John Carlos BaezApr 10

In the town of Quartzsite I picked up a beautiful chunk of labradorite. This mineral creates an eerie blue shimmer in the sunlight - a phenomenon called 'labradorescence'. Reading up on it, I discovered it's a form of feldspar. 60% of the Earth's crust is feldspar, and I know so little about this stuff!

Turns out there are 3 fundamental kinds of feldspar:

• orthoclase is potassium aluminosilicate
• albite is sodium aluminosilicate
• anorthite is calcium aluminosilicate

Then there are lots of feldspars that contain different amounts of potassium, sodium and calcium. We get a triangle of feldspars with orthoclase, albite and anorthite at the corners.

But not all points in this triangle are possible! There's a big region called the 'miscibility gap', where as you cool the molten mix it separates out!

And there are also subtler problems. When you cool down the feldspar called labradorite, it separates out a little, forming tiny layers of two different kinds of stuff. When the thickness of these layers is the wavelength of visible light, you get a weird optical effect: labradorescence!

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Show threadJohn Carlos BaezApr 10

Below is a photo of labradorite - not mine. You really need a movie to see the strange shimmer as you turn a piece of labradorite in the sunlight.

In fact there are 3 kinds of feldspar that separate out slightly as they cool and harden, forming thin alternating layers of two substances:

• The 'peristerite gap' produces layers in feldspars with 2-16% anorthite and the rest albite - these layers create the beauty of moonstone!

• The 'Bøggild gap' produces layers in feldspars with 47-58% anorthite and the rest albite - these are labradorites!

• The 'Huttenlocher gap' produces layers in feldspars with 67-90% anorthite and the rest albite - these are called bytownites.

The physics and math of all this stuff is fascinating.

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Show threadJohn Carlos BaezApr 10

What I'd like to understand better:

Feldspar crystals are complicated structures built from tetrahedra, and aluminum and silicon have to be distributed among these tetrahedra. This distribution is determined by the relative amounts of potassium, sodium and calcium, and it in turn controls the symmetry of the crystal, which can be either 'monoclinic' or the less symmetrical 'triclinic'.

There's a whole body of work - by Salje, Carpenter, and others - applying Landau's theory of symmetry-breaking phase transitions to map out the space of different possible feldspar crystals!

(3/n)

Show threadThorium
@johncarlosbaez You should check out zeolites, feldspathoids, and melanophlogite https://www.mindat.org/min-2630.html.
Melanophlogite

First recognized natural clathrasil (or silica clathrate); compare bosoite and chibaite - the other natural clathrasils.

Apr 10 at 5:48pmWeb
Show threadJohn Carlos BaezApr 10
@Thorium - I once did a shallow dive into the incredible diversity of zeolites. There are some nice online galleries of their crystal structures. I know nothing about the other two.