Identification of minerals is not easy at first. Just like people each mineral
has a unique set of characteristics that distinguish it from another mineral.
These characteristics that we can see are called 'physical properties'.
It requires some practice to see the subtleties of some of the properties. Part
of the reason we do this in the lab is to hone those senses and learn to discriminate
between subtle differences that may not be apparent to the untuned senses.
Some minerals are easily recognizable by a single property once it has been seen.
Others may look quite a bit alike and it may take more investigation to distinguish
between them.
But there will always be some differences between two different minerals, and the
same mineral will always have the same properties regardless of the outward appearance.
What makes the whole process difficult is that the outward appearance of a particular
mineral may not always be the same. Because of the great differences in the conditions
of formation, the same mineral may have many different appearances.
Mineral
Each mineral is a specific combination of atoms arranged in a specific way. Although
we can not see the atoms, the inner composition and structure of the mineral leads
to its unique set of physical (and chemical) properties.
The properties that are used to identify minerals are those that are dependent upon
that composition and structure of the atoms that comprise the mineral.
Although nearly all minerals contain varying amounts of impurities, the amounts are
generally too small to affect the overall physical properties of the individual specimen.
More advanced techniques can detect the small differences in properties caused by
different amounts of impurities or variations in composition, but these will not
affect the 'hand specimens' that we will examine in the lab.
Color
Color is the most noticeable and least reliable of all of the physical properties.
In some minerals the color is distinctive and immediately diagnostic because they
only come in one color.
Other minerals may come in a wide range of colors so that color is less of a diagnostic
tool.
It is easy to be fooled by color. Not everyone agrees on what to call a certain
color, and not everyone perceived color in the same way.
Your 'orange' may be another person's 'vermillion','pink', 'salmon', or something
else.
Cleavage
Cleavage is the tendency of a crystal to break along certain flat planes. Although
most crystals do not easily break, many specimens show the effects of already having
been broken.
Look in the "Earth Revealed" textbook on page 232, figure. 9.20 to see
what is meant by a direction of cleavage.
A 'cleavage plane' is the orientation of that flat surface, such that two sides of
a cleaved crystal that are parallel would be the same 'direction' of cleavage.
To see cleavage, look for 'flat' surfaces. Some minerals will have "good"
cleavage and you will be able to see a 'flash' of light on the entire surface like
a mirror as you rotate move it around under a light or next to a window. Look at
specimens C and E for example.
On some specimens you will see a 'blocky' appearance. Look at specimen F for example.
Use the magnifier. To use it hold it as close to your eye as you can and look through
it. Then bring the sample up closer and closer until it is in focus.
Look at specimen D. It is white and does not look shiny. But if you move it around
you will see three different 'flashes'. Upon closer inspection you will see that
those flashes correspond to flat surfaces that are like steps.
Specimen A does not look shiny, but again rotating it around in the light will reveal
those flashes across the entire surface.
Other minerals may have cleavage that is 'fair' to 'poor', meaning that it will be
harder to detect.
On sample J, for example you will see that one side 'catches' the light and seems
to 'light up' when it catches the light a certain way.
It has 'poor' cleavage.
It is sometimes difficult to distinguish cleavage from a crystal face.
Specimen B is a well-formed crystal. There the flat surfaces are crystal faces rather
than cleavage.
Although cleavage and crystal faces may appear similar, the more you look the more
you will see the differences.
In geological terms 'hardness' means 'scratchability'. A hard mineral will scratch
one that is softer and be scratched by one that is harder.
To determine hardness a geologist uses the "Moh's Scale" of hardness, which
is nothing more than a standard set of common minerals arranged in order of hardness
from 1 to 10, with 10 being the hardest.
Since most of us do not carry a collection of Moh's minerals with us we can use common
objects such as fingernail, penny, and glass to estimate hardness.
To determine the hardness of a specimen you will need to try to use it to try to
scratch the test object (glass, penny, steel rod) and then use the test objects to
scratch the specimen.
You may have to press very hard to get one to scratch the other. Use the magnifying
lens (loupe) if you cannot see whether or not there is a scratch.
Also be sure that you are seeing a scratch and not a streak of the softer substance.
Luster
Luster is subjective. It refers to the way light is reflected from the surface of
a mineral (or other substance).
Not everyone will agree on a particular word to describe luster, but everyone would
agree that a piece of saran wrap has a different luster from a satin scarf or a nylon
stocking.
Geologists use terms to describe luster that remind them of other more common things.
These have been 'refined' throughout the years and may not always be the way that
you would personally describe them.
The easiest distinction is between 'metallic' and 'nonmetallic' lusters. Metallic
luster is opaque and shiny like metal. However most minerals have nonmetallic lusters.
Lusters are described in terms of comparisons to common substances: vitreous (glassy),
resinous (the luster of resin), pearly, silky, dull, earthy.
Look at specimens B and E. These are 'glassy' or 'vitreous' because they have a glassy
appearance. It is easy to see because they are colorless.
Now look at specimens A, C, I. They are also glassy, but not as obviously so as B
and E.
Streak
The streak of a mineral is its color when finely powdered.
It is much more reliable than the color of the mineral specimen itself.
To obtain the streak, rub the mineral very hard
against the white streak plate in the lab mineral kit. Pretend that you are writing
with the mineral like chalk on a sidewalk, but press very hardly.
The color on the streak plate may be subtle, and it helps to use natural lighting
(from a window) to see the true color.
Some colors are greately affected by the type of light and a weak color may be
invisible in certain kinds of lighting.
The comments about color and different perceptions above under the 'color' heading
also apply for streak, only more so because the streak is more subtle in most cases
than the mineral color.
Minerals that are harder than the streak plate will not leave a streak (greater
than 7), and the harder the mineral the more force is necessary to get a streak on
the plate.
