I have wondered for some time whether this was muscovite or phlogopite; I more or less convinced myself this morning that it must be muscovite. Its source is a granite pegmatite near Magnetawan, Ontario (45.6651, -79.6440). I'm not going to post a Google Earth image because I don't remember exactly where the location was, but I believe it was north of the town.
When I looked up the mineral data, I found muscovite is a potassium mica (which I had remembered), and phlogopite is a magnesium-rich mica (which I had not remembered). Given that granite does not have much magnesium in it, chemically it would make sense for this to be muscovite. Furthermore, muscovite is slightly softer than phlogopite. I can scratch this with my fingernail (H=2.5), which puts it at the very lower end of the phlogopite hardness; again, it looks more likely to be muscovite. The conclusive test would be to see if it decomposes in sulfuric acid (phlogopite) or not (muscovite), but I don't have any of that kicking around my apartment.
The problem, as you can see from the two above pictures, is that this is pretty dark. Actually the two photos above are misleading: the sample is not really as dark as these make it look. But I think of muscovite as being silvery- silvery gray, and this sample is more of a smokey gray. Phlogopite is described as being a yellowish brown, and this sample always seemed closer to that than to what I consider a "normal" muscovite color. However, when you hold it up to light, it is quite translucent (this piece is a millimeter or two thick; it would be quite easy to use a razor and split this into 15 to 20 thinner sheets)
And in fact, this site is an old mine where the mica was the ore. We tend to think of "ore" as pertaining to metallic resources, but any rock material that can be extracted for a profit is technically an ore. Some of the "books" of mica here are 10 inches (25 cm) in diameter and a couple of inches (5 cm) thick. The mica was at one time used as a high-temperature "glass" for lamps and wood stoves, or other situations where one wanted or needed to be able to see the flame or light, but still wanted the heat source enclosed. There was a lamp in my grandparents' front stairwell which, though it had been converted to electric, still had the original
muscovy-glass panes.
The problem, as you can see from the two above pictures, is that this is pretty dark. Actually the two photos above are misleading: the sample is not really as dark as these make it look. But I think of muscovite as being silvery- silvery gray, and this sample is more of a smokey gray. Phlogopite is described as being a yellowish brown, and this sample always seemed closer to that than to what I consider a "normal" muscovite color. However, when you hold it up to light, it is quite translucent (this piece is a millimeter or two thick; it would be quite easy to use a razor and split this into 15 to 20 thinner sheets)
From the appearance alone my first thought would have been biotite. Phlogopite is the Mg end-member of the biotite solid solution series and is more commonly confused for muscovite because it isn't as dark in color as the Fe end-member, annite. Is there any reason you ruled out a more Fe-rich biotite? These, after all, are quite common in granitic rocks.
ReplyDeleteWe tend to think of "ore" as pertaining to metallic resources, but any rock material that can be extracted for a profit is technically an ore.
ReplyDeleteI didn't know that. Thank you for that info. Seriously.
Ron- as I mentioned in the post, because of the flash (I think) the first two pictures make this look *much* darker than it actually is. The third picture also shows how translucent it is. Biotite can pass some light, but not like that. If I peel off a thin sheet, it's almost transparent. The texture is wrong for biotite, too.
ReplyDeleteDean- all in a day's work. ; )
It can't be a muscovite because then it would be wearing a thick fur hat and carrying shopping bags, trudging through the snow.
ReplyDelete