Minerals are inorganic solids with a definite chemical composition and crystal structure. They are classified by their chemical composition and crystal structure. Minerals can be identified in a laboratory by their physical and chemical properties. Some of these properties include hardness, color & streak, luster, and density. Let's discuss each one a bit:
Hardness relates to how hard or soft a material is. The softest mineral is Talc, which has a hardness of 1; the hardest mineral is Diamond, which has a hardness of 10. Hardness is measured on a logarithmic scale, called the Moh's Hardness Scale, relative to known materials. Common materials used to test hardness are your fingernail (hardness of about 2.5), a penny (hardness of 3), an iron nail (hardness of about 5), a pane of glass (hardness of 6.5), and a steel file (hardness of about 8). The official full hardness scale is available through the American Federation of Mineralogical Societies and it can be compared with the hardness of most known minerals to identify a mineral by its hardness.
2) Color & Streak
Two different colors are recorded for a sample. The color of the mineral sample itself is called Color; and the color of the crushed mineral powder, identified by the mark it leaves behind when scratched against a white ceramic tile, is called Streak. Some samples are also examined under special lights to determine if they fluoresce or "glow-in-the-dark".
If you wish to practice identifying these properties virtually, you may do so at the Virtual Color Testing Lab and the Virtual Streak Testing Lab. You can also try to complete these Exploring Color and Exploring Streak practice exercises.
Luster is the scientific word for shininess. When examining the color and general appearance of a mineral, its sheen can also be noted, using specific science terms like metallic - shiny like a metal, or dull - like most rocks. It is important to note that luster is separate from a mineral's optical properties - how the mineral interacts with light; although these may also be considered.
If you wish to practice identifying these properties virtually, you may do so at the Virtual Visual Properties Testing Lab. You can also try to complete these Exploring Visual Properties practice exercises.
Fracture describes the edges formed when the mineral is cut or broken; and cleavage describes the flat, smooth, planar surfaces cut or broken mineral crystals have. The ability of a material to cleave and the number of planes of cleavage and type of fracture will depend on its internal arrangement, or crystal structure. Some minerals will produce a spark or glow - called triboluminescence - when broken. When examining the cleavage and fracture, one can also talk about how likely a mineral is to break when stress is applied; this is done using terms like malleable, ductile, or brittle; although they are different properties.
In advanced mineralogy classes, you will also examine the crystal structures which give minerals their properties. If you wish to practice visually identifying crystal structures virtually, you may do so at the Virtual Crystal Structures Lab. You can also try to compete these Exploring Crystal Structures practice exercises. There is a resource called the database of crystal structures online. Furthermore, you will practice identifying body-centered, edge-centered, and all sorts of other crystal shapes and arrangements. You'll examine the chemistry of ionic compounds that leads to this crystal structure. You may even do so in a laboratory, using X-Ray diffraction or an electron microscope and the properties of specific elements and compounds.
Density = mass/volume. Although the density of a mineral can directly be calculated from the mass and volume of a specimen, typically whether it floats or sinks in water is enough information for classification.
6) Other Unique Properties
Certain minerals can also be identified by other special properties, which are unique to them!
Some minerals have a unique feel to them. Diamond, for example, feels cold because of its thermal properties; while talc feels greasy or oily. A few minerals are radioactive and can be identified using a Geiger counter to detect their radioactivity. A few minerals - like magnetite and pyrrohite - are ferromagnetic, meaning they will readily attract iron. These are the minerals magnets are frequently made out of.
Lastly, some ionic minerals - like chalcanthite - dissolve easily and quickly in water. A few of these - like calcite - have a salty taste to them, and some of them - like aragonite - will react with acid, forming bubbles. A few minerals - like pyrite - also have a distinct smell or taste, other than salty. Note: You should never lick or taste an unknown mineral sample until you've ruled-out the possibility that it might be poisonous!
If you wish to practice identifying these properties virtually, you may do so at the Virtual Special Properties Testing Lab. You can also try to complete these Exploring Special Properties practice exercises.
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