Physical Properties of Minerals
1) What is a mineral?
A mineral is a naturally occurring inorganic crystalline substance, whose compositions are either fixed or vary between certain fixed limits. This excludes, for example, artificial diamonds, coal, volcanic glass. What makes each mineral unique is a combination of its chemical composition and the internal arrangement of its constituent atoms.
The chemical composition may be fixed, as in quartz (SiO2) in which, for every single atom of silicon there are 2 atoms of oxygen. Or it may be variable for example the mineral olivine, which varies between two formulae; Mg2SiO4 and Fe2SiO4. Some minerals have a single chemical composition but a different arrangement of atoms in their three-dimensional structure. This is called “polymorphism” and is well seen in the polymorphs of carbon: graphite and diamond. One is grey/black and very soft, the other is usually colourless and is the hardest natural substance.
There are many different classes of minerals, but only the most common will be examined. These are: silicates (containing the SiO44^- anion), carbonates (with the CO3^2- anion), halides (which contain F^-, Cl), sulphides (with S^2-), sulphates (SO4^2-), oxides and native metals. The following are examples of each of these classes, which will become familiar in this lecture.
The extent to which atomic structure controls the outward shape of a substance is called its crystallinity. For example, quartz can develop in 4 ways. Firstly, as a crystal showing crystal faces, which is termed CRYSTALLISED.
Secondly, as irregular grains of quartz with fully developed internal structure, but not displaying crystal faces, which is termed CRYSTALLINE.
Thirdly, as a finely distributed crystalline aggregate called chalcedony whose grains are only visible under powerful magnification, this is termed CRYPTOCRYSTALLINE.
Fourthly, as a precipitate such as OPAL where there is no regular arrangement of atoms which is termed AMORPHOUS.
3) Physical Properties
What physical properties can be used to identify minerals?
In hand-specimen, minerals can be identified using a combination of physical properties, and it is these that will be studied in this lecture and practical.
a) Crystal Form/Habit
The characteristic shape of an individual crystal of a mineral is called its “habit”. Sometimes it is possible to identify a mineral from its habit alone, for example, quartz often forms a six-sided column with a set of pyramid-like faces at the top. We would describe this as prismatic.There are many common terms to describe the habit of crystals for example:
- acicular (needle-like);
- bladed (elongate crystals flattened in one direction);
- botryoidal (rounded masses looking like bunches of grapes);
- reniform (kidney like);
- fibrous (groups of parallel thread-like crystals);
- massive (no regular form);
- platy (very flat crystals);
- prismatic (elongated crystals with well developed prism faces);
- tabular (crystals slightly flattened in one direction).
A fully developed form is referred to as euhedral; the opposite, an
irregular form, is called anhedral.
A diamond is considered to be very hard because no other mineral can scratch it. Quartz can scratch a large number of other minerals. A scale has been devised to describe the hardness of minerals; Mohs’ scale. It varies from 1 (softest) to 10 (hardest). To test the hardness of an unknown mineral a specimen set of minerals called a hardness set is used. Starting with a hard specimen such as corundum the user tries with each in turn to scratch the unknown mineral until one is found that will not scratch the unknown mineral. The hardness of the unknown mineral is between the number of the mineral that will scratch it and the one that will not.
It is useful in the field to know the hardnesses of some everyday objects. For example, a fingernail has a hardness slightly over 2, and can scratch minerals of hardness 2 or less. Teeth have a hardness of around 4 and a copper coin 4.5 to 5; a steel knife blade is 6 and a hard file is around 7. Silicate minerals vary in hardness – the softest is talc (1) the hardest is topaz (8). Oxides, sulphides and many native minerals are very soft (less than 3), but exceptions include corundum Al2O3 which has a hardness of 9.
c) Specific Gravity / Density
Some minerals have an unusually high or low density (mass per unit volume), although most are between 2.5-3.0 g/cm^3. Specific gravity, or relative density, is the ratio of the mass of a mineral to the mass of an equal volume of water. However, it is difficult to estimate in the field except in some cases. Some common ore minerals are particularly heavy, for example, galena PbS (7.5), cassiterite SnO2 (6.9). This is because of the high atomic weight of the elements Pb and Sn. One commonly occurring mineral, barite (barytes) BaSO4, is also unusually heavy (4.5) compared with other superficially similar minerals such as calcite.
Colour is rarely a reliable indicator of the identity of a mineral. This is because in many minerals, impurities elements in trace amounts) can change the colour drastically. An example is a quartz, which is colourless when pure (“rock crystal”) but occurs in the coloured forms of amethyst (purple), rose quartz (pink), smokey quartz (grey) and others. Fluorite (CaF2) also occurs in purple, blue, green and yellow varieties. Some minerals are less variable, and in some cases, the colour is characteristic, for example, malachite (green), pyrite (golden), Galena (silver-grey). Nevertheless, it is better to identify these minerals on other characteristics in addition to the colour, for example, using cleavage or habit.
A slightly more reliable method of determining a mineral’s true colour is to use its streak. This is the name given to the powdered material left behind when a mineral is rubbed on an unglazed porcelain plate. Most pale-coloured minerals have a white streak which is of little use in identification, but the method is very useful with dark coloured opaque minerals such as hematite (Fe2O3) which has a characteristic red/brown streak, or pyrite (FeS2) which has a characteristic black streak.
f) Degree of transparency
Terms such as transparent, translucent and opaque are used to describe the degree to which a mineral can transmit light. However, this often depends on the thickness of a specimen and other factors such as internal inclusions. It is, thus, not a particularly useful guide for mineral identification. However, most ore minerals (pyrite, galena) are opaque, while many silicates are translucent to transparent.
Cleavage in minerals is the splitting or breaking of a crystal along planar surfaces which are determined by the crystal structure. There are often only a small number of possible cleavage planes in a mineral, whereas there can be many possible crystal faces. Cleavage of mica is along sheets, so that parallel smooth flat surfaces can be seen. This is due to weak bonds between the sheets in the structure of mica. As mica can only cleave in this one plane, it is said to have only one cleavage direction. Other minerals such as calcite have three excellent cleavage directions, giving a rhomboid shape. Distinctive patterns of cleavage are good identifying marks for many minerals. Galena and halite both have three good cleavages at 90°, yielding almost perfect cubes.
This was a rather boring post, but informative 🙂