Last modified 9 / 06

GeoMan's Mineral ID Tests

 

Metallic | H<2.5 | H 2.5 to 3.5 | H 3.5 to 5.5 | H >5.5 | Glossary | Tests | Index

Rock Summary | Igneous | Sedimentary | Metamorphic

 

Background Information

In simple terms, our planet is just a big ball of very hot stuff which, at the surface where it is exposed to space, has cooled to form crystalline solids (igneous rocks) composed of minerals. Minerals in turn are combinations of elements, which are made of smaller particles, which are made of smaller particles, which are made of..... who knows what.

To be classified as a mineral, a substance must be an inorganic, naturally formed solid, with a specific chemical formula and a fixed internal structure. For example, coal is not a mineral (it's organic), but snow meets all five (5) requirements and therefore is a mineral.

 

Mineral Identification Tests

Luster: The quantity and quality of light reflected from the surface. Most identification schemes begin with a simple classification based on luster. Because of this, luster is the first fundamental test to be made when identifying any mineral. Most are relatively obvious, but some minerals can exhibit a range of lusters (ex. Hematite).

Metallic: looks like a metal. Metallic minerals are commonly shiny and opaque

Non-metallic: doesn't look like a metal. There are many subtle differences in the non-metallic lusters, but most are relatively dull, and are often transparent to translucent on thin edges.

Color: Obvious, but not always definitive. Sulfur is (almost) always yellow, and there are a few others, but not many minerals have a fixed color. Small amounts of impurities can drastically change a mineral's color.

Streak: The color of the powdered mineral. The test is usually performed by scraping the mineral across a piece of unglazed porcelain. Streak can be definitive. Good examples include hematite (always red-brown no matter what form it's in) and chromite (distinguished from the hundreds of other black minerals by its chocolate-brown streak).

Hardness: Hardness is the resistance of a mineral to scratching. It does NOT refer to how easily the mineral is broken. Hardness is a measure of the bonding strength between atoms. If these bonds are strong, the mineral is not easily scratched. Minerals with weaker bonds are more easily scratched. Pencil "lead" is softer than paper, so it writes. Try writing with a steel-tipped pencil and you'll see what I mean. Now the pencil rips the paper. This is clearly related to the relative hardness of each substance. Hardness in minerals can vary due to impurities, but is usually definitive. We determine the relative hardness of minerals using a scale devised by mineralogist Friedrich Mohs. The scale assigns hardness to ten common index minerals, and is based on the ability of one mineral to scratch another.

Mohs Hardness Scale
Hardness Index Mineral
 1  Talc
 2  Gypsum
 3  Calcite
 4  Fluorite
 5  Apatite
 6  Orthoclase
 7  Quartz
 8  Topaz
 9  Corundum
 10  Diamond

Since few of us carry these index minerals around in our pockets, we can use more common articles to get a rough estimate of hardness.

GeoMan's Field Hardness Kit
Hardness Item Comments
2.5 Fingernail We all have 10 of these to wear out before taking off our shoes.
3.5 Penny Sometimes all we have left.
4.5 Knife The hardness of a blade can vary wildly depending on the quality of your knife. Check yours before going into the field so you'll know.
5.5 Glass There's usually a chunk of glass available.
7 Quartz A piece of quartz can be found almost everywhere.

Breakage pattern: How a mineral breaks is determined by its internal structure, and is therefore very important (and nearly always diagnostic). Unfortunately, it can also be the hardest to determine (sorry). There are two (2) major subdivisions: fracture and cleavage.

Fracture: The mineral just breaks, leaving an uneven surface. Most are irregular but there are some special cases (ex: the conchoidal fractures common to quartz and glass)

Cleavage: The mineral splits along closely spaces parallel planes, leaving a mirror surface which will flash at you if rotated in the light. Cleavage is controlled by the internal crystalline order of the mineral. A mineral can have 1, 2, 3, 4, or 6 planes of cleavage. If more than one (1) plane is present, it is important to note the angles between the cleavages. Cleavage can be obscured, but is definitive when present. Terms such as Perfect, Good in 2 directions, Poor, etc. are used to indicate the quality or strength of the cleavage. Cleavage can be tough to distinguish from external crystalline form, and it's always a shame to break a good "crystal" when checking for cleavage.

Specific Gravity: Defined as "the weight of a specific volume of a mineral divided by the weight of an equal volume of water (at 4°C.)" Since water is always 1.0, it's the same number as density without any units (they cancel). This is almost impossible to measure in the field, but a rough approximation and be determined.

Effervescence (the Fizz test): Minerals containing calcium carbonate (CaCO3) will generally react when exposed to weak acid (usually hydrochloric acid (HCl), but even vinegar will work). Carbon dioxide (CO2) is released and the mineral or rock literally "fizzes." Some may need to be powdered (increases surface area) before any reaction can take place.

Magnetism: Magnetite is naturally magnetic. Don't put a chunk near your computer!

Taste: Some minerals have a distinctive taste. Notable examples include Halite (rock salt), and Chalcanthite (a copper sulfate - be careful with this one!!). I don't generally recommend the taste test.

Smell: Some minerals have a distinctive odor. Sulfur is a good example.

 

Metallic | H<2.5 | H 2.5 to 3.5 | H 3.5 to 5.5 | H >5.5 | Glossary | Tests | Index

Rock Summary | Igneous | Sedimentary | Metamorphic

 

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