Monday Mineral: Labradorite

Somehow, I just keep coming back to this mineral (see here and here, in the "fixed composition" paragraph). No apologies though; it's an important one. And in case you didn't click on the second of the above links, I'll remind you that it's a plagioclase feldspar, sodium-calcium aluminosilicate, with 50-70% calcium and 30-50% sodium. In simpler terms, it's the calcium-richer side of the middle of that series. Overlooking the glare from the flash, this sort of non-descript dark color is typical for coarse labradorite.In porphyritic volcanic rocks (those with a generation of larger crystals from slow cooling in a magma chamber , and one of very small crystals from the ultimate eruption and subsequent quick cooling), it often appears lighter in color. As should be clear from the above picture, this particular sample has been polished. It was a gift from my friend Cassidy, so I don't know where it's from.Another picture showing the somewhat dull appearance of this mineral. But wait! what are those little sparkly bits? Where are those colors coming from? Time for some wonkishness...
I lifted this diagram from some astronomy lecture notes, of all places; if this is infringement let me know, and I will politely remove them. What this diagram tells you, basically, is that feldspars form with two broad swaths of composition: sodium-potassium rich (alkali feldspars), and sodium-calcium rich (plagioclase feldspars). Na and K trade off because they have similar charges. Na and Ca trade off because they have similar ionic radii. They do not have the same charge, and this is compensated by Na taking one more Si (+4) and one less Al (+3), while calcium does the reverse. Remember, labradorite would occupy a fifth of the bottom swath, from the middle, near the "o" in "Plagioclase" to the right, to maybe the divide between the "s" and the "e."

The above is schematic, but basically approximates what a minerologist would call the "hot" feldspar curve. I have modified it with blatant disregard for hard data below, but the general principles, I think, are accurate. If the feldspar cools slowly (note we are dealing with a fairly large crystal here; this sample cooled very slowly underground), some of the mid-range compositions are not stable; an initially crystallizing feldspar at the composition of the "o" will "exsolve." Some regions will become Na-enriched, and other regions Ca enriched. So instead of a single continuous composition of "o," it ends up with alternating regions of composition "Pl" and regions of composition "se." And if those regions are in a size range approximating the wavelength of visible light, there are some amazing optical effects...
Note that this is not the intrinsic color of the mineral; it's an effect of scattering and interference from zones of slightly different compositions. It's like the "rainbow" effect of an oil slick on water. In the latter case, the layer of oil is thin enough to produce the same optical interference and resultant colors: the oil isn't actually "rainbow-colored." You can only see these colors at particular angles to the crystal lattice, which is why I had to catch this flare at an odd angle, and so much is out of focus. But it really is quite beautiful. This stone is often used in sculptures, and as facade and decorative stone in buildings- I've noticed it's used quite often for jewlery stores.

I should also make it clear that this effect is quite different from the brillant colors that sunstones can take on. In the case of Oregon's sunstones, the colors apparently arise from microscopic inclusions of metallic copper, but that may not hold true for all sunstones. The sunstones are phenocrysts in volcanic rock; they have initially formed pretty slowly, but their final cooling was quite rapid by geologic standards. They would not have undergone the gradual cooling necessary to get the alternating lamellae of varying compsitions.