Minerals and Rocks

Geology & Plate Tectonics Fundamentals

Course 1 · Chapter 2 · Minerals and Rocks: Identifying and Classifying

Pick up a pebble from an Edinburgh beach and you are holding a record of how it was made. Reading it begins with two steps: say which minerals it contains, then say how they are arranged. This chapter covers how minerals are identified with simple tests, which few minerals make up most of the crust, and how the three rock families of Chapter 1 are sorted into named types. Hardness values, densities and names are from general knowledge and flagged "check" where they matter.

What Is a Mineral?

Five conditions

A mineral is naturally occurring, solid, has a definite chemical composition, has an orderly internal crystal structure, and is normally inorganic (check; textbooks differ slightly on the last point). Ice in a glacier counts, water in a river does not.

Crystal structure

The atoms are arranged in a repeating pattern. That pattern decides how a mineral breaks, how hard it is, and the shape of its crystals when they have room to grow.

Mineral or rock?

Quartz is a mineral. Granite is a rock made of quartz, feldspar and mica. Limestone is mostly the single mineral calcite, which shows that a rock can consist of one mineral (check).

Why so few minerals matter More than five thousand minerals have been named (check), yet a handful of silicate minerals make up most of the crust: feldspars, quartz, micas, pyroxenes, amphiboles and olivine. Silicates are built from silicon and oxygen, the two most abundant elements in the crust. For most field geology, a short list is enough.

Identifying a Mineral

Property	How to test it	Example (check)
Hardness	What scratches what, using the Mohs scale of 1 to 10, with a fingernail (about 2.5), a coin and a steel knife as everyday tools	Talc is 1, quartz 7, diamond 10
Streak	Colour of the powder when rubbed on unglazed porcelain	Haematite looks grey or black but streaks red-brown
Lustre	How light reflects: metallic, glassy, pearly, dull	Pyrite is metallic, quartz is glassy
Cleavage and fracture	Whether it splits along flat planes or breaks unevenly	Mica peels in sheets; quartz breaks with curved fracture
Density	How heavy it feels for its size	Galena feels unusually heavy
Reaction to acid	Fizzing in weak acid (use with care)	Calcite fizzes; quartz does not

Colour alone is a poor guide, because the same mineral can come in many colours. Use several tests together, and say how confident the result is.

Classifying Rocks

Family	Main clue	Examples (check)
Igneous	Interlocking crystals; size shows cooling speed; no layers or fossils	Granite (slow, coarse), basalt (fast, fine), pumice (frothy glass)
Sedimentary: clastic	Grains of broken rock, sorted by size and cemented	Conglomerate, sandstone, shale
Sedimentary: chemical or organic	Mineral crystals or shells and plant remains	Limestone, rock salt, coal
Metamorphic: foliated	Minerals lined up in bands or sheets by pressure	Slate, schist, gneiss
Metamorphic: non-foliated	Interlocking crystals without banding	Marble, quartzite

Two questions get you far in the field: are the grains or crystals interlocking, or separate and cemented? Are they in layers? Interlocking and unlayered usually means igneous or non-foliated metamorphic. Separate grains in layers usually means sedimentary.

Reading Texture

Crystal size. Large crystals formed slowly, usually deep; fine grains formed fast, near the surface.
Grain size and sorting. In sedimentary rock, large rounded grains suggest strong flow and short travel; well-sorted fine grains suggest gentle water or wind.
Fossils. Shells and plant remains show a sedimentary origin and often the environment, such as a shallow sea.
Banding. Alignment of minerals suggests directed pressure, as in schist and gneiss (check).

A British comparison Edinburgh gives a ready field kit. Arthur's Seat is volcanic, with fine-grained basalt and coarser rocks from the same volcano (check); Salisbury Crags beside it is an intrusion of dolerite that cooled underground (check); and the sandstone of the Old and New Town buildings is sedimentary, quarried locally in the past (check). A walk from the Crags to a stone tenement wall covers two of the three rock families.

Check the details Mohs hardness numbers and mineral lists differ a little between sources, and rock names depend on exact composition, which needs a lab to confirm. Do not collect from protected sites, and test with acid only with proper safety advice.

Exercises

Exercise 1

You find a glassy, grey mineral that scratches glass and does not fizz in acid. Say which tests you used, what the most likely mineral is, and how sure you are.

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Exercise 2

Explain the difference between granite and gneiss in terms of minerals and texture, and why both can look similar at first sight.

→ Solution

Exercise 3

Describe a two-question field key (yes or no) that separates igneous, sedimentary and metamorphic rocks, and say one rock it would misclassify.

→ Solution

Quick Reference

Mineral: natural, solid, definite composition, ordered crystal structure
Tests: hardness, streak, lustre, cleavage, density, acid reaction
Common silicates: feldspar, quartz, mica, pyroxene, amphibole, olivine
Igneous clue: interlocking crystals, no layers
Sedimentary clue: cemented grains, layers, fossils
Metamorphic clue: banding or recrystallised texture