Minerals: Crystal Structure & Identification Properties: Sixth Grade Science
Minerals are the naturally occurring, inorganic chemical building blocks of all rocks, identifiable by distinct crystal structures and diagnostic physical properties.
The Five Criteria of a Mineral
In geology, the word "mineral" has a strict scientific definition. To be classified as a true mineral, an earth material must satisfy five specific criteria simultaneously:
- Naturally Occurring: Formed exclusively by natural geological processes on Earth or cosmic bodies; human-made synthetic gemstones (such as lab-grown cubic zirconia) are not minerals.
- Inorganic: Formed by non-living geological processes, never consisting of living organic tissue or carbon-hydrogen biological compounds (coal, pearls, and amber are organic and therefore not minerals).
- Solid: Exists as a solid at standard Earth surface temperatures and pressures; liquid petroleum and liquid water are not minerals, though solid glacial ice is a true mineral!
- Definite Chemical Composition: Can be expressed by an exact chemical formula (e.g., quartz is SiO2, halite is NaCl, pyrite is FeS2).
- Orderly Crystalline Structure: The internal atoms are arranged in an organized, geometric, three-dimensional repeating lattice.
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| THE FIVE MINERAL CRITERIA |
| |
| 1. Naturally Occurring (Not artificial or manufactured in a lab) |
| 2. Inorganic Substance (Not living tissue; never alive) |
| 3. Definite Solid State (Maintains shape at room temperature) |
| 4. Fixed Chemical Formula (Consistent atomic composition throughout) |
| 5. Internal Crystal Order (Repeating 3D atomic geometric lattice) |
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Diagnostic Physical Properties for Mineral Identification
Geologists use systematic diagnostic physical tests in the field and laboratory to identify unknown mineral specimens:
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| MINERAL IDENTIFICATION TESTS |
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| Diagnostic Property | Testing Procedure | Diagnostic Significance & Example |
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| Luster | Visual inspection | Metallic (reflects like polished |
| | of reflected light| metal) vs. Non-metallic (glassy, |
| | | pearly, earthy, vitreous, dull) |
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| Streak | Scrape across an | The color of a mineral's fine |
| | unglazed porcelain| powder; highly reliable! Pyrite is|
| | streak plate | gold, but its streak is black. |
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| Hardness | Scratch test using| Resistance to scratching; ranked |
| | Mohs Hardness | from 1 (Talc) to 10 (Diamond) |
| | Scale reference | |
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| Cleavage vs. | Inspect broken | Cleavage: Breaks along smooth flat|
| Fracture | surfaces and | atomic planes; Fracture: Breaks |
| | fracture patterns | irregularly (conchoidal, jagged) |
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| Specific Gravity | Ratio of mineral | Heaviness relative to water; |
| (Density) | mass to water vol.| Galena (PbS) feels extraordinarily|
| | | heavy due to dense lead atoms. |
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The Unreliability of Mineral Color
A common novice mistake is attempting to identify a mineral solely by its exterior color. Color is the least reliable diagnostic property for two reasons:
- Many different minerals share the exact same color (e.g., gold, pyrite, and chalcopyrite all appear shiny golden).
- A single mineral can occur in a vast rainbow of colors due to tiny trace impurities (e.g., pure quartz is colorless, but trace iron turns it purple amethyst, trace titanium creates rose quartz, and radiation creates smoky quartz).
Streak: The Power of True Powder Color
While surface color varies, a mineral's streak is remarkably constant. When scraped firmly against a white porcelain streak plate, the mineral's surface fractures into a fine powder. The color of this powder is an inherent physical property unaffected by surface weathering. Pyrite (fool's gold) has a brassy gold luster, but produces a distinctive greenish-black streak; real gold leaves a shiny, bright yellow streak. Hematite can appear silver, metallic, or black, but always leaves a diagnostic reddish-brown rust streak.
The Mohs Hardness Scale
In 1812, German mineralogist Friedrich Mohs established a relative hardness ranking from 1 to 10 using ten common reference minerals. A mineral can scratch any mineral with an equal or lower number on the scale.
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| THE MOHS HARDNESS SCALE |
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| Hardness Reference Mineral Common Field Testing Tools |
| 1 Talc (Softest) |
| 2 Gypsum <--- Human Fingernail (2.5) |
| 3 Calcite <--- Copper Penny (3.5) |
| 4 Fluorite |
| 5 Apatite <--- Steel Pocketknife / Nail (5.5) |
| 6 Feldspar <--- Glass Plate (5.5 - 6.0) |
| 7 Quartz <--- Steel File (6.5) |
| 8 Topaz |
| 9 Corundum (Ruby/Sapph) |
| 10 Diamond (Hardest) |
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Cleavage Versus Fracture: Atomic Bonds in Fracture
How a mineral breaks reveals the underlying geometry of its internal atomic bonds:
- Cleavage: The tendency of a mineral to split cleanly along smooth, flat, parallel planes where chemical bonds are weakest. Mica splits into paper-thin flexible sheets (1 direction of cleavage); halite cleaves into perfect cubes (3 directions at 90 degrees); calcite cleaves into slanted rhombohedrons.
- Fracture: Occurs when atomic bonds are equally strong in all directions, causing the mineral to break along irregular, rough, or curved surfaces. Quartz breaks with smooth, shell-like curved hollows called conchoidal fracture (identical to broken glass).
Exercises and Solutions
Exercise 1: Evaluating Materials Against the Five Mineral Criteria. Evaluate each of the following materials against the five scientific mineral criteria and determine whether it qualifies as a true mineral: (a) A synthetic cubic zirconia gemstone grown in an industrial furnace; (b) Solid naturally occurring glacial ice extracted from an Antarctic ice sheet; (c) Liquid petroleum crude oil pumped from an underground reservoir; (d) A lump of coal mined from a sedimentary rock bed.
Solution 1: (a) Synthetic cubic zirconia is not a mineral because it is manufactured artificially in an industrial lab, violating the requirement to be naturally occurring. (b) Glacial ice is a true mineral. It is naturally occurring, inorganic, solid at its native temperature, has an exact chemical formula (H2O), and possesses an orderly hexagonal crystalline lattice. (c) Petroleum crude oil is not a mineral because it is a liquid (not solid), has an organic biological origin derived from decayed prehistoric marine plankton, and lacks an orderly crystal structure. (d) Coal is not a mineral because it is organic rock composed of ancient compressed plant matter and lacks a definite chemical formula.
Exercise 2: Identifying an Unknown Mineral with Diagnostic Tests. A student tests an unknown shiny mineral and records the following laboratory data: Luster: Brassy, metallic; Hardness: Scratches a copper penny (3.5) and a glass plate (5.5), but is scratched by a quartz crystal (7.0); Streak: Leaves a greenish-black powder line on a porcelain plate; Cleavage/Fracture: Breaks with uneven, jagged fracture; Chemical Reactivity: Does not fizz when hydrochloric acid is applied. (a) Determine the approximate Mohs hardness value of the mineral; (b) Use the streak and luster data to distinguish whether the specimen is real gold or pyrite (fool's gold); (c) Identify the unknown mineral.
Solution 2: (a) Because the mineral scratches glass (hardness 5.5) but is scratched by quartz (hardness 7.0), its hardness is between 6.0 and 6.5 on the Mohs scale. (b) Real native gold has a hardness of only 2.5 to 3.0 (can be scratched by a copper penny) and produces a distinctive shiny golden-yellow streak. Pyrite has a hardness of 6.0 to 6.5 and produces a diagnostic greenish-black streak. (c) The unknown mineral is pyrite (iron disulfide, FeS2).