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Earth Resources: Mineral Identification
Introduction Animal, Vegetable, or Mineral?
If you are like most Americans, at some time, you have played the guessing game “Animal, Vegetable, or Mineral?” The premise behind the game is that one individual receives a certain number of questions in which to guess what material object another person has chosen. Of course, the first question is the aforementioned one, as every object that we can think of is supposed to fall within one of these three broad classification schemes. Alas, this is not really true. One problem is that some objects fall into 2 or more of these categories (some viruses have a crystalline stage, some animals undergo photosynthesis, etc.). However, the idea behind the game is that everything made of matter will fall into a classification of either living or non-living, with minerals being the catch all for non-living.
The other problem with the game is that a mineral is much more than just a non-living object. What it is exactly, though, will raise a debate amongst geologist. A check of different textbooks will find many different definitions for mineral. For the purposes of this activity, we are going to define a mineral as a substance that is naturally occurring, inorganic, crystalline in nature, and has a definite chemical make-up. The first of these criteria means that anything man-made is not considered a mineral. This is somewhat problematic, as mankind has developed ways of creating certain gemstones in the lab that are almost indistinguishable from their natural counterparts. For instance, industrially created diamonds are used for many different tools, such as diamond-tipped saw blades. The second criterion is not without its problems, too. Certain minerals such as graphite, diamonds, and calcium carbonate can and do have biological origins. Graphite and diamonds can come from plant matter. Calcium carbonate is the chemical that makes up seashells. By convention, they are usually included amongst minerals.
The third and fourth criteria are less problematic. The fact that a mineral must have a crystalline structure eliminates all liquids. It also eliminates all glasses, as these are amorphous solids with no definite atomic arrangement. The chemical make-up does come with one caveat: some minerals are allowed to have substitutions of certain chemicals in their molecular structure. As an example, hornblende is a complex mixture of hydrous ferromagnesium silicate that can various proportions of calcium, aluminum, and sodium within it. These substitutions usually just change the color of the mineral and do not radically alter the other properties of the mineral. Fig. 1: Calcite crystal ( USGS )
Identification
To accurately identify a mineral and be 100% certain, an individual would have to run a number of laboratory tests on a sample. They would have to run an X-ray diffraction analysis of the material to find out what its true crystalline shape is. A ground-up and prepared sample would have to be put through a chemical analyzer to determine its chemical formula. Both of these procedures would take a lot of time and money, and some of the sample would be destroyed in order to perform the analysis. For these reasons, we rarely run such test unless there is a great need to know the answer for sure.
Instead, most minerals are identified by their physical properties. Since minerals have a definite chemical make-up and crystalline shape, one can usually identify them things like their hardness, color, or crystalline shape. Some of the more common properties used to identify them are listed below.
- Hardness – One of the most common properties upon which to base identification, this is a measure of the scratchability of a mineral. It is evaluated on the basis of the Mohs’ hardness scale, which identifies the hardness of certain keystone minerals on a 1-10 scale. The scale is 1 for talc, 2 for gypsum, 3 for calcite, 4 for fluorite, 5 for apatite, 6 for orthoclase, 7 for quartz, 8 for topaz, 9 for corundum, and 10 for diamond, the hardest substance known to humans. The principle behind the scale is that any substance that is higher in number is able to scratch a substance of a lower number. topaz will scratch quartz, fluorite will scratch gypsum, and diamond will scratch them all. For further reference, it should be noted that the average human fingernail is about a 2 ¼, a copper coin is a 3 ½, a steel nail is about a 5 – 5 ½, and glass is about a 5 ½ – 6. To do the hardness test, you will sometimes need to use considerable force. You should try to minimize the scratching of the mineral by limiting the size of the mark. Further, you should wipe the mineral after the scratch test to make sure that it did indeed scratch, and that you are not just seeing powdered residue on the surface left behind by the device with which you performed the test.
- Luster – This is the appearance of the mineral surface in reflected light. This test can be very hard to perform, as dirt on the surface or an uneven surface will skew results. The test is best carried out when you are looking at a large crystal face. The different categories are metallic (reflect a considerable amount of light and look like a metal surface), adamantine (brilliant, like a polished jewel), vitreous (glassy), resinous, pearly, silky, and earthy (dull, very little reflection).
- Color – While this seems to be a very simple property, it is far from easy to use this property. Impurities can greatly change the color of a mineral. Dirt or other substances on the surface can also give a false reading. Color is also very subjective. What one person would call green, another might call grayish. This property is most reliable for metallic minerals, and fails a lot for transparent minerals. As an example, gold and iron pyrite often look very similar in color (see Fig. 2). This is one reason why iron pyrite is often called “fool’s gold”.
Fig. 2: Gold (left) and iron pyrite (USGS)
- Streak – This property is the color of the mineral residue when it is powdered. Amazingly, this property is usually much more reliable than color. To create a streak, one would usually use a mortar and pestle to crush a small sample. However, the most used tool for measuring this is to use a piece of white, unglazed pottery. Since pottery has a hardness of about 6, this tool is unusable for minerals that have hardnesses of 6 and greater.
- Cleavage – This is the tendency of a mineral to split along certain planes. A great example of a mineral that has excellent cleavage would be mica, which cleaves along flat planes to give very thin sheets. Other minerals such as halite will have several different faces upon which they will cleave, while some other minerals such as quartz have no cleavage (and yes, geologist are know to make bad, sexist jokes during the discussion of this property).
- Crystalline shape – This is the geometric pattern that a lone crystal of the mineral will have. To see this pattern, though, the crystal needs to be reasonably large and not convoluted by many crystals growing over one another. Oftentimes, all that one sees is just a face or two of the crystal. This might be enough if the shape of the crystal is simple.
- Fracture – This is the shape of a mineral when it is broken. This occurs for minerals like quartz that do not have cleavage. The different types of fractures are conchoidal (concave breakage reminiscent of glass), splintery, or uneven.
- Specific gravity – This is the density of the mineral compared to water (1 gm/cm^3 ). Most minerals will have a specific gravity in the 2.5-3.5 range. Some, such as the natural metal ores and few other minerals rich in metals, will have specific gravities much higher than this. Others, such as halite and gypsum, will be much less than this. To determine this property, one needs a graduated cylinder with water in it and a mass scale. Putting the mineral in the graduated cylinder will tell one the volume of the mineral by the amount of water it displaces. Putting the mineral on the scale will give its mass, which when divided by its volume in cubic centimeters, gives its specific gravity.
ESA21: Environmental Science Activities Activity Sheet
Mineral Identification
Name:
# Mineral Identifying Characteristics
DESCRIPTIVE MINERAL TABLE
Minerals with Metallic Luster
Name and Composition
Hardness Color Streak Features
Graphite C
1 Silver gray
Black Marks paper like a pencil, greasy feel, light in weight. One perfect cleavage. Molybdenite MoS 2
1-1 ½ Blueish- gray
Greenish- gray
Soft, flexible, shiny plates (one perfect cleavage), often with hexagonal outline. Marks paper. Galena PbS
2 ½ Silver- gray
Black Cube or octahedron crystals, cubic cleavage, bright metallic luster, heavy. Native Copper Cu
2 ½ - 3 Copper- rose
Copper- rose
Copper-rose color on fresh surfaces; greenish-gray surface film where altered. Heavy and malleable. Rare crystals; usually in compact masses. Often has a pale green surface coating of malachite. Native Gold Au
2 ½ - 3 Gold, white- gold, rose
Same as color
Color varies with impurities. Extremely heavy. May be gouged or sliced with a knife. Dissolves in aqua regia. Rare small crystals, and dendrites; nuggets in sedimentary deposits Native Silver Ag
2 ½ - 3 Silver- white
Silver- white
Tarnishes dark gray. Irregular fracture. Very heavy. Sectile. May occur as dendrites (see Gold) and wires in calcite and other minerals. Usually tarnished blackish-gray. Bornite Cu 5 FeS 4
3 Rose to brown
Gray-black lrridescent alteration coating common; brittle conchoidal fracture. “Peacock ore.” Chalcopyrite CuFeS 2
3 ½ Brass- yellow
Greenish- black
Often tarnished irridescent or chalky greenish-blue. Brittle, fairly soft, usually massive. Conchoidal fracture. Pyrite FeS 2
6 Light brass- yellow
Black Occurs in cubes with grooved faces, and pyritohedrons with 5-sided faces. Called “fool’s gold,” much lighter than true gold. Poor cleavage; fragile. Magnetite Fe 304
6 Black Black Magnetic, granular or octahedral crystals common. No cleavage. Specular Hematite Fe 203
6 Shiny steel-gray
Dark red Glittering flakes or wavy sheets. Streak is distinctive. Tendency to flake obscures true hardness.
Minerals with Non-Metallic Luster
Name and Composition
Hardness Color Streak Features
Talc Mg 3 Si 4010 (0H) 2
1 White, pale green
Pearly Extremely soft; soapy feel. Impurities may increase apparent hardness. One perfect cleavage; often in scaly masses. Kaolinite AI 2 Si 2 O 5 (OH) 4
1 – 2 ½ White, cream Earthy, dull Soft, powdery texture. Smells earthy when damp. Usually in clay-like masses with dull appearance. Native Sulfur S
1 ½ - 2 ½ Yellow Resinous, greasy
Color, low hardness, light in weight. Detectable sulfur odor. Often in well- developed blocky crystals, or as a fine coating on volcanic rock.
Azurite Cu 3 (C0 3 ) 2 (OH) 2 Malachite Cu 2 C03(OH)
3 ½ - 4 Azure blue and bright green, respectively
Dull or velvety
Colors and association distinctive; both effervesce in hydrochloric acid. Azurite often in radiating masses. Malachite frequently in curved masses exhibiting color banding in shades of green. Dolomite CaMg (CO 3 ) 2
3 ½ - 4 White, yellow, pink
Vitreous to pearly
Slowly effercesces in cold dilute acid when powdered. Pale pink color is indicative. Often associated with calcite. Usually in rhombohedral crystals; perfect rhombohedral cleavage. Fluorite CaF 2
4 Colorless, all pastels, deep purple
Vitreous Crystals often cubic or octahedral. Color banding common. Octahedral cleavage. Usually fluorescent in ultraviolet light. Colemanite Ca 2 B 6 O 11 ~ 5H 2 O
4 ½ Colorless, white
Vitreous May be in stubby, glassy crystals, or in compact granular masses. Perfect cleavage. Apatite Ca 5 (P0 4 )3F
5 White, blue, brown
Vitreous Will not scratch glass. Commonly in 6- sided prisms. Green, Blue, Yellow. One poor cleavage. Scheelite CaWO 4
5 White, yellow, brown
Vitreous Will not scratch glass. Heavy. Flu- oresces. Good cleavage, crystal faces may be grooved. Goethite HFeO 2
5-5½ Dark rusty brown, ochre yellow
Dull, earthy Streak distinctive yellow-brown. Often spongy, porous or earthy; also bladed, fibrous. Also called limonite. Often occurs in cubes and pyritohedrons as an alteration of pyrite. Hematite (earthy) Fe 203
5 Dull brownish red to bright red
Sub-metallic to earthy
Characteristic red-brown streak. Often earthy and too powdery for accurate hardness test. May be granular or oolitic. Crystals rare; no cleavage.
Rhodonite MnSiO 3
6 Pink to deep rose
Vitreous Massive, dense or granular aggregates often have black veins. Color and hardness diagnostic. Blocky crystals, nearly 90° cleavage. Hornblende 5½-6 Greenish-black Vitreous Barely scratches glass. Shiny on cleavage faces; opaque; often splintery at edges. Usually massive; occasionally in chunky crystals. Two directions of cleavage at 124° and 56°. Auglte 6 Dark green Vitreous to dull
Stubby prismatic crystals. Usually duller and greener than closely related horn blende. Two cleavages at 87° and 93°, and uneven fracture. Orthoclase Feldspar KAISi 3 O 8
6 WhIte, pink
Vitreous Two good cleavages. Will scratch glass. Wavy internal pattern and pink color distinguish it from plagioclase when present. May be massive, or in large, well-developed coffin-shaped crystals. Plagloclase Feldspar NaAISi 3 O 8 CaAl 2 Si 2 O 8
6 WhIte, gray
Vitreous Two good cleavages. Will scratch glass. “Record grooves.” Rectangular cleavage faces often seen In igneous rocks.
Spodumene LiAISi 2 O 6
6½ Colorless, white, lavender
Vitreous Elongated prismatic crystals. Associated with lepidolite, tourmaline, beryl. Deep grooves often parallel long crystal faces. Perfect prismatic cleavage. Olivine (Mg, Fe) 2 SiO 4
6 ½ - 7 Olive green Vitreous Crystals often appear as glassy green beads, isolated or in masses. Color distinctive. Conchoidal fracture. Epidote Ca 2 (Al, Fe) 3 Si 3012 (OH)
6 ½ - 7 Light to dark green
Vitreous Usually a dull avocado massive; crystals are dark green, with striations and well developed cleavage. Quartz Family SiO 2
7 Colorless White Gray, brown Pink Purple Yellow
Vitreous to greasy
Crystals are 6-sided prisms, often with terminations and steps perpendicular to crystal length. Conchoidal fracture; no cleavage. Crystals may be in clusters, or line cavities in rock; some weigh several hundred pounds.
Chalcedony (Quartz) (petrified wood, flint, chert, agate, jasper)
7 Variable Waxy Massive, dense, often bumpy masses; waxy surface. Color banded or mottled appearance common. Not wholly crystalline. May line rock cavities to form geodes, or replace organic material to “petrify” wood, shell or bone. Staurolite FeAl 4 Si 2010 (OH) 2
7-7½ Brown Vitreous Usually found as prismatic crystals; often twinned to form crosses. Crystal faces are pitted and rough. Cruciform twinning is diagnostic when present. Tourmaline 7-7½ Black, brown, green, pink, blue yellow
Vitreous to dull
Typically in elongated crystals with grooved faces and rounded triangular cross section. Common variety shiny black. Crystals often occur in parallel or radiating groups. No cleavage. Garnet Fe 3 Al 2 (Si0 4 ) 3
7-7½ Brown, red; also purple, green, yellow, black, pink
Vitreous to
resinous
Commonly in shades of red. Dodecahedral crystals have diamond- shaped faces. Color and hardness aid identification. No cleavage. Transparent to opaque. Beryl Be 3 Al 3 Si 6 (^018)
7½ - 8 Colorless, white, pink, blue, light green, emerald green
Vitreous Commonly pale green, and in 6-sided prisms with flat terminations. Harder than quartz. Poor cleavage.
Topaz Al 2 SiO 4 (OH,F) 2
8 Colorless, white, golden yellow, light blue
Vitreous Distinct glassy prismatic crystals with perfect basal cleavage exhibit ing diamond-shaped cross section. Internal rainbows. Striations on crystal faces. Corundum Al 203
9 Gray, all pastels, red, dark blue, brown
Vitreous to greasy
Commonly in barrel-shaped 6-sided crystals, tapered or with flat ends. Extremely hard. No cleavage.
Diamond C
10 Colorless, pastels, blue, yellow, gray, black
Adamantine to greasy
Octahedral crystals with greasy luster. Hardest known substance. Two directions of cleavage.
From General Geology of the Western United States – A Laboratory Manual by Bassett and O-Dunn, pp. 6- 18, Peek Publications, Palo Alto, CA, 1980.
