Colorless cassiterite from Bolivia

Is there such a thing? I have a specimen out of a micromineral collection I acquired labeled as "colorless cassiterite, Potosi, Bolivia".



Also on the specimen are these off-white or tan needles. What might they be?



Thanks!

Steve Stuart

Steve, yes, there certainly is such a thing as colourless cassiterite and, since pure SnO₂ should be colourless, it's surprising that colourless cassiterite doesn't occur in nature more often. I suspect that your piece is from LLallagua, and that the needles are tourmaline.

Steve,

Firstly all I can say is that I would love to acquire such a piece.
As Alfredo states it is surprising that more is not found. I suggest that the answer to that is that it is simply misidentified. It is not expected to be clear. May even be thought of by miners, at first glance, as valueless Quartz. Some of the recent Chinese material is very transparent - though I have not seen colourless - perhaps some will show up. It doesn't make it more valuable per se just very unusual.

Cheers



Edited 1 time(s). Last edit at 02/20/2012 11:54AM by Keith Compton.

Steve,
I live in Cornwall, with tin mines everywhere all around me, and cassiterite is fairly common.
I can confirm that colourless cassiterite does exist, though not as frequently found as the brown stuff.

All the best
Eric

United Kingdom, Cornwall

Thanks for the responses. Just to be clear, my images have a 2 mm FOV, so these are micros! Llallagua sounds very plausible after reading the MR article in Vol. 37(2).

Steve

Does anyone know what usually makes cassiterite brown/black? Lead, iron, ...?

Fe³⁺, I think. Hydrothermal vein cassiterite usually contains iron, sometimes a little indium too. Pegmatite cassiterite, on the other hand, often contains a little Nb+Ta.

Here is another possible cassiterite on the same small micro-specimen. FOV is estimated at 1 mm; this is a crop of a 2 mm FOV image. Can anyone confirm visually that it is cassiterite?



Thanks!

Steve

Looks tetragonal, so yes. I've often seen this shape before in black cassiterites, but not yet in a colorless one. (But then keep in mind that colorless cassiterite in general is far far less common than black ones.) Nice find.

Yes, that was my impression. I did orient the crystal to look down the c-axis and the cross-section look square to me. I'll post the photos on Mindat.

You are all going to suffer a colorless cassiterite story from me.

Back in the late 1950s a 25 pound pure lump of cassiterite was brought in to the U.S. Bureau of Mines Seattle Office.

It was found by a road grater operator near the Monroe, Washington city dump. The noise of the encounter was loud enough that the grator operator got out of his cab and pulled the specimen out of the road bed, and he delivered to the USBM where the late Elwin Magill identified it as cassiterite.

Since tin was a strategic metal at that time, he initiated a very detailed prospecting program to find its source. Careful and exhaustive stream sediment prospecting was conducted all up and down the nearby drainages. Nothing but a trace of tin in a roadcut near Skyomish was ever found.

The source of that boulder remained a mystery until around 1990 when Larry Jeffers, owner of a mineral shop in Seattle's Pioneer Square named Semantics Underground showed me a flat of cassiterites from Tin City, Alaska.

BINGO ! That was the stuff ! Light brown cassiterites grading to colorless exactly like the piece that Magill had saved on his dusty shelf.

The theory was that 60 years ago a miner returned to Monroe from Alaska had tossed out a lump of cassiterite into the Monroe city dump.

And thus another example of the scourge of field collectors was born. Specimen dumping hundreds of miles from their true source.

Just like back in 1965 when my little brother found an amazonite cluster on top of the dump of the Ibex Mine in Leadville, Colorado.

Pack dumping. It's a problem. But haven't we all done it?

p.s. I sell that colorless cassiterite as a electron probe reference material to this day. Better than synthetic tin oxide.

~Bart