There's gold in them thar backyards.

May 21, 2010 3:05 PM

Is there a name for the flaky, layered clay/soil deposit with a striking metallic sheen found in my childhood neighborhood in Northern Virginia?

In the neighborhood I grew up in in Burke, Virginia, which is in Fairfax County, there was an interesting clay/soil in our yards (as compared to the less interesting red-brown clay also found there). As kids we called it "fool's gold," because of its color and sheen, although obviously it was nothing like iron pyrite.

It was easy to dig up in dry, solid chunks. It could be broken into layers, like shale, so we could divide the chunks into sheets, but it was nowhere near as hard as shale. It was soft enough that we could use fist-sized chunks of it like chalk on the sidewalks, yet was firm enough that we could draw with it without it breaking much, though it was softer than chalk.

Because of the layers, I assume it was sedimentary. Some googling turned up "mudstone" - does that sound like what it was? I've long wondered if it had a particular name, if there was more to know about how it was formed, and where it is geographically distributed.

posted by jocelmeow to science & nature (12 answers total)

Mica?
posted by emyd at 3:09 PM on May 21, 2010

I also think it sounds like mica. Here's a better picture of it. At a place I lived in southern VA (piedmont) when I was a kid, we had red clay mixed with this stuff. (Never tried drawing with it.)
posted by nangar at 3:28 PM on May 21, 2010

Definitely mica.
posted by whimsicalnymph at 3:32 PM on May 21, 2010

miiiiiicaaaaaa! virginia represent!
posted by soma lkzx at 3:36 PM on May 21, 2010

Hmmm. Neither of those pictures of mica look very much like it, but I'll bet that when they go to take a picture of mica, they find the absolute best example of it, right? Is it possible that what I had had more not-mica mixed in with it?
posted by jocelmeow at 3:38 PM on May 21, 2010

Try looking at micaceous schist. That name came up when I looked at a Fairfax County soil map (pdf) Also check the Fairfax soil descriptions here.
posted by stefanie at 3:42 PM on May 21, 2010 [1 favorite]

I had always though schist was really hard and not splittable. The mica I'm thinking of had much smaller bits than the pictures linked upthread, and was really easy to find around baseball fields. This is information from the 10-year-old version of myself, though, so not necessarily trustable.
posted by soma lkzx at 3:59 PM on May 21, 2010

You probably played with sedimentary soil mixes that contained mica. The rocks in the North Carolina Appalachians look just as you've described. Layers of dirt and shiny stuff. Easy to break. The shiny stuff is definitely mica. The other parts of the soil / rocks can a whole host of other minerals.
posted by whimsicalnymph at 4:06 PM on May 21, 2010

Stefanie and whimsicalnymph, I think you've got something there. The structure and shape of this looks like it, though the color is very different...mine was a more uniform golden color. I suppose mixing with reddish clay could end up that color.

Stefanie, good find on that soil map and Fairfax soil description. The soil description page, which repeatedly mentions "flakes of mica" states that the sample was taken south of Braddock Road and east of Route 123, which is almost exactly two miles north of where I lived.
posted by jocelmeow at 4:18 PM on May 21, 2010

Another Virginian here chiming in to say that it sounds like mica. When I was little my parents had a basement dug under our house and the dirt that was dug out had lots of mica in it. My sister and I would find pieces and peel the layers off.
posted by cropshy at 6:21 PM on May 21, 2010

Thanks, everyone! "Mica & its sedimentary friends" sounds like a satisfactory answer. I'm going to read more about mica and would be interested to know more about where deposits of it are found, if anyone knows that off the top of his head.
posted by jocelmeow at 9:47 AM on May 22, 2010

Oh, just for fun, I also wanted to mention something that occurred to me last night. My neighborhood friends and I also drew and colored with palm-sized chunks of bark mulch out of our yards' planting beds. This was simply before there those big sticks of sidewalk chalk were available. An artist interested in a sentimental sort of project could surely do something interesting with the brown and gold values of bark and sedimentary soil containing mica.
posted by jocelmeow at 9:57 AM on May 22, 2010