Minerals and Fire Trucks: A Lesson about Color and Identification

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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
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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.

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Its red.
Its a truck.
But its 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
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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.
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Here we go!
Its red and its 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.
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.
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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, well get there in a minute
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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)
Lets 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.
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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.
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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)
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This mineral has a rather feldspar-like pink
color.
However, this mineral forms thin platy crystals,
has a greasy feel, and is very soft all
properties which means its not feldspar.
The pink mineral is talc, which is not typically
pink.
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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.
Thats also a K-feldspar, a green variety known
as amazonite. (borrowed from the Texas Memorial
Museum again)
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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.