Science, Maths & Technology

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An introduction to minerals and rocks under the microscope

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# 1.7 Summary of Section 1

1. Matter exists in the form of gases, liquids and solids, and the arrangement of atoms becomes progressively more ordered from gases to solids. The stability fields for the three states of a chemical element or compound are shown in a pressure-temperature plot known as a phase diagram.
2. Physical characteristics of minerals evident in hand specimen include crystal shape, colour, lustre, cleavage, density and hardness. Colour may be misleading, as minute amounts of impurities can affect the colour of some minerals. Cleavage, density and hardness are strongly related to the underlying atomic structure.
3. Atoms are bonded together by three different mechanisms: metallic bonding in which a 'sea' of electrons holds the metal cations strongly together, giving dense, closely packed structures; ionic bonding where electrons are transferred between atoms, producing positive and negative ions that are strongly attracted to each other; and covalent bonding where electrons are shared, resulting in open, low-density crystal structures, which are strongly bonded. About 90% of all minerals are essentially ionic compounds.
4. Crystals may have several different types of defect that can strongly influence the mineral's physical and chemical properties.
5. Many geological processes - rock formation, rock deformation, weathering and metamorphism - are controlled by processes operating at much smaller scales, such as the movement of atoms in crystals (diffusion), the breaking of atomic bonds within crystal structures, the initiation and growth of new crystals and phase transformations.
6. Various types of crystal twinning exist, and in each case the twin is a single crystal that consists of two or more regions in which the crystal lattice is differently orientated.
7. The external shape of a crystal (i.e. the arrangement of crystal faces) is controlled by its internal structure. Crystals are composed of atoms arranged in repeating patterns that can have two-, three-, four- or six-fold symmetry. Each repeating pattern is located at a lattice point. A three-dimensional crystal lattice is a structure formed by countless numbers of identical tiny building blocks, called unit cells. Unit cells have a box shape, which can be defined by the length of the three sides of the unit cell (a, b and c) and the angle between the axes of the unit cell (α, β and γ). Variation in the shape of the unit cell results in different symmetry elements (rotation axes and reflection planes), but all crystals may be ascribed to one of seven crystal systems.
8. When looking at crystal symmetry, the angles between faces are more important to consider than the absolute sizes of individual faces, as conditions during the growth of a crystal often prevent some faces from developing as perfectly as they might, or from developing at all.