Minerals: Properties and Identification
What Is a Mineral?
Every rock you pick up is made of one or more minerals. Minerals are the building blocks of geology. They form naturally, have a fixed chemical composition, and their atoms arrange themselves into a repeating three-dimensional pattern. Geologists identify minerals not by guessing but by testing a small number of measurable physical properties. Learn those properties and you can name most of the minerals on your data sheet with confidence.
Colour and Streak
Colour is the first thing you notice about a mineral, but it is one of the least reliable identification tools. Many minerals come in several colours because tiny amounts of impurities change the appearance. Quartz, for example, can be colourless, white, pink, purple or smoky grey depending on trace elements present during crystallisation.
Streak is far more reliable. When you drag a mineral across an unglazed porcelain streak plate, it leaves a powder. That powder colour is constant for a given mineral regardless of surface colour variations. Haematite is a perfect example: its surface can look silver-grey or red, but its streak is always red-brown. Galena, the lead ore, always gives a grey streak. Calcite always gives a white streak.
Haematite always produces a red-brown streak. Galena gives a grey streak. Calcite gives a white streak. Streak is a more reliable identifier than surface colour because it is unaffected by impurities.
Hardness and the Mohs Scale
Hardness is tested by attempting to scratch one mineral with another, or with a fingernail (hardness about 2.5), a copper coin (about 3.5) or a steel file (about 6.5). A harder mineral will scratch a softer one; a softer one will not mark the harder. The Mohs scale runs from Talc at 1 (scratched by a fingernail) to Diamond at 10 (scratches everything).
The ten reference minerals in order are: Talc (1), Gypsum (2), Calcite (3), Fluorite (4), Apatite (5), Orthoclase feldspar (6), Quartz (7), Topaz (8), Corundum (9), Diamond (10). In exam questions you must use these exact values. A common error is giving quartz as hardness 6 or calcite as hardness 2 — memorise the scale accurately.
The Mohs hardness scale: Talc 1 — Gypsum 2 — Calcite 3 — Fluorite 4 — Apatite 5 — Orthoclase 6 — Quartz 7 — Topaz 8 — Corundum 9 — Diamond 10. Quartz (7) is harder than a steel file (about 6.5) and will scratch glass easily.
Cleavage and Fracture
Cleavage occurs when a mineral breaks along planes of weakness in its crystal lattice. The resulting surfaces are flat and often shiny. Mica breaks into thin flat sheets — it has perfect cleavage in one direction. Feldspar has cleavage in two directions at approximately right angles, producing blocky fragments. Calcite has cleavage in three directions that are not at right angles, producing rhombohedra (diamond-shaped chunks).
Minerals without regular cleavage show fracture instead. Quartz has a conchoidal (shell-like, curved) fracture. Galena has perfect cubic cleavage in three directions at right angles, giving bright flat surfaces that make it look metallic. On the data sheet, pay attention to cleavage descriptions — they distinguish similar-looking minerals.
Lustre
Lustre describes how a mineral surface interacts with light. It is described using standard terms: metallic (galena, haematite), vitreous or glassy (quartz, olivine), pearly (mica), silky (some gypsum varieties), and dull or earthy (some haematite). Metallic lustre always indicates either a native metal or a metallic ore mineral.
The minerals on your data sheet and their most exam-relevant properties:
Quartz — colourless to white, vitreous lustre, hardness 7, conchoidal fracture, no cleavage. Extremely common in granite and sandstone.
Feldspar (orthoclase) — pink or white, vitreous, hardness 6, two cleavage directions at right angles, often showing a blocky shape. Essential mineral in granite.
Mica (biotite / muscovite) — black (biotite) or silvery (muscovite), pearly lustre, hardness about 2.5, perfect single-direction cleavage into thin flexible sheets.
Olivine — olive green, vitreous, hardness 6.5–7, conchoidal fracture, common in basalt and peridotite.
Augite — black to dark green, vitreous to dull, hardness about 5.5–6, two cleavage directions nearly at right angles (compare with hornblende which has cleavage at 60° and 120°).
Calcite — white to colourless, vitreous to pearly, hardness 3, three cleavage directions (rhombohedral), reacts vigorously with dilute hydrochloric acid.
Garnet — red to brown, vitreous, hardness 6.5–7.5, no cleavage (conchoidal fracture), rounded crystal form. Found in metamorphic rocks.
Halite — colourless to white, vitreous, hardness 2.5, cubic cleavage in three directions at right angles, salty taste.
Haematite — silver-grey to red-brown surface, metallic lustre, hardness 5–6, red-brown streak.
Galena — silver-grey, bright metallic lustre, hardness 2.5, perfect cubic cleavage, grey streak, very high density (the main ore of lead).
The Acid Test
Dropping dilute hydrochloric acid onto a mineral surface is a quick and definitive test for calcite. Calcite reacts vigorously, fizzing as carbon dioxide gas is released. This is because calcite is calcium carbonate, and the acid decomposes it. Other carbonate minerals may react slowly or only when powdered. Minerals that are not carbonates, such as quartz, feldspar or mica, do not react at all. In exam questions, if a mineral fizzes with dilute acid, you should identify it as calcite unless the question gives you strong evidence for a different carbonate.
Colour is unreliable for identification. Students often name a mineral solely by colour and are caught out when quartz appears purple (amethyst) or when two minerals share the same colour. Always use at least two properties.
Cleavage surfaces are not the same as crystal faces. Crystal faces form during growth; cleavage surfaces form when you break the mineral. Both may look flat and shiny, but they reflect different internal structures.
Only spec-listed minerals earn marks. Hornfels is a rock, not a mineral, and anorthite is not on the data sheet. Name minerals using only the list on your data sheet.
Pulling all these properties together, a systematic approach always works best in your exam. Observe the lustre first (metallic or non-metallic), then test hardness, examine cleavage, check streak colour, and apply the acid test if a carbonate is possible. Each property narrows the field until only one mineral fits. This approach connects directly to the next lesson, where we look at how these minerals originally formed and where they occur in different rock types.