A mineral is a naturally occurring solid with a definite chemical composition and a
Answer: A
Composition and structure together define the species, which is why the same substance formed artificially is not strictly a mineral. Colour varies with trace impurities and is one of the least reliable identifying properties.
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A rock differs from a mineral in being
Answer: B
Correct answer: an aggregate of one or more minerals
A mineral is a naturally occurring solid of definite composition and ordered internal structure; a rock is an assemblage of mineral grains held together. Most rocks contain several minerals, though some - a pure limestone, a quartzite - contain essentially one, which is why the definition says one or more.
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Streak is a more reliable identifying property than colour because streak is the colour of the
Answer: C
Drawing the specimen across unglazed porcelain removes the effect of surface films and of the coarseness of the crystal. Haematite is the classic case, appearing black or grey in the mass yet always giving a red-brown streak.
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Cleavage differs from fracture in following
Answer: D
A mineral cleaves along the planes where the bonding is weakest, so the surfaces are flat and reproducible whatever direction the blow comes from. Fracture is irregular breakage where no such plane exists, as in quartz.
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Quartz breaks with a conchoidal fracture rather than cleaving because its structure is a
Answer: A
Correct answer: framework of tetrahedra bonded equally in all directions
Cleavage requires planes of weaker bonding, and in quartz every oxygen is shared between two tetrahedra so the silicon-oxygen bonds run in all directions with equal strength. There is no weak plane for a break to follow, so a fracture takes the shell-like conchoidal form instead - the same reason flint and glass break as they do.
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Mica cleaves into thin elastic sheets because its structure consists of
Answer: B
The bonds within a sheet are strong and those between sheets weak, so the mineral splits into flakes in one direction only. That perfect basal cleavage is what makes mica a plane of weakness in any rock containing much of it.
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On the Mohs scale, hardness 1 and hardness 10 are represented respectively by
Answer: C
The scale is ordinal rather than proportional, so diamond is very much more than ten times as hard as talc. Quartz at 7 is the practical dividing line, since it will scratch window glass and a steel knife will not scratch it.
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A mineral that can be scratched by a steel knife but not by a copper coin has a hardness of about
Answer: D
Correct answer: 4 to 5
A copper coin is about 3.5 on the Mohs scale and a steel knife about 5.5, so a mineral scratched by the knife but not by the coin lies between them. The field tests are chosen for exactly this purpose: fingernail at about 2.5, coin at 3.5, knife at 5.5 and glass at about 5.5 bracket the common minerals.
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Effervescence with dilute hydrochloric acid identifies a mineral as a
Answer: A
Correct answer: carbonate
Dilute hydrochloric acid liberates carbon dioxide from a carbonate, producing the visible effervescence. It is the quickest field test there is, and it separates calcite and the limestones from everything else at a stroke. Dolomite is the qualification: being a carbonate it reacts, but only slowly unless powdered or warmed.
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The basic unit of every silicate mineral is the
Answer: B
Correct answer: silicon-oxygen tetrahedron
Every silicate is built from a silicon atom surrounded by four oxygens at the corners of a tetrahedron. The whole classification of the silicates follows from how those tetrahedra are joined - isolated, in chains, in double chains, in sheets or in a framework - and so, in turn, do their cleavage and their hardness.
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