Minerals

What makes something a mineral. A mineral is a natural, solid substance found in the Earth's crust with a fixed chemical composition — the same defining property that marks out any pure substance in chemistry. This fixed composition is the reason minerals slot neatly into the element/compound framework already built earlier in this chapter, rather than needing a category of their own.

Rocks versus minerals. Rocks are not themselves minerals — a rock is typically a mixture of minerals, several different minerals physically combined together, each one keeping its own separate identity and its own fixed composition inside the rock. Granite, for instance, is a mixture that includes minerals such as quartz and mica side by side. This is exactly the same idea used earlier in the chapter for lab mixtures like iron filings stirred with sulfur: the components sit together without losing their individual identities, and in principle they could be picked apart again.

Native minerals — the elements. A small set of minerals occur as pure elements, called native minerals. The examples given in this chapter are gold, silver, sulfur, and carbon. These are minerals precisely because they are naturally occurring solids with fixed composition — but their composition happens to be a single element, so they fall on the "element" side of the framework.

Most minerals — the compounds. The larger group of minerals are compounds: quartz, calcite, mica, pyroxene, olivine, and talc are the examples named in this chapter. Each has a fixed chemical composition made of more than one element chemically combined — the same defining test used earlier in the chapter for compounds like water, formed from hydrogen and oxygen locked together in a fixed ratio.

From the ground to the store shelf. Two everyday links tie minerals to familiar products. Cement is made using the compound minerals calcite, quartz, alumina, and iron oxide. Talcum powder is made from the compound mineral talc, simply ground into a fine powder. Both examples show the same journey: a naturally occurring compound mineral, processed by grinding, mixing, or heating, becomes a manufactured product used every day.

This topic has no numeric formula to derive — the classification is purely conceptual — so it is worth walking carefully through why the framework applies to minerals in exactly the way it applies to laboratory substances, since that is the reasoning skill being tested.

Step 1. Recall what "fixed chemical composition" actually distinguishes. Earlier in the chapter, a pure substance — whether element or compound — was defined by having one fixed composition throughout, while a mixture could vary in the proportions of its components. A mineral is defined as having a fixed chemical composition, so by that definition alone a single named mineral (quartz, gold, talc) must be a pure substance, never a mixture.

Step 2. Decide element or compound by asking how many elements are present. If a mineral's fixed composition consists of just one element — as with gold, silver, sulfur, and carbon — it is a native mineral, an element. If its fixed composition consists of two or more elements chemically combined — as with quartz, calcite, mica, pyroxene, olivine, and talc — it is a compound mineral. This is the identical test used earlier in the chapter to sort laboratory substances: count the elements in the fixed composition.

Step 3. Recognise why a rock breaks that pattern. A rock is not one substance with one fixed composition — it is several minerals occurring together, and the proportions of quartz, mica, and other minerals can differ from one rock sample to the next, and even from one part of a rock to another. Because the composition is not fixed and the components (the individual minerals) keep their own separate identities, a rock satisfies the definition of a mixture, exactly like iron filings mixed with sulfur or salt dissolved in water.

Step 4. See the two-level structure this produces. At the level of an individual mineral, the element/compound test applies directly. At the level of a rock made of several minerals, the mixture test applies. A single rock sample can therefore be described at two levels at once: it is a mixture overall, built from components that are themselves either elements or compounds.

Question: A geology sample is described as "a rock containing quartz, mica, and a small amount of native gold." Classify the rock overall, and classify each of its listed components.

Step 1. Classify quartz. Quartz is named in this chapter as an example of a mineral that is a compound, so quartz = compound.

Step 2. Classify mica. Mica is also named in this chapter as an example of a mineral that is a compound, so mica = compound.

Step 3. Classify native gold. "Native" signals a native mineral — gold occurring as a pure element rather than combined chemically with other elements — so gold = element.

Step 4. Classify the rock as a whole. The rock contains three different minerals (quartz, mica, gold), each keeping its own separate identity and fixed composition, simply combined together physically rather than chemically combined with each other. By the mixture test from Step 4 of the reasoning above, the rock as a whole is a mixture of minerals.

Final answer: quartz = compound, mica = compound, gold = element (native mineral); the rock overall = a mixture.

Competitive stretch

Question: Talcum powder is made from the mineral talc, and cement is made using calcite, quartz, alumina, and iron oxide. Is either everyday product itself best classified as a compound, or as something else?

Step 1. Talc, on its own, is a compound mineral — but talcum powder is talc ground into a fine powder, not chemically altered, so pure talcum powder shares talc's classification as a compound.

Step 2. Cement is different: it is made using several compound minerals together (calcite, quartz, alumina, iron oxide), combined and processed rather than being a single fixed-composition substance. That combination of several starting minerals makes cement best described as a mixture (or, after processing, a manufactured material built from several compounds) rather than a single compound.

Step 3. The contrast matters for the exam: "made from one mineral, unchanged in identity" (talcum powder from talc) keeps the mineral's own compound classification, while "made using several minerals combined" (cement) produces a mixture-like material, the same logic that made a multi-mineral rock a mixture in the worked example above.

• Assuming all minerals are elements because they occur "naturally" — most named minerals in this chapter (quartz, calcite, mica, pyroxene, olivine, talc) are actually compounds; only gold, silver, sulfur, and carbon are native, pure-element minerals.
• Confusing "rock" with "mineral" — a rock is typically a mixture of several minerals, while each individual mineral within it has its own fixed composition and is itself an element or a compound.
• Thinking a mineral's "fixed chemical composition" makes it automatically a mixture — a fixed composition is the hallmark of a pure substance (element or compound); it is the rock, not the mineral, that varies in composition and is therefore the mixture.
• Trying to memorise detailed mineral or ore chemistry, or metal-extraction processes — this chapter stays at the naming level (which minerals are native elements, which are compounds); detailed metallurgy is beyond Class 8 scope.

Ask the tutor: Want to try classifying a few more real rocks and products you encounter around the house using this same element/compound/mixture test?