This picture is a manganese X-ray map--an image showing where the metal manganese is concentrated in a sample of rock. An electron microprobe beam was aimed at the rock, and sensors measured the X-rays that bounced back. Where lots of manganese X-rays bounced back, the pixels in the map are colored bright white; where there's some manganese but not a whole lot, the pixels show up in shades of gray. Black pixels indicate no measurable amounts of manganese. The microprobe had to run all night long to map this one little rock sample, which measures just over 1 square centimeter.
Who cares about manganese? Geologists studying metamorphic regions of the world--places where rocks were once buried 10 miles deep or even more. Why study stuff like that? Because it's really fun, and because metamorphic history tells a story about how the planet got to be the way it's gotten to be.
Manganese is an elegant element to map because under conditions of high temperature and pressure, it always wants to make a garnet, and garnets are among the most useful minerals to study, very sensitive to changes in temperature and pressure. The areas of this map that show presence of some manganese--that are gray or white--are in fact garnet. The areas that are black are minerals other than garnet. What look like two black eyes near the middle of the garnet are inclusions of other minerals within the garnet. What looks like a very bright white "nose" in the middle--well, that's the exception that proves the rule, a grain of a fairly rare mineral called manganiferous ilmenite. Ilmenite usually contains titanium and iron, but in this grain, much of the iron has been replaced by manganese. If you want to know why, you'll have to speak up; this message is already way too long.
Metamorphic garnets are typically very high in manganese right in the middle, at their cores, where they first start to grow. As they grow larger and larger, they require less and less manganese. Normally, you'd find the least manganese at the outer edge of the garnet--the garnet rim on a map like this should be dark gray, almost black. But the rim of this garnet is bright white--chock full of manganese. Also, it's funny-shaped, kind of fractal, an intricate crust of little garnets embedded around the edges of a big ol' lumpy garnet in the middle.
This garnet is not normal. It doesn't have the usual kind of metamorphic history. It started out normal, with higher manganese in the middle gradually decreasing as it grew. But I think it kept on growing and growing far beyond the size we see here--it became a huge garnet, a normal-looking one, with the usual pattern of manganese decreasing from core to rim, till there was just a tiny bit left at the rim.
Then the garnet's world started to change. It was pushed and shoved part of the way closer to the surface, and/or the mountains sitting on top of it partly eroded away. At lower temperature and pressure, garnet was no longer chemically stable. Garnet likes it hot. It doesn't like room temperature, as a matter of fact, that's much too cold--but it takes a lot of energy to un-make a garnet once it's made, so at room temperature, garnet stays garnet. I believe that the garnet in this X-ray map spent much of its "life" enduring pressure-temperature conditions not quite hot enough for it to "want" to remain a garnet, but still pretty hot--hot enough tto motivate the garnet to gradually become something else. The iron, magnesium, calcium, and aluminum near the rim of the crystal all got sucked out to make different minerals, minerals such as chlorite and mica that are more stable at slightly cooler temperatures than garnet. The manganese got left behind, and it backed up just inside the garnet rim, because other minerals don't like manganese and manganese doesn't like other minerals. It's loyal to garnet, unlike iron and all those other elements. As the garnet shrank, the manganese level got higher and higher, reaching levels that show up bright white on the map.
Then the garnet's world changed again. Temperature rose. Pressure increased. Conditions became once again favorable for garnet growth--and there was all that huge store of manganese right near the rim, just itching to make more garnet. There was so much manganese available in one convenient location that the new garnet grew in a flash, nucleating hundreds of new baby garnet grains on top of each other all over the rim of the old core garnet.
If you can make up stories like this from manganese X-ray maps, they'll give you an advanced degree in geological science. Truth.
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