A. Definitions
What is a mineral? What makes it unique? These are questions that can be used to open up the subject to the student. They will often confuse the term with “rock” or think only of the more valuable forms such as rubies or diamonds. They may also want to include coal because they have heard it called a mineral resource, or pearls since they seem to fit in with the other gems. So a discussion of just what a mineral is would be in order.
Minerals are natural substances that are inorganic and not the result of any living process, therefore ruling out coal, oil, or pearls. It must also have a specific chemical formula, made up of atoms in a definite ratio. In addition the atoms must have a definite and specific arrangement in space. It is because of these characteristics that minerals have unique properties which can be used to differentiate them form one another.
B. Terms of Identification
Some of a mineral’s properties are easily determined by simple visual examination: crystal shape, color, and luster. The beginning student will generally try to identify a mineral by its color first. In the case of some minerals this may be successful—malacite is always green, zaurite is always blue, etc. But for most minerals the surface color can vary tremendously depending on impurities included in the crystal structure.
Many minerals are made up primarily of elements which impart no strong color of their own and only minute amounts of a coloring agent can have striking results. Some color guidelines are: red may indicate the presence of chromium or hematite, green can indicate chlorite or chromium, and blue an indicate the presence of titanium or titanium and iron. The presence of copper ions can result in shades of green or blue and manganese can result in shades of red. It is sometimes helpful to determine the color of a mineral’s streak by rubbing the sample on an unglazed porcelain streak plate. This powdered residue is often more accurate in indicating true color and many mineral identification handbooks include a list of streak colors.
Hardness
of a mineral is shown by its resistance to being scratched. This is related to the crystal structure in that the more tightly bonded the atoms the harder the surface resistance to being etched will be. Diamond is the hardest mineral but is not readily available for student experimentation. corundum is the next step down and it inexpensively available. In 1812 Friedrich Mohs devised a rough scale of hardness that is invaluable in mineral identification. Diamond at number 10 is the hardest, and talc at number 1 is the softest. The intervals between the numbers are not equal, however, and the difference between corundum at 9 and diamond at 10 is greater than the entire range of 1 to 9!
Mohs Scale of Mineral Hardness
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1. Talc
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2. Gypsum
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3. Calcite
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4. Fluorite
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5. Apatite
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6. Feldspar
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7. Quartz
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8. Topaz
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9. Corundum
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10. Diamond
Cleavage
is a reflection of the electrical forces acting between the atoms which result in the crystal breaking along atomic planes that are parallel to crystal faces. The children are asked to look for these flat faces and simply to indicate whether the mineral has “good” cleavage or instead breaks with rather raggedy edges and is therefore declared to be fractured. Or if the student has already looked at crystal shape pictures and constructed paper models of them, they can try naming the crystal system it belongs to.
(figure available in print form)
Luster
is that mineral property which indicates the way light is reflected from the surface of the sample. Some luster terms are: glassy, metallic. greasy, pearly, or satin-like. These are all terms the students are familiar with and can also be used in conjunction with an identification manual.
Another means of identifying minerals is
specific gravity
. This measurement can be the most helpful identifying characteristic of all as it is apt to be the most reliable. The students can have a very interesting lab built around this property. Directions for this lab are in Appendix 1, Activity 4.
The story of Archimedes and his quest for a way to determine the value of the king’s crown is a sure-fire attention-getter to start the lesson. According to the legend, in about 250 B.C. Archimedes was given the task of determining if a crown belonging to King Hiero was pure gold or only an alloy of gold and silver. It is said that upon easing into his tub the bath water spilled over the edge and it came to him that the volume of water lost was the same as the volume of his body, and he could use the same technique to determine the volume of the crown. Since it was known that gold and silver have different densities, the only thing that would remain would be to take an accurate measure of the weight of the crown and divide this by the volume of the crown. The resulting density figure could be compared with the density of gold, and the truth would be known. It is said that with this revelation, Archimedes leaped from his bath and ran, forgetting the state of his undress, through the streets of Syracuse in Sicily exclaiming, “Eureka! — I have found it!” on his way to the palace! A sad footnote to the story is that the crown was indeed not the pure gold it had been portrayed as, and the unfortunate merchant met an uncomfortable end. Or so they say.
Carried one step further, the concept of specific gravity is based on the physical law that an object immersed in water loses as much weight as an equivalent volume of water would weigh. With a spring balance and a water pan the students can determine the specific gravity of a variety of minerals. Experience has shown that fairly large specimens will give the best results. See Appendix 1 for directions for labs on this and related topics; Activities 4, 5, and 6.