Mineral Properties & Identification ‧₊˚
The main physical properties of minerals include:
Colour is often the most eye-catching feature of a mineral. Some will always have a similar colour, such as Gold, whereas some minerals come in a variety of colours, like Quartz & Calcite. The presence of certain elements will usually determine a specific mineral's colour. A great example of this is the beautiful green Malachite, which gets its inherent colour from copper in its atomic structure. There are many minerals which instead get their brilliant hue from slight additions of colour-causing elements. For example, pure Quartz is colourless, however Amethyst, a purple variety of quartz, has a violet colour caused by traces of the element iron. The amount of iron present in the Amethyst determines the intensity of its colour. Certain minerals exhibit a colour change, fade or darken when exposed to light, heat or radiation. These methods are commonly utilised in the gemstone industry to artificially enhance the colour of many gemstones & minerals.
To simplify such a wide, complex topic, we will look at three main groups of minerals identified on the basis of the property of colour:
Hardness plays an important role in the mineral identification process. It is a way to measure the strength of the structure of the mineral relative to the strength of its chemical bonds. Minerals with small atoms, packed tightly together with strong bonds throughout, tend to be the hardest minerals.
In simple terms, hardness is defined by how well a substance will resist scratching by another substance, which can be better understood by using the Moh's Scale of Hardness. The scale consists of numbers one through ten, with one being the softest & ten being the hardest. Each number represents a different mineral - each one harder than the previous.
The process of testing a mineral’s hardness begins by "scratching" one mineral with the other (or another material in which the hardness is known). Using the above scale as a guide, we can conclude that both Gypsum & Talc are soft enough to be scratched by a fingernail. Suppose a mineral scratches Fluorite, but not Apatite - it is clear that this mineral has a hardness between 4 & 5. A Topaz will leave a scratch on Quartz or any other mineral under a 8, but not on a 9 or 10. To determine if you have been given a piece of fake glass instead of real Quartz, use a 6, like Feldspar, to test whether the hardness is that of glass (5.5) or quartz (7).
A streak is the colour of a crushed mineral's powder, that can be determined by preforming a streak test, in which a mineral is scraped across a piece of unglazed porcelain known as a "streak plate." This process can be a helpful identifying factor, particularly when needing to distinguish between two minerals with a similar appearance. An example of where a streak test is useful is in the case of Gold & Chalcopyrite. While these minerals may look similar, they have vastly different streaks - with Gold displaying a bright yellow & Chalcopyrite showcasing a greenish-black streak instead.
Specific Gravity is a measurement that determines the density of minerals - as in how heavy it is by its relative weight to water. Water has a specific gravity of 1.0 so, if a mineral has a specific gravity of 2.7, it is 2.7x heavier than water. Minerals with a specific gravity under 2 are considered light, between 2 & 4.5 average, & greater than 4.5 heavy. As an example, Gold has a specific gravity of 19, meaning Gold is 19x heavier than water & therefore considered a heavy mineral.
Lustre describes how a mineral appears to reflect light, & how brilliant or dull the mineral is. The terms used to describe lustre are:
Tenacity describes a mineral’s reaction to the stress of crushing, breaking, bending, or tearing. The different forms of tenacity are:
- Brittle - a mineral that is hammered & results in fine powder or crumbs. Most minerals are brittle. E.g. Quartz, Calcite & Sulphur.
- Sectile - a mineral that can be separated with a knife. E.g. Gypsum.
- Malleable - a mineral that can be flattened by pounding with a hammer. All true metals are malleable. E.g. Silver.
- Ductile - a mineral that can be stretched into a wire. All true metals are ductile. E.g. Gold.
- Flexible but inelastic - any mineral that can be bent, but continues to remain in the new position after. E.g. Copper.
- Flexible & elastic - a mineral that is bent, but springs back to its original position. E.g. Mica.
In mineral terms, cleavage describes how a crystal breaks when subject to stress on a particular plane. If part of a mineral breaks due to stress, & that broken piece retains a smooth plane or crystal shape, the mineral has cleavage. This is often measured by three factors:
- Quality of Cleavage - Cleavage can be categorised into five qualities: perfect, good, poor, indistinct, or none. Perfect cleavage refers to a mineral that cleaves without leaving any rough surfaces; such as Calcite, Muscovite, Lepidolite or Barite. Some minerals, like Quartz, have no cleavage at all, as their broken surfaces appear fractured & rough rather than smooth.
- Number of Sides Exhibiting Cleavage - Many minerals
exhibit cleavage only on one side, though some may exhibit different quality cleavage on different crystal sides. Minerals that cleave in one direction include Biotite & Muscovite. Two direction minerals include Orthoclase & Plagioclase, which cleave approx. 90 degrees from each other. Three direction cleavage is seen in minerals such as Calcite, whereas four direction cleavage can be recognised in minerals such as Fluorite.
- Cleavage Habit - Different habits of cleavage exist on different minerals, depending on their mode of crystallisation.
These forms of cleavage are:
Fracture refers to the characteristic mark left when a mineral chips or breaks. While cleavage & fracture both have to do with the positioning of atoms in a mineral & how it breaks when put under stress, fracture deals more with the "chipping" shape of a mineral rather than the break of a new crystal face. All minerals exhibit a fracture.
There are several terms to describe various mineral fractures:
Fluorescence & Phosphorescence
Fluorescence is a phenomenon that causes a mineral to "glow" when exposed to a specific wavelength of light such as ultraviolet light, electron beams or x-rays. Only about 10% - 15% of minerals have a fluorescence that is visible to humans & these minerals must contain 'activators' (cations of metals such as tungsten, lead, titanium, uranium etc.) in specific concentrations. There are two classified ultraviolet wavelengths: longwave & shortwave. Some minerals can fluoresce the same colour in both wavelengths, others fluoresce in only one wavelength, & others can fluoresce different colours in different wavelengths. Common fluorescent minerals include Fluorite, Corundum & Calcite.
Phosphorescence is a phenomenon exhibited by several fluorescent minerals where the mineral continues to glow even after the UV light source has been removed. The glow slowly fades after several seconds or minutes. Examples of phosphorescent minerals include Celestite, Sphalerite, Fluorite.
Triboluminescence & Thermoluminescence
Triboluminescence is a flash of light produced when a material is subjected to impact, friction or breakage. These flashes of light produced are often white or orange, but other colours are possible. About 50% of crystalline materials are thought to exhibit this property. Some examples of triboluminescent minerals include Quartz, Sphalerite, Fluorite, Calcite, Muscovite, & some varieties of Common Opal.
Thermoluminescence is luminescence that arises upon heating of a mineral. This emission of light can be used to date materials containing crystalline minerals to a specific heating event. This is especially useful for identifying the age of ceramics, as it determines the date of firing, as well as for lava or sediments.
Three electrical properties are applicable to minerals:
- Conduction - the ability of a mineral to conduct electricity, which can be found in only a very small number of minerals. These conductors can be placed between a wire carrying electricity, & the electricity will pass through. Examples of conductive minerals include Graphite, Galena, Silver & Copper.
- Pyroelectricity - the ability of a mineral to develop electrical charges when exposed to temperature changes, either when heated or cooled. As a result of this change in temperature, positive & negative charges move to opposite ends of the crystal through migration & so, an electrical charge is established. Examples of pyroelectric minerals include Tourmaline & Quartz.
- Piezoelectricity - the ability of a mineral to develop electrical charges when put under stress, such as being rubbed or struck repeatedly. Examples include Tourmaline, Quartz, Topaz & even cane sugar.
Several minerals react when placed within a magnetic field, with certain minerals being strongly attracted to the magnet (ferromagnetism), & others being weakly attracted (paramagnetism). In almost all cases, the presence of the element iron as a component of the mineral's chemical structure is responsible for its magnetic properties. Common minerals that carry an attraction to magnetic fields include Magnetite, Hematite & Pyrrhotite.
The mineral Bismuth displays an interesting property known as diamagnetism, meaning it is repelled from magnetic fields. Another fascinating mineral is Lodestone, which can act as a magnet, generating magnetic fields all on its own.
Light & Colour Effects
Light & colour effects refer to the manner in which light interacts with the crystalline structure, inclusions, & internal structure of a mineral. The reflection, diffraction, absorption, &/or diffusion of that light results in a beautiful & magical display of optical phenomena. There are several types of light & colour effects, that often play an important role within the gemstone industry: