When Minerals Click "Save"

When Minerals Click "Save"

How garnets act as snapshots of changing conditions in metamorphic environments.

Have you ever missed the peak of a sunset and walked out of your house right at the tail end? You have an idea that something really cool was displayed up in the sky, but there is no real way to know how bright, how pink, or how awesome it was? We have all experienced this or a similar time where the scene/person/information you wanted is not available to you because you weren't there. For many earth processes, geologists don't have direct ways to peer into the system. We can't walk into a volcano chamber to see how fast the hot rock flows in, or make your to the core of the planet to grab a piece and see what its made of (despite what Journey to the Center of the Earth would have you believe)!

Sunset mid-changing colors. The garnet picture below is from this island!

But imagine if you had a camera- and you could put it anywhere. And that camera had the ability to record lots of information. You would then have more insight into that place at that time. And if you put a bunch of those pictures and informational records together, you could piece together a story! In our sunset analogy, you would see how the colors changed or how the temperature varied. Well the mineral garnet (the pretty, pinkish-red birthstone for January), acts just like that recorder, a little spy in the heart of some geologic settings. Why are garnets so special, you may ask? Garnet is found in many metamorphic rocks, rocks that got really hot or were put under pressure and changed in response. And because these garnets are found all over the world, comparing conditions on a global scale becomes possible. The mineral structure (or how the mineral is put together on the tiny atomic scale) is particularly good in acting as a "safehouse" for rare elements which contain key information to what was going on when the garnet was forming. 

Garnet grain with different amounts of an element.

The breaking up of the grain also share insight into

how beat up the rock got.

Take a look at the picture to the right. This is a how a garnet looks up close. There is a lot this garnet is trying to tell you! First, focus on the colors. This is a sort of map, where the warmer the color, the more of a certain element there is in that region. These maps can be made for many different elements, and when that information is put together, you can tell the chemical history of the rock as the mineral grew. Next, the shape. You can see that it has a defined shape, possibly squarish or hexagonal. Because the edges aren't eaten away, geologists can say that the mineral was able to grow in its normal pattern, without any space restriction. Another aspect is how broken up the grain is. It almost looks as though you took a small hammer to it. Being able to see how the mineral has been broken and deformed is another key piece of information about what the rock had to go through from the time it was made to the time it ended up on the surface where we could grab it. When scientists can piece all that information together, they are presented with a clear view into the story of formation for that rock, and on a bigger scale, the overall region. Having a team of historians in the rock that are good at telling stories is very helpful when you're attempting to piece together the past!