5 August 2011

Purple everywhere! Samples from the Harding Pegmatite

Posted by Jessica Ball

All right, I’m finally getting to this post (cross-country driving trips aren’t good for keeping up with posting, apparently). My last post about the Harding Pegmatite Mine near Dixon, New Mexico had some lovely photos of the mine, but not so many of the rocks and minerals close-up. The samples I have are a little far from home (seeing as I’ve dragged them to the East coast from New Mexico), but they’re still as impressive as they were at the mine!

First, my favorite sample: one with beryl, potassium feldspar, and apatite all in one.

Apatite (blue-gray), potassium feldspar (pink), and beryl (white), with a little sprinkling of muscovite.

This is the first time I’ve seen beryl (beryllium aluminum silicate) in-situ, and it was hard to pick out at the mine, because there’s so much milky quartz lying around. The beryl here is massive rather than forming distinct crystals, and the best way to tell it apart from the quartz is by the lustre (how light reflects off the mineral): this beryl has more of a waxy or greasy lustre, while the quartz tends to be more vitreous (glassy). The apatite is a beautiful dark blue-gray here; no nice crystals, but a pretty striking color.

Apatite, beryl, and a little k-spar

One of the most striking things about the mine was the abundance of colorful micas – namely purple lepidolite and the more pinkish “rose muscovite”. It was sometimes hard for me to tell the difference between the two – I think they’re both lithium-rich, although the rose muscovite seems to have smaller crystals and form crusts here, while the lepidolite appears to come in larger mica “books”.

Lepidolite (darker purple) on the left and “rose muscovite” (the pinkish purple) on the right, with matrices of beryl, smokey quartz and some apatite. (I think!)

Rose muscovite?

Larger crystal sheets, but pinker – maybe the rose mica?

I’m pretty sure this is lepidolite

There’s more than just pegmatite at the mine, however – the country rock is pretty neat too. It’s metamorphic, with (I believe) mica schist below the pegmatite and amphibolite above. Both rock types are abundant in the tailings, but I mainly picked up amphibolite. Here’s a neat sample with little bits of biotite mica:

Amphibolite with biotite flakes

Naturally, when I mentioned that mica shows up in cosmetics, it precipitated a round of sparkly vampire jokes.

Sparkles!

This sample looked a little more schist-y to me, but came from an amphibolite-rich tailings pile.

Boudinaged (?) quartz layers

Finally, if you hike around the back of the mine works, you can find a lovely little pit full of calcite. Some of it is close to “optical grade”, which means it’s clear enough to use in optical instruments. It’s hard to find the clear kind, though (according to the caretaker it was depleted by the younger mine visitors, who find it more appealing than the other minerals).

Not-quite-optical-grade calcite crystals

This one is almost “optical grade”, and you can see some double refraction (birefringence), which causes a double image of fractures and imperfections within the crystal.

More subtle birefringence

There were (of course) lots of other minerals there, but I had to be selective when packing my rock samples (gas mileage is important when you’re driving 2000 miles in a trip). Since it was the first time I’d seen many of these outside of a museum or mineralogy lab, I’m glad I got to pick up samples of these!


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