September 14, 2012
So you’re trying to pick feldspars, eh?

Well I don’t know about you but that’s what I’ve been doing for the last few days. Sounds easy enough, but in practice it can be very hard telling one vitreous mineral from another, especially when the sample looks like this:

Are they quartz? Feldspars? If they are feldspars, which ones?! 

Those are the questions that you have to ask yourself when you are picking minerals for isotope analysis because if you pick the wrong one, you’re fucking screwed. You’ll have wasted a lot of time and effort, and not to mention everyone will think you’re retarded. 

For what I’m doing (Rb-Sr dating) you need a mineral with a high Rb:Sr ratio, which in our case is muscovite, and a mineral which has a low Rb:Sr ratio. For most of my samples, this is plagioclase feldspar. 

The problem is, that a lot of my samples have whats called ‘smokey’ quartz - ie, quartz which is not clear and looks cloudy, very similar to the colour of the feldspars I’m looking for…

The best way to tell the difference between quartz and feldspar is a trick I learned in my first year, and it’ll all to do with cleavage - the way a rock reflects light when rotated, due to the arrangement of atoms in it’s crystal lattice.

Quartz glistens, but feldspar flashes.

This quartz, if you were to rotate it under a light, would reflect the light at different times/angles. Thus showing it has no cleavage planes.

Whereas feldspar has two good cleavages which intersect at 90 degrees to each other. So if you were to rotate it, the light would reflect off the whole surface at the same time, creating a ‘flash’ of light across the plane.

Mineralogy: it’s easy when you know how.

March 30, 2012
Vug filled with feldspar and quartz crystals near Butte, Montana
submitted by CampBenCh

Vug filled with feldspar and quartz crystals near Butte, Montana

submitted by CampBenCh