IMPERIAL COLLEGE DEPARTMENT OF EARTH SCIENCE & ENGINEERING Lecture 2: PHYSICAL PROPERTIES OF MINERALS 1. Crystal habit and crystalline aggregates The habit of a crystal is its general shape produced by the relative development of its different crystallographic forms. The habit is a function of the conditions of crystallisation and the crystal structure and thus can be highly variable. The form of mineral aggregates is obviously a function of the habit of the individual crystals making up the aggregate. Crystal Habit Crystalline Aggregate Equidimensional Granular Fine grained equidimensional Massive or compact Prismatic (or columnar) Bladed Acicular Parallel or radiated Fibrous Foliated Platy Radiated Concentric 2. Twinned Crystals Contain two or more parts of the crystal which are physically continuous but in different crystallographic orientations, symmetrically related to each other. Generally classified into simple or contact twins, repeat or polysynthetic twins and interpenetrant twins. They form during crystal growth /transformation or by deformation 3. Density Function of packing density of atoms in the crystal structure AND the mass of individual atoms. 4. Hardness The resistance of minerals to mechanical pressures Mohs Scale of hardness 1. Talc 6. Orthoclase 2. Gypsum 7. Quartz 3. Calcite 8. Topaz 4. Fluorite 9. Corundum 5. Apatite 10. Diamond. Fingernail 2-3; Copper coin 3.5; knife blade 6 5. Cleavage Many minerals possess a tendency to split along one or more planes determined by the crystal structure. The ease with which a mineral may be cleaved, and the number and relation of the planes are valuable aids to identification Among minerals characterised by a single perfect cleavage are the micas. The cleavage can be quantitatively described as: Perfect e.g. mica, calcite, galena Good e.g. feldspars Distinct e.g. chalcopyrite Indistinct e.g. olivine 6. Fracture Unlike cleavage planes fracture surfaces are uneven and are not parallel planes throughout the crystal nor are they related to the crystal faces. Described qualitatively, e.g. conchoidal, even, uneven, splintery. 7. Properties dependent on light 1. COLOUR Some minerals have a distinctive colour due to their composition or physical structure. These are known as idiochromatic minerals, eg. malachite - green; native sulphur - yellow. Other minerals may show a wide variety of colours according to impurities present or slight differences of composition. These are known as allochromatic minerals, e.g. varieties of quartz, fluorite. STREAK is the colour of the mineral in a powdered form, and is determined by scraping the mineral on a streak plate of unglazed porcelain. The streak is usually constant for a particular mineral, e.g. iron oxide minerals - haematite Fe203 (red streak), magnetite Fe304 (grey streak) 2. LUSTRE is the appearance of the surface of the mineral in reflected light that is largely independent of the colour of the mineral. The following kinds are distinguished: a) Metallic lustre - shown by the native metals and by the metallic sulphides and oxides, e.g. pyrite (FeS2), haematite (Fe203). b) Non-metallic lustre - many different varieties are recognised. i) Vitreous - lustre of glass, eg. quartz. ii) Resinous - e.g. zinc blend, opal. iii) Pearly - e.g. dolomite, talc. iv) Silky - fibrous minerals, eg. asbestos, satin spar gypsum v) Adamantine - e.g. diamond vi) Greasy - e.g. weathered halite. vii) Dull or earthy - minerals with no lustre, eg. kaolinite.