Thursday, June 30, 2011

It is mostly what holds you up

kw: rocks, mineralogy, gemstones

My screen saver is a picture gallery program, and at present it is pointed at images of mineral crystals. Some I took, mostly at museums but a few of my own specimens. Others were gathered from all over the internet. I have the timer set to three minutes, so the pictures frequently start up when I'm getting a little think time at the keyboard.

After lunch today, seeing the minerals, I got thinking about feldspar. Most people seldom or never heard the word, yet feldspar minerals are the most common on Earth. Maybe you know that most tombstones that aren't made of marble are of granite, either whitish or reddish. The light-colored mineral that makes up most of the composition of granite is feldspar. In granite, the white or pink feldspar is usually a type of orthoclase.

Feldspar is a kind of junkyard mineral, because it has such a variable composition, variable chemistry. There are two "solid solution series" that make up the main feldspar family of minerals. Sodium feldspar, or Albite, is one end member of both series. The composition of pure albite would be NaAlSi3O8. The other end member of the alkali feldspar series is Orthoclase or potassium feldspar. The composition of pure orthoclase would be KAlSi3O8. In this series, most orthoclase contains a little sodium, and most albite contains a little potassium, but mixes near the middle are very rare. More common is orthoclase in which a tiny bit of iron substitutes for aluminum, making it pinkish.

A whitish or yellowish orthoclase that cleaves exceptionally well is also called microcline, so named because its two cleavages are very nearly at 90°, just a tiny angle (a micro-cline) shy of a perfect right angle. If the microcline contains lead instead of iron, it can be a lovely turquoise color, and is called Amazonite:

I have sometimes wondered why doesn't adopt this mineral as a mascot (Image from Wikimedia Commons). Amazonite is sometimes polished into cabochon gemstones, though it is a bit fragile. Also, its hardness is equal to that of glass, so it scratches rather easily, because sand and silt contain quartz which is one step harder (7 versus 6).

Microcline, usually of the white variety, tends to crystallize in large crystals. A very coarse-grained granitic rock called pegmatite is often mostly microcline, with the other nice mineral specimens in between. The largest known crystal on Earth, a microcline crystal in a miles-long pegmatite in Russia, has been mined for road-grading gravel for decades – that one crystal! It isn't in pretty condition, and wasn't recognized as a single crystal until about thirty years ago.

There is one other feldspar stone that is sometimes made into gems, Labradorite. It is a member of the Plagioclase feldspar series. One end member is albite, and the other is Anorthite, or calcium feldspar. The composition of pure anorthite would be CaAl2Si2O8. This series mixes readily enough that there are several mineral names for intermediate compositions. Labradorite is right in the middle, 30-50% albite and 50-70% anorthite. By the way, that ancient moon rock that was dated near 4.5 billion years old was nearly pure anorthite. A labradorite that is 60% anorthite would have the chemical composition Ca0.6Na0.4Al1.8Si2.4O8.

This labradorite specimen has been polished. It reflects light with this bluish hue in a way similar to opal, because of a series of tiny offsets in the crystal structure that cause constructive interference for blue light. (Image from Wikimedia Commons.)

Labradorite is sometimes called Moonstone, but there are other minerals also called moonstone when polished. It is as fragile as polished microcline, but with care it makes a lovely gem. I have a tie pin with a labradorite cabochon, about 12x16mm. It was polished with the "blue angle" set a little upwards so it glows when the light comes from above. At other angles it looks black.

Most of the time, our contact with feldspar is by walking on it, almost any time there is a gravel driveway or walkway. These special varieties show a pretty side of this most common mineral family that makes up 60% of the crust of the Earth.

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