The definition of a mineral, and the diagnostic physical properties (hardness, cleavage, fracture, lustre, colour, streak, density and crystal habit) used to identify common rock-forming minerals in hand specimen.

What this dot point is asking

WJEC wants you to define a mineral precisely and then identify common rock-forming minerals in hand specimen using their diagnostic physical properties. You must know what each property is, how it is tested, and which minerals it separates. This is a Component 1 skill: the practical paper hands you specimens and photographs to identify, so the properties have to be at your fingertips.

The answer

What is a mineral?

A mineral is a naturally occurring, inorganic solid with a definite chemical composition (or a narrow composition range) and an ordered internal atomic structure that gives it a characteristic crystal form. Each part of that definition matters: synthetic crystals are not minerals because they are not naturally occurring, coal is excluded because it is organic, and a glass is excluded because its atoms are not arranged in an ordered lattice.

The diagnostic physical properties

Mineral identification rests on a small set of properties you can test on a hand specimen with a hand lens, a steel blade, a streak plate and dilute acid.

Hardness is the resistance to scratching, measured on the Mohs scale from 1 (talc) to 10 (diamond). The scale is relative: a higher mineral scratches a lower one. Useful reference points are a fingernail (about 2.5), a copper coin (about 3.5) and a steel blade or knife (about 5.5). Quartz at 7 scratches a blade; calcite at 3 is scratched by it.

Cleavage is the tendency to split along planes of weak bonding, producing flat, reflective surfaces. It is described by the number of cleavage directions and the angles between them. Mica has one perfect cleavage and peels into thin sheets; the feldspars have two cleavages at about 90 degrees; calcite has three cleavages at about 75 degrees, giving rhombs; halite has three at 90 degrees, giving cubes.

Fracture is how a mineral breaks where it has no cleavage. Quartz breaks with a smooth, curved conchoidal fracture like the inside of a shell. Other fracture types are uneven, splintery or earthy.

Lustre is the way the surface reflects light. The main classes are metallic (like polished metal, as in pyrite and galena) and non-metallic, which is subdivided into vitreous (glassy, as in quartz), pearly (mica), resinous, silky and earthy (dull).

Colour is the most obvious property but often the least reliable, because impurities and weathering change it. It is only diagnostic for a few minerals with a consistent colour, such as the green of olivine or the brassy yellow of pyrite.

Streak is the colour of the powdered mineral, found by rubbing it on an unglazed porcelain tile. It is far more consistent than the bulk colour and is especially useful for metallic minerals: haematite gives a red-brown streak whatever its lump colour.

Density (relative density or specific gravity) is the mass compared with an equal volume of water. Most rock-forming silicates are around 2.6 to 3.0; the metallic ore minerals are much denser, so galena (about 7.5) feels noticeably heavy in the hand.

Crystal habit is the characteristic external shape a mineral takes when it grows freely: quartz forms six-sided prisms with pyramidal ends, pyrite forms cubes, and the micas form thin platy flakes.

Working through an identification

Examples in context

Quartz versus calcite in the field. A geologist mapping a limestone that contains pale veins uses the acid bottle first: the rock fizzes but the vein crystals may not, separating calcite cement from quartz infill in seconds. Streak in ore prospecting. Prospectors distinguish red-brown haematite from grey-black magnetite partly by streak (both can look dark) and by magnetism, properties that scale up directly to identifying iron ore in the field. Cleavage and rock strength. Engineers care that mica's single perfect cleavage creates planes of weakness, so a mica-rich rock splits and weathers more readily than a quartz-rich one, linking a hand-specimen property to slope stability.

Try this

Q1. State the four parts of the definition of a mineral. [2 marks]

• Cue. Naturally occurring, inorganic, definite chemical composition, ordered internal (crystalline) structure.

Q2. A specimen has a hardness of about 3, three cleavages at 75 degrees and fizzes with dilute acid. Identify it. [1 mark]

• Cue. Calcite.

Q3. Explain why density is a useful property for distinguishing galena from quartz. [2 marks]

• Cue. Galena has a relative density of about 7.5 and feels heavy in the hand, whereas quartz is about 2.6, so a piece of galena is far heavier than a quartz piece of the same size.