down the line. A trove of seashells known only to occur in Polynesia will surface in the center of what is now Kansas someday, and bones from foxes and wolverines may find their way to New Zealand or an East Indian shore. We pick things up and take them home because they're pretty or unusual, or sometimes simply because they appeal to us in some mysterious way. Thus it was that this two-pound hunk of rock managed to creep inside my backpack thirty years ago, and thence to a shelf in my home where it remained a mystery. It was unusual in its original location (perhaps relocated there by another collecting creature like me), a unique find no rockhound could pass by. Thirty years its composition remained a mystery until I met USGS geologist Tom Sisson, who offered to put a sample through testing in order to assuage my curiosity.

Diaspore falls among the minerals referred to generically as bauxite. I quote the authority: "It's diaspore: AlO(OH). I analyzed for Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, Cl, F, S, Sr, and Ba. All I came up with were Al and a little F. The analytical totals were quite low and the mineral damages under the electron beam, which is typical of hydroxides that form at low temperature (we can't analyze for O or H easily). Since the only cation of abundance is Al (excluding possible H), it must be gibbsite (Al(OH)3), diaspore (AlO(OH)), or boehmite (also AlO(OH) but different structure). It's too hard and the habit is wrong for gibbsite or boehmite, but both are right for diaspore."

Believe it or not, I have a couple of people waiting in the wings to hear the result of this analysis. The rock has been returned to my bedroom windowsill where now when my eye falls upon it, I can say, "Diaspore!" and feel richer for knowing.