Articles from Lapidary Digest
Streaking Minerals - Streak Testing!
Dr. Bill Cordua
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Streak tests are easy tests, helpful in mineral identification. The streak
is simply the color of the powdered mineral. It doesn't matter how the
mineral is powdered - you can scrape off some with a nail or pound the
mineral to bits with a hammer. More commonly, mineralogists use a streak
plate, a piece of unglazed porcelain usually cut in a square or hexagon
a few inches across. Streak plates have a hardness of about 6.5, so if
you want to test the streak of anything harder, get out the hammer! They
can be bought from most mineral supply houses. For example, the latest
Ward's Natural Science Establishment catalog lists them at 10 for $2.90.
When they get dirty they can be cleaned by scrubbing them off with an old
toothbrush. I often use some sand with the water to scour off resistant
streaks. If they get too dirty - heck, toss them out - they cost less than
30 cents each. When I was a kid, I used the back of old bathroom tiles
to make an even cheaper streak plate.
Why do a streak test instead of just looking at the color of the bulk mineral?
The color of a larger chunks of mineral can really vary, depending on what
trace element impurities may be present. Calcite, for example, can be any
color of the rainbow ( and a few that aren't on any rainbow). But calcite
always has a white streak. So why don't the impurities color the streak?
They do, but only to a slight extent. This is because light going through
a small grain of a mineral has less chance to interact with the impurities
than light going through a big chunk of the material. Powdering the material
thus minimizes the effect of the impurities.
Streaks are most useful in the oxides and sulfides. Silicates and carbonates
generally have white or light colored streaks. The oxides are fun to streak.
Hematite's red streak is distinct from goethite's yellow-brown streak and
pyrolusite's coal black streak. Sphalerite is another mineral that can
be lots of colors, but gives a yellow streak.
The streak of rocks is generally not distinctive. They usually give a light
streak that reflects their dominant silicate or carbonate composition.
If they give a red or brown streak, it suggests the presence of iron oxides.
Of course, if the rock is coarse grained, you can try the streak test on
the individual mineral grains.
Mineral databases and texts sometimes list the streak colors and some times
don't. It depends on the tastes of the author and the data available. All
minerals have streaks (you can powder anything if you put your mind to
it) but they may not be too distinctive (hundreds of minerals have white
streaks). I think that when a new mineral is described, the streak should
always be included. After all, the material had to be powdered in order
to do its microprobe or x-ray analysis, so all some one needs to do is
remember to record the color. That would be a real help to those of us
who don't have well-equipped analytical labs in our basements.