All that glitters …

At the end of my last post on granular segregation and placer mineral
deposits, I included a photo of a sand sample from Yosemite that is reputed to
contain flakes of gold, with a slightly enigmatic candidate grain. Lockwood at
Outside the Interzone and
the Lost Geologist kindly
commented with testing advice - press the grain with a needle and, because gold
is relatively soft and malleable, it will deform, the needle leaving an imprint;
mica, being flaky and brittle, will split apart. I couldn’t find the same grain,
but found a similar one, and gave it a poke with a needle. The point left a dent

• aha, I thought, my hopes rising. But, on applying further pressure, the grain
  fragmented, flying apart into tinier flakes. It was muscovite, a mica, coloured
  in this case yellow, and slightly rotten from the processes of weathering -
  hence the dent, and its deceptively lustrous appearance. I should have known -
  these are reallyyoung sand grains, the recent sawdust of the crumbling
  Sierran granites, and the mica minerals (including an almost perfectly hexagonal
  biotite grain, left) form a significant membership of this granular tribe -
  they’ll rot out of existence as time goes by.

So, all that glitters is not gold. Of course, pyrite, the shiny yellow iron
sulphide mineral, is the traditional “fool’s gold” (left in the photo above) but
I’m not that foolish: if I found something that looked like the placer
gold shown on the left, below, I like to think that I might have leapt to the correct
conclusion. But the little muscovite grains (above, right) do sparkle in a most
alluring way - I came across the second photo, below right, which purports to show
placer gold nuggets and, while I’m no mineralogist, they look a bit suspicious.
But then most prospectors know a lot more about what they’re doing than I
do.

My wife would agree. When we got married, she found the old picture below and
felt that, with a geologist for a husband, this would be illustrative of the way
her life would evolve (note the contents of the woman’s bag). We still,
ever-hopefully, have this picture on the wall, but alas, so far … SIGNATURE

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2009/06/all-that-glitters-.html