Mineral Properties and Identification
Minerals are the building blocks of rocks, and each one has a unique combination of physical properties. Learning to identify minerals means learning to test and observe these properties systematically. This section covers the main properties geologists use, the techniques for testing them, and a few special properties like habit and specific gravity that round out the identification toolkit.
Physical Properties of Minerals
Color is the first thing you notice about a mineral, but it's actually one of the least reliable identification tools. Some minerals do have a consistent, diagnostic color (sulfur is almost always yellow), but many minerals vary widely because of trace impurities. Quartz, for example, can be clear, purple, pink, smoky, or milky white depending on tiny chemical differences.
Streak is far more dependable. Streak is the color of a mineral in powdered form, tested by dragging the mineral across an unglazed porcelain plate (called a streak plate). Even though hematite can appear silver, black, or reddish-brown as a hand sample, its streak is always reddish-brown. That consistency is what makes streak more useful than color.
Luster describes how a mineral's surface interacts with light. The first question is whether the luster is metallic (looks like polished metal, like pyrite) or nonmetallic. Nonmetallic lusters have several subtypes:
- Vitreous: glassy, like quartz
- Resinous: waxy or resin-like
- Pearly: soft sheen, like talc
- Silky: fibrous sheen, like some forms of gypsum
- Adamantine: brilliant and sparkly, like diamond
- Earthy/dull: no shine at all, like kaolinite
Hardness measures a mineral's resistance to scratching, ranked on the Mohs Hardness Scale from 1 (talc, very soft) to 10 (diamond, hardest natural mineral). This is one of the most useful diagnostic properties. For instance, topaz (hardness 8) will scratch quartz (hardness 7), but not the other way around. You'll use common reference objects to estimate hardness in lab (more on that below).
Cleavage is the tendency of a mineral to break along flat, predictable planes. These planes exist because bonding is weaker in certain directions within the crystal structure. Cleavage is described by the number of planes and the angles between them. Mica has one excellent cleavage plane (it peels into sheets), while feldspar has two cleavage planes meeting at roughly 90°.
Fracture describes how a mineral breaks when it doesn't follow cleavage planes. Common fracture types include:
- Conchoidal: smooth, curved surfaces (like broken glass); quartz is the classic example
- Hackly: jagged, sharp edges; seen in native copper
- Splintery: breaks into thin splinters, like kyanite in certain directions
- Uneven/irregular: rough, irregular surfaces
Mineral Identification Techniques
Dichotomous keys are flowcharts that walk you through a series of yes/no questions based on observable properties. At each step, you test or observe one property, and the answer sends you down a specific branch until you arrive at an identification. These keys work well, but they require careful, accurate observations at each step.
Hardness tests involve scratching your unknown mineral against objects of known hardness to narrow down where it falls on the Mohs scale:
- Fingernail (~2.5): if your nail scratches the mineral, it's very soft
- Copper penny (~3.5): scratches minerals softer than 3.5
- Glass plate (~5.5): most common mid-range test
- Steel nail (~6.5): if the mineral scratches steel, it's fairly hard
Always try to scratch in both directions. If mineral A scratches mineral B but B can't scratch A, then A is harder.
Acid tests use a drop of dilute hydrochloric acid (HCl) placed on the mineral surface. Calcite fizzes vigorously because the acid reacts with calcium carbonate to produce carbon dioxide gas. Dolomite fizzes only when powdered (scratched surface). Quartz and most silicate minerals show no reaction at all. This test is especially handy for telling carbonate minerals apart.
Magnetic tests simply involve holding a magnet near the mineral. Magnetite is strongly magnetic and will visibly attract a magnet. Pyrrhotite is weakly magnetic. Most other minerals show no magnetic response, so a positive result here is very diagnostic.
Types of Mineral Habits
Mineral habit refers to the characteristic external shape a mineral tends to grow in, controlled by its internal crystal structure and the conditions during growth. Habit isn't always diagnostic on its own since some minerals can display different habits depending on how they formed (pyrite can be cubic, octahedral, or even framboidal). Still, habit provides useful supporting evidence during identification.
- Prismatic: elongated crystals with well-developed parallel faces, like tourmaline or beryl
- Tabular: flat, tablet-shaped crystals dominated by broad flat faces, like mica or graphite
- Fibrous: slender, needle-like crystals forming parallel or radiating bundles, like asbestos or satin spar gypsum. This habit reflects strong bonding along one direction and weaker bonding in others.
- Equant: roughly equal dimensions in all directions, like garnet
- Bladed: flat and elongated like a knife blade, like kyanite
- Acicular: very thin, needle-like crystals, like natrolite
- Dendritic: branching, tree-like patterns, like some native copper or manganese oxide coatings on rock surfaces
Significance of Specific Gravity
Specific gravity (SG) is the ratio of a mineral's density to the density of water. It's a dimensionless number that tells you how many times heavier a mineral is compared to an equal volume of water. A mineral with an SG of 3.0 is three times denser than water.
Two main factors control a mineral's specific gravity:
- Atomic mass of the elements present: minerals made of heavier elements have higher SG. Barite (, SG ≈ 4.5) is noticeably heavier than quartz (, SG ≈ 2.65) because barium is much heavier than silicon.
- Packing efficiency of atoms in the crystal structure: tighter atomic packing yields higher SG. Diamond and graphite are both pure carbon, but diamond's tightly packed structure gives it an SG of ~3.5, while graphite's loosely bonded sheets give it an SG of ~2.2.
Measuring specific gravity uses the Jolly balance method (or a similar displacement technique):
- Weigh the mineral in air.
- Weigh the mineral suspended in water.
- Calculate using the formula:
The denominator represents the buoyant force, which equals the weight of water displaced by the mineral.
Why it matters for identification: SG helps distinguish minerals that look similar but have different compositions. Magnetite (SG ≈ 5.2) feels noticeably heavier in your hand than hematite (SG ≈ 5.0), and both are much heavier than similarly dark-colored minerals like hornblende (SG ≈ 3.2). With practice, you can even estimate SG just by hefting a sample, a technique geologists call "heft."