After growing several crystals and investigating some of their properties, students may have whetted their curiosity and may want to investigate further. They might want to know how crystals are classified or what makes them valuable. The following are some criteria used by minerologist in identifying crystals. Students can use these same steps in the classroom to identify crystals supplied to them by their teacher or some mineral found on a field trip or a rock that they may have at home and want to know what it is.
The procedure outlined below can be found in any general book on minerals. Some of this information comes from Alan Holden’s books (see bibliography).
A. Properties of Crystals
There are many ways used in identifying crystals. Some are easily identified by their appearance. Others are identified by their smell (sulfur-P, taste (salt), or by how they sound when tapped. The quickest way to identify a mineral is by its appearance and feel. After one becomes familiar with a crystal one can often identify it again upon seeing it. In order to be more accurate you should test it. There is a good test that has been devised to determine a mineral’s identity. A mineral is tested in the following order:. Luster, hardness, streak, heft(relative weight), and the shape that the crystal has.
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1. Luster
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Luster refers to the way light is reflected from the mineral surface, There are two types of luster, metallic and nonmetallic. If it looks like a metal, it has metallic luster. Pyrite and galena have metallic luster. Nonmetallic luster is dull, pearly, silky, glassy or brilliant.
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Diamond has a brilliant luster. Nonmetallic luster may also be transparent.
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2. Hardness
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This is one of the more useful properties. Hardness is a mineral’s resistance to being scratched. The harder mineral always scratches the softer mineral. Frederick Mohs, a German Mineralogist, worked out a scale of hardness used in mineral identification. The minerals are arranged in order of increasing hardness. Each mineral is assigned a number between one and ten. A mineral with a higher number will always scratch a mineral with a lower number.
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Moh’s Scale of Hardness
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1-Talc
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6-Orthoclase
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2-Gypsum
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7-Quartz
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3-Calcite
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8-Topaz
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4-Flourite
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9-Corundum
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The hardness of an unkown mineral is found by scratching its edge against the surface of each reference mineral. If the reference mineral scratches the unknown then the reference mineral is harder than the unknown. If the unknown scratches a reference mineral then the unknown is harder. If they do not scratch each other then they have the same hardness. The number of hardness can be compared to the known list of mineral hardness in order to arrive at a possible identification. A table will be included at the end of this paper which will give samples.
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3. Streak
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Can you streak the mineral? Streak is the color of the powdered mineral. This is a useful property in identifying minerals that have color streak other than white. Too many minerals have the streak of white. In this case streak is not helpful. To find the streak(color) rub the unknown across a piece of unglazed porcelain. The color on the porcelain is the streak. A sample list in the Appendix will give the streak of some minerals. In this way the possible list of the unknown will be narrowed.
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4. Shape
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What is the shape of the mineral? Shape refers to the geometric pattern. Is the rock cubed, rectangular, hexagonal, pointed, etc. This refers to the ideal crystal arrangement and is not always apparent. The shape of the crystal is classified in one of six crystal systems. The shape of an unknown can be compared with the models of the crystal systems below.
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CRYSTAL SYSTEM
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(figure available in print form)
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5. Cleavage or Fracture
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Does the mineral have any broken surfaces? Whether a mineral cleaves or fractures is also useful in determining its identity. Minerals cleave if they break along smooth, flat planes. Cleavage planes may meet in angles that form geometric patterns similar to the crystal patterns. A fracture is a break along an irregular surface. A hammer and a sharp edge may be necessary. In some cases, mica for instance, you might be able to cleave the mineral with your hands.
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6. Color
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This property is useful in identifying a limited number of minerals as most minerals are mixed.
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7. Specific Gravity
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Is the mineral heavy? This is useful in recognizing heavy-minerals and jewels. Specific gravity refers to the ratio of the mineral’s mass to the mass of an equal amount of water. For example, galena(a lead mineral) has a specific gravity of 7.5. This means that a one cubic centimeter sample of galena is seven and a half times heavier then an equal amount of water. Heavy minerals can be roughly judged by picking the mineral up and tossing it in your hand, hence, heft.
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8. Special Property
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Does the mineral have some unique or special property?
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Some minerals have some unique property by which they can be identified. For example, you can taste halite(salt), smell sulfur and tap jade for the bell like ring. Test your mineral for any of these.
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9. Activities in identifying Minerals
Materials Needed
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Mineral samples
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Glass (for water displacement)
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Streak plate
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White paint
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Brush (small)
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Ink (black)
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Magnifying glass
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Pan balance
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Graduated cylinder
Water
Recording Chart
(figure available in print form)
Procedure
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a. Paint a small circle on each sample.
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b. Put a number on each rock
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c. Find the luster, hardness, streak, shape, cleavage-fracture, color, specific gravity and any special property for each sample. Record these on a chart like the one on the previous page.
Findings
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a. Were you able to identify any of the samples?
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b. Which tests were hard and which easy?
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c. Which property is most useful in identifying?
Follow-Up If you are having difficulty with this activity a fascinating way to reinforce this is by visiting the Peabody Museum. There is a large computer-type machine which the public is encouraged to use. By supplying some information the computer will zero-in until it identifies the unknown mineral. One merely supplies the properties the computer does the rest.
