The physical properties of minerals can be determined readily by inspection or by simple tests. Because the physical properties are determined in hand specimens, they are important in the recognition of the of minerals in the field. The chief physical properties are colour, streak, lustre, hardness, habit, cleavage, fracture, odour, feel, tenacity, fluorescence, magnetism, specific gravity, and crystal form.
The correct identification of minerals is made with the help of a polarizing microscope. This involves grinding the minerals or rocks into very thin slices and allowing polarized light to pass through them. In this way their optical properties are studied and minerals are identified. Opaque minerals, such as ores, are studied under the ore microscope in the reflected light. The optical properties of minerals are discussed in the end.
The colour of minerals is due to the absorption of certain wavelengths of light by atoms making up the crystals. The remaining wave-length of white light that are not absorbed give rise to the colour seen by the observer. Thus dark coloured minerals absorb most of the light, whereas red minerals reflect or transmit the red light and absorb all others.
Some minerals possess characteristics and fairly constant colour, for example, the lead-grey of galena, bass-yellow of pyrite, and green of chlorite. But in other cases such as quartz, the colour is variable and can not be relied on as a guide to identify minerals.
Presence of small amount of impurities can give a variety of colors to a white and colourless mineral. For example, the colour of amethyst and rose-quartz is due to due to the presence of titanium or manganese in traces. The most common colouring impurity is hematite. It imparts red color to many minerals including some felspar, calcite and jasper.
Some minerals, when viewed in different directions show irregular changes in colour tints. This is called ”play of colour”. The term ”opalescence” is applied to mineral, which show milky appearance. For example, opal. When bands of prismatic colours are seen on the surface of a minerals, it is said to show ”iridescence”.
The colour of the mineral powder is called ”streak”. It is more consistent and reliable than the body colour of the mineral. The streak is obtained by rubbing a mineral against on an un-glazed porecelain plate, called ”streak plate”. The study of streaks is most useful in case of coloured minerals, which often give a much lighter streak then their body colour. For example, hematite which appears almost black, gives a red coloured streak. However the streak is less useful for identifying most most of the silicate, carbonate, and transparent minerals because they give white streak.
Lustre is a very characteristic and useful property of minerals. It is a measure of the reflectivity of the mineral surface. The lustre may be defined as the general appearance of a mineral surface in reflected light. The various types of lustre are as follow;
- Metallic Lustre: Minerals, which have the appearance of metal, are said to have a metallic lustre, e.g. pyrite and galena.
- Sub-metallic Lustre: The feebly displayed metallic luster is called the ”submetallic lustre”, e.g chromite and hematite.
- Adamentine Lustre: A hard brilliant lustre like that of a diamond, ”adamantine lustre”. It is due to the mineral’s high index of refraction, e.g. transparent cerussite.
- Vitreous Lustre: It is the lustre exhibited by the broken glass, e.g. quartz.
- Pearly Lustre: It is the lustre exhibited by the pearls, e.g. muscovite, talc, and calcite.
- Silky Lustre: It is the lustre exhibited by the silk fibres. Minerals, which crystallize in fibrous habit commonly show silky lustre, e.g. asbestos and fibrous gypsum.
- Resinous Lustre: It is the lustre exhibited by the resin, e.g. sphalerite.
- Greasy Lustre: It is the lustre exhibited by the resin, e.g. talc and nepheline.
- Dull or Earthy Lustre: Minerals showing no lustre are said to possess dull or earthy lkustre, e.g. kaolin.
Hardness is one of the most useful diagnostic properties of a mineral. It is defined as the resistance of a mineral to abrasion or scratching. Hardness is determined by rubbing a mineral of unknown hardness against one of known hardness. A numerical value is obtained by using the ”Mohs scale of hardness”. In this scale, there are ten minerals, which are arranged in the order of their increasing hardness.
- Tale = Scratched by a fingernail
- Gypsum = Scratched by a fingernail
- Calcite = Scratched by a knife
- Fluorite = Scratched by a knife
- Apatite = Scratched by a knife
- Orthoclase= Scracely scratched by a knife
- Quartz = Scarcely scratched by a knife
- Topaz = Not scratched by a knife
- Corundum = Not scratched by a knife
- Diamond = Not scratched by a knife
In the absence of harness testing minerals, the following materials may be used to determine approximate hardness. (i). A fingernail will scratch up to about 2.5 (e.g. not calcite). (ii). A window glass will scratch up to about 5 (i.e. not a felspar). (iii). A penknife will scratch up to about 6.5 (i.e. not quartz).
The habit of a mineral may be defined as the size and shape of the crystals, and the structure or form shown by the crystal aggregates and cryptic crystalline masses. The chief habits shown by minerals are as follow;
- Acicular: Minerals showing needle-like crystals. For example, natrolite.
- Fibrous: Minerals showing an aggregate of long thin fibres, for example, asbestos and satinspar.
- Foliated: Minerals with platy habits commonly occur as foliated aggregates containing thin separable sheets, for example, muscovite and biotite.
- Bladed: Minerals showing bladed habit occur as small knife blades, for example, kyanite.
- Tabular: Minerals showing broad flat surfaces, for example, felspar.
- Columnar: Minerals showing columnar crystals, for example, tourmaline. The term stalactitic refers to columnar forms of minerals, such as calcite and aragonite.
- Botryoidal: Minerals showing aggregate of round masses resembling a bunch of grapes, e.g chalcedony.
- Reniform: Minerals showing kidney-shaped form, e.g. kidney iron ore.
- Granular: Minerals, which occur as aggregate of equidimensional grains, for example, chromite.
- Pisolite: Minerals, which occur as aggregate of rounded grains of a pea size, for example, bauxite.
- Oolitic: Minerals showing an aggregate of bodies resembling fish roe. In this case the rounded grains are of the size of a small pinhead.
- Massive: When non-crystalline or cryptocrystalline minerals occur as structureless mass, their habit is described as ”massive”, for example, flint.
If a mineral breaks along a flat plane, it is said to possess a cleavage, if it breaks with an irregular surface, it is said to show a fracture. Thus the cleavage may be defined as the tendency of a mineral to break more easily with smooth surfaces along planes of weak bonding. Hence cleavage is the property, which is related to the atomic arrangement within the mineral. Because the cleavage always occurs parallel to a possible crystal face, it is designated in terms of crystal face to which it lies parallel, such as cubic, octahedral, prismatic, basal, and so on. k For example, galena has cubic cleavage, fluorite has octahedral cleavage, mica has basal cleavage, and calcite has rhombohedral cleavage.
Depending on the ease with which a crystal cleaves and the perfection of the surface obtained, the cleavage is classified as perfect, good, poor, and indistinct. Examples of minerals, which show perfect cleavage are mica, galena, calcite, and fluorite. Quartz has no cleavage at all.
Minerals, which do not exhibit cleavage, break with an irregular surface. The nature of this broken surface is called ”fracture”. In case of fractures, the breaking should be in any other direction than the cleavage. Unlike the cleavage, the fracture does not produce a smooth surface. The common types of fractures are as follow:
- Conchoidal Fracture: It is a curved fracture surface showing concentric lines like a shell. Quartz and glass show conchoidal fractures.
- Even fracture: It is a fracture surface, which is almost flat. Flint shows even fractures.
- Uneven fracture: It is a fracture surface, which is irregular and rough. A large number of minerals show uneven fractures.
- Hackly fracture: It is a fracture surface, which is rough with sharp and jagged points. Native metals show a hackly fracture.
Some minerals give a characteristic smell, when rubber, breathed upon or heated up. The chief types of orders are as follow:
- Arsenical: The arsenical odor is like the smell of garlic. Orpiment or other arsenic minerals give an arsenical odor.
- Sulfurous: This order is like the order of burning sulfur. Pyrite gives a sulfurous odor.
- Argillaceous Odor: This order is like the order of clay. Kaolin gives an argillaceous odor.
Feel is the sensation upon touching or handling a mineral. The different types of feel are greasy, soapy, rough, and harsh. An example of a greasy feel is talc, that of soapy is kaoline, and that of rough is bauxite.
The tenacity of a mineral denotes the degree or character of cohesion. Tenacity is classified as follows:
- Sectile: Minerals, which may be cut with knife, but slices are not malleable.
- Malleable: Minerals, which flatten under hammer.
- Flexible: Minerals, which may be bent.
- Elastic: Minerals that spring back after bending.
- Brittle: Minerals, which break easily on bending.
- Friable: Minerals, which crumble easily.
- Pulverulent: Minerals, which are powdery and have little or no cohesion, e.g. clay or chalk.
Some minerals when exposed to light or ultraviolet light, produce a colour quite different their own. Thus green or colourless flourite shows a blue or purple colour in ultraviolet light. This property of minerals is known as ”fluorescence”. The other minerals, which often show fluorescence are calcite and scheelite.
Some minerals glow and emit light when they are placed in ultraviolet light or certain other electrical radiations. The glow induced in the mineral may continue for few seconds, or minutes, after the removal of the cause. This property of minerals is called Phosphorescence. The example of minerals, which show phosphorescence are diamond and sphalerite.
A few minerals are attracted by a magnet. Of these minerals, magnetite and pyrrhotite are the most common examples. The magnetite that possesses attracting power and polarity is called ”Iodestone”.