Minerals and Fire Trucks: A Lesson about Color and Identification a non-sanctioned production of Robert M. Reed Jackson School of Geosciences The University of Texas at Austin version 0.2

Color and Mineral ID In identifying rocks and minerals, color can be an important factor. It should not; however, be the only factor. I've always had trouble getting students, or anyone else for that matter, to understand this key point. So as a way of explaining it, let's talk fire trucks. Yes, fire trucks. When somebody asks you how to identify a fire truck, I'm guessing that one of the first things that you come up with is red color. It's a good starting point. But there's a whole lot more to it than that.

It’s a truck. It’s red. But it’s not a FIRE truck. More important than the red color are features like hoses, ladders, sirens, flashing lights, axes, and guys (or gals) in rubber boots and strange hats. A dalmatian can be a pretty good clue as well. And if you find a red truck in a fire house, that's a dead give-away. This truck lacks all those features, so it's not a fire truck

Red truck again. But still a long, long way from a FIRE truck. It could be a municipal vehicle, and it could have guys (or gals) in boots and strange hats. But still no hoses or ladders, and the load of trash is highly uncharacteristic of a fire truck. And if a truck like this has got a dalmatian, well, it's liable to be a former dalmatian.

Here we go! I realize that to anyone even vaguely exposed to modern society, this is obviously a fire truck. You don't think about it, you just look at it and know. However, a whole series of visual clues (or properties) are being processed by the brain to make this determination. Even though you probably don't even realize that you are processing any clues, you are doing it on a subconscious level. It’s red and it’s a truck But it has also got: hoses sirens ladders a nice sign on the door that says "Fire Dept." All in all, that's extremely diagnostic. Even the lack of a dalmatian isn't troubling, this would be the elusive fire truck.

Still a fire truck? Of course. It still has got: hoses sirens ladders a nice sign on the door that says "Fire Dept." You can still do a heck of a job putting out fires with it. So, fire truck ID is about the features. and...wait...the same is true for minerals? yeah, we’ll get there in a minute

Still a fire truck? Despite the fact that the fire men might not want to ride in it? Of course. Color is not diagnostic for fire trucks. (or for minerals either) Let’s rephrase that a bit, color can be an important tool for identifying things, but it's not conclusive. Check the other properties, they have to be right.

To apply this idea to rocks and minerals, let's take the example of potassium (K) feldspars (one of the necessary ingredients in a granite). K-feldspars typically range in color from pink to light orange to almost red When you see a mineral that is these colors, K-feldspar is a good first suspect. In fact, all of the large pink or orange minerals above are K-feldspars.

However, there are a few pink-orange-red minerals out there that aren't K-feldspar. Both calcite and quartz can be pink in some rare cases. [Yes, it would help if I had pictures of a few of these, I'm working on it.] The pink mineral is rhodochrosite a type of manganese carbonate (I “borrowed” this image from the Texas Memorial Museum)

This mineral has a rather feldspar-like pink color. The pink mineral is talc, which is not typically pink. However, this mineral forms thin platy crystals, has a greasy feel, and is very soft; all properties which means its not feldspar.

There are even times when K-feldspar is white or pale green. It's the plain yogurt, 4- door sedan, Wonder Bread of feldspars, but it's still a K-feldspar. Yes, that's a feldspar. That’s also a K-feldspar, a green variety known as amazonite. (“borrowed” from the Texas Memorial Museum again)

Actually, the "color" problem in mineral identification is worse for black or white minerals (much more of them around) but there wasn't much I could do with white trucks. The mineral equivalents of hoses, sirens, dalmations, etc., are properties like crystal shape, hardness, luster, streak, magnetism, and so forth. [In the feldspar above, the lighter white streaks are a characteristic known as exsolution lamellae which make me confident this is a K-feldspar and not some other variety of feldspar.] Although finding a pink mineral in a granite is pretty much the equivalent of finding a red truck in a fire house. So, to repeat the main point, in mineral identification as well as trucks, color is a good starting point, but should be backed up by careful identification of other properties.