Minerals are naturally occurring inorganic substances with a defined chemical composition and crystalline structure. They exhibit a variety of physical properties that are valuable for identifying and classifying minerals. Some of the most useful physical properties for mineral identification include color, streak, luster, hardness, cleavage, fracture, crystal form, and density.
While mineral color can sometimes be helpful, it is not completely reliable for identification, as different minerals can be similar colors. More diagnostic properties like streak, the color of the mineral’s powder, and luster, or how a mineral reflects light, are better indicators of a mineral’s identity. Hardness, which refers to a mineral’s resistance to scratching, can be quantified using the Mohs hardness scale. The tendency for a mineral to break along cleavage planes or fracture is also key to its identification. Crystal form, or the shape a mineral naturally forms into, gives clues about its internal structure. Finally, density relates to a mineral’s specific gravity or heaviness.
Geologists employ these physical properties to identify unknown mineral samples, like those collected in the field. By combining observations of color, luster, hardness, cleavage, crystal form, density and other properties, the identity of a mineral can be determined. For example, a green mineral with perfect cleavage that is easily scratched could be the mineral fluorite. The diagnostic physical properties of minerals thus allow for their complete characterization and classification.
Hardness
Hardness refers to a mineral’s resistance to abrasion or scratching. It is an important diagnostic property that allows minerals to be differentiated from each other. The hardness of a mineral depends on the strength of its chemical bonds. The harder the mineral, the more tightly bonded its molecules are.
Mohs Hardness Scale
In 1812, the German mineralogist Friedrich Mohs devised a scale to measure hardness using 10 reference minerals, ranging from the softest mineral, talc, to the hardest, diamond. The Mohs hardness scale arranges minerals in order of increasing hardness, with each mineral able to scratch all those below it. When identifying an unknown mineral specimen, its hardness can be determined by observing whether it’s scratched by a mineral of known hardness.
Testing Hardness
Common objects used to approximate Mohs scale minerals include fingernails (2.5), copper pennies (3.0), knife blades (5.5), and glass plates (5.5). If an unknown mineral is softer than the knife blade but harder than the copper penny, its hardness likely falls between 3 and 5. Field hardness testing kits contain various Mohs scale minerals to precisely determine an unknown specimen’s hardness.
Hardness and Mineral Identification
Hardness varies greatly among mineral species and is one of the most diagnostic properties for identification. For example, diamond is extremely hard at 10 on Mohs scale, while talc is very soft at 1. Quartz has a hardness of 7, distinguishing it from softer minerals like calcite (3) or harder minerals like topaz (8). The hardness of a mineral thus provides key insights into its identity.
Luster
Luster describes how light is reflected from a mineral’s surface. This optical property varies between mineral species based on differences in their chemical composition. Luster can range from bright, reflective metals to dull, earthy rocks.
Types of Luster
Some of the principal categories of luster include:
- Metallic – Reflective like metal, e.g. pyrite
- Sub-metallic – Slightly reflective, e.g. hematite
- Vitreous – Glass-like sheen, e.g. quartz
- Pearly – Iridescent like pearl, e.g. talc
- Silky – Soft-fibrous luster, e.g. gypsum
- Dull or earthy – No shine, e.g. kaolinite
Identifying a mineral’s type of luster aids identification by narrowing down mineral possibilities.
Luster in Mineral Identification
Luster is a key distinguishing factor between minerals like hematite and magnetite, which have metallic and sub-metallic luster, respectively. Chalcopyrite has a brassy metallic sheen, while pyrite has a pale, metallic luster. Luster combined with other properties like hardness can definitively identify mineral specimens.
Comparing the Different Types of Luster
| Luster Type | Description | Examples |
|---|---|---|
| Metallic | High reflectivity like metal | Gold, copper, galena |
| Sub-metallic | Slightly reflective metallic | Hematite, magnetite |
| Vitreous | Glassy, transparent luster | Quartz, topaz |
| Pearly | Iridescent, pearl-like sheen | Talc, gypsum |
| Silky | Soft, silk-like fibrous luster | Asbestos, satin spar gypsum |
| Resinous | Clear and glassy, amber-like | Sphalerite, amber |
| Greasy | Looks oil-stained or wet | Stibnite |
| Adamantine | Brilliant sparkly luster | Diamond, cerussite |
| Dull/Earthy | No reflectivity, soil-like | Kaolinite, bauxite |
| Waxy | Looks like plastic or wax | Jadeite, nephrite |
Color
Color is the most obvious physical property of minerals. While some mineral types are restricted to a narrow range of colors based on their composition, many minerals occur in a rainbow of hues. Thus color alone is not a reliable identifier, as different minerals can occur in the same color. However, it can still contribute to mineral identification.
Color and Mineral Formation
Mineral color results from the presence of certain chemical elements and physical processes affecting the mineral. For example, iron produces reds, greens, and browns; copper yields blues and greens; and sulfides often cause yellows, browns or blacks. Processes like radioactive damage, oxidation, and crystal defects also impact color.
Example Minerals with Same Color
While several minerals may occur in the same color, they often differ in other properties. For instance, emerald and dioptase are both deep green, but dioptase has a much lower hardness of 5 compared to emerald’s 7.5. Azurite and sodalite can form similar rich blues, but azurite has a vitreous luster while sodalite is more sub-vitreous.
Color as Identification Aid
Though unreliable on its own, color is still a useful first step in mineral identification. Green crystals could indicate olivine or dioptase, while blue crystals may be azurite or sodalite. The mineral possibilities can then be narrowed down by testing hardness, luster, and other properties. But color should not be the sole factor in mineral ID.
Streak
Streak is the color of a mineral’s powder when it is dragged across a rough, unglazed porcelain plate. This is a more reliable identifier than color, as minerals with the same color often have different colored streaks. Streak testing is thus an important technique in mineral identification and classification.
Streak Testing Procedure
To perform a streak test, scrape the mineral across the porcelain plate with pressure. This abrades a streak of fine mineral powder on the plate, revealing the mineral’s true color. Clean the plate between tests to avoid cross-contamination.
Streak Color Variance
Minerals with dark colors like hematite and magnetite have red and black streaks respectively, contrasting with their metallic gray appearance. Azurite has a deep blue color but produces a lighter blue streak. Streak thus reveals the true mineral hues.
Streak in Mineral ID
Streak allows differentiation between minerals with similar colors but different composition. For example, chalcopyrite and galena have identical gray hues, but chalcopyrite has a black streak compared to galena’s darker gray streak. Streak is a highly useful identification tool.
Cleavage and Fracture
Cleavage and fracture both describe the characteristics of how a mineral breaks, but with key differences. Observing a mineral’s cleavage and fracture helps identify its internal structure.
Cleavage
Cleavage refers to the tendency of a mineral to break cleanly along smooth, flat planes corresponding to zones of molecular weakness. The weak bonds between molecules in these zones allow the mineral to separate into cleavage fragments.
Fracture
In contrast, fracture describes the irregular, jagged breaking of a mineral in any direction without cleavage planes. Fracture occurs when force is applied without any structural weaknesses present.
Cleavage and Fracture Patterns
The angles between cleavage planes are consistent for each mineral based on its crystal structure. For example, fluorite cleaves in four directions forming octahedral fragments, while mica only cleaves in one plane. Thus, observing a mineral’s cleavage or fracture pattern aids identification.
Crystal Form
Crystal form or habit refers to the characteristic shape or physical symmetry of the crystal faces and angles of a mineral. This reflects the regular, repeating geometric pattern of molecules in the crystal lattice.
Crystal Systems
The basic external symmetry and angles of crystals derive from their internal ordered arrangement of atoms and molecules. There are seven crystal systems minerals form within:
- Cubic – Cube forms like pyrite
- Tetragonal – Square prisms like zircon
- Hexagonal – Hexagonal prisms like beryl
- Orthorhombic – Rectangular prisms like topaz
- Monoclinic – Parallelogram prisms like gypsum
- Triclinic – No symmetry, uneven like turquoise
- Trigonal – Triangular prisms like calcite
Crystal Habit in Mineral ID
Identifying the crystal form helps determine which crystal system it belongs to, indicating the mineral type. For example, perfect cubic fluorite crystals signal the cubic crystal system. Hexagonal quartz crystals fall into the hexagonal system. Knowing the crystal system narrows possibilities during mineral identification.
Density
Density or specific gravity measures a mineral’s mass per unit volume. Denser minerals contain heavier elements or tightly packed crystal lattices. Density assists identification by differentiating light versus heavy minerals.
Measuring Density
A mineral’s density is measured by first determining its mass using a scale. Then its volume is measured by water displacement into a graduated cylinder. Dividing the mass by the volume yields density.
Density and Composition
Mineral composition directly influences density. Metallic minerals like galena and gold are very dense. Halite is lighter than metals despite its ionic bonding. Loosely packed silicates like pumice have extremely low density. Thus, density provides clues about a mineral’s makeup.
Using Density in Identification
Measuring density quantitatively distinguishes mineral specimens. For example, the dark minerals galena and hematite can seem visually similar, but galena has a density around 7.5 while hematite is around 5. This along with other tests confirms their identity.
Frequently Asked Questions
Why is color not a reliable identifier for minerals?
Many different mineral species can occur in the same color, so color alone does not definitively identify a mineral. Additional tests like hardness, luster and streak are needed to distinguish between similarly colored minerals.
How accurate is the Mohs hardness scale?
The Mohs scale is a relative scale based on scratch tests against reference minerals. It allows reasonably accurate hardness determinations within 1 hardness unit. Precise measurement requires advanced hardness testing machines.
Can cleavage patterns be seen in all mineral specimens?
Cleavage is only visible on fresh crystal faces. In eroded or irregular mineral samples, cleavage may not be observable. Fracture patterns must be used instead in these cases.
What causes color variations in the same mineral?
Chemical impurities, exposure to radiation, structural defects and oxidation state changes can all produce color variations within a mineral species. For example, quartz can be colorless, purple, pink, red, yellow and more.
Are all properties required to identify an unknown mineral?
Only a few key properties are needed for identification in most cases. Hardness, luster, streak and cleavage alone can fingerprint many mineral specimens. Density, crystal form and other traits provide supplemental data.
Can minerals change their physical properties?
Minerals are static in their chemical composition and structure, so their inherent properties do not change over time. However, external processes can alter appearance. For example, cleavage surfaces can become eroded or coated.
Why is streak more useful than color for mineral ID?
Streak reveals the true intrinsic color of the powdered mineral, as pigments and impurities are averaged out. Colored specimens may appear one shade but produce a different streak color.
Can minerals form in crystal systems other than the main seven?
There are seven primary crystal systems that encompass all minerals, but more detailed sub-systems exist as well. For example, trigonal is a sub-division of the hexagonal system.
How are minerals with varying hardness identified?
Hardness varies in different crystal faces of some minerals. Always test hardness on a consistent surface. Move to a fresh surface if the hardness seems to change. Take an average of multiple measurements.
Conclusion
Mineralogists utilize physical properties to conclusively identify unknown mineral samples, just as a detective uses clues to solve a case. Properties like hardness, luster, streak, cleavage, crystal form, and density all contribute key insights into a mineral’s identity. While individual properties alone cannot prove a mineral’s type, deductive reasoning using multiple properties provides an accurate diagnosis. By combining each piece of evidence – color, hardness, density, etc. – the mineralogist can definitively characterize each unique mineral specimen.
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