Introduction to Mineralogy

Introduction to Mineralogy

• Minerals are the building blocks of rocks
• Rock-forming minerals are minerals that form rocks
• Other terms used are industrial minerals and ore minerals

Defining a Mineral

• Minerals are naturally occurring, inorganic solids.
• Minerals are made by geological processes and have a crystalline structure and a definable chemical composition.
• Examples of minerals include copper, ruby, diamond, olivine, and calcite
• Examples of non-minerals include steel, obsidian, synthetic sapphire, sugar, plastics, and amber.

Atoms and Crystals

• Minerals are made of atoms attached to each other by atomic bonds, and crystals are often used to describe a mineral.
• A Crystal is a single continuous piece of crystalline solid, typically bounded by flat surfaces called crystal faces.
• Crystal structures are not random processes and are defined by their chemical formula for example quartz is SiO2
• Elements combine into minerals and minerals into rocks

Atomic Bonds

• Atoms share or donate electrons to bond together, forming covalent or ionic bonds
• Sharing electrons makes covalent bonds, donating makes ionic bonds.
• In covalent bonding, atoms share electrons, resulting in strong bonds, termed electron sharing
• In ionic bonding, atoms donate or accept electrons. This makes positively charged ions (cations) and negatively charged ions (anions) that attract each other. This is termed electron transfer
• In metallic bonding, electrons are delocalized and shared, allowing them to move and conduct electricity. This is termed electron delocalisation
• Bond types influence properties such as strength, hardness, solubility, conductivity, and examples include Halite, fluorite, Diamond, Copper, silver and gold.

Bonding and Packing

• Bonding and packing in minerals is related to valency, bond strength, melting point, and hardness.
• Periclase (MgO) has Mg2+ O2- valency 2, four times stronger bonds, a melting point of 2800°C, and a hardness of 6.5.
• Halite (NaCl) has Na+ Cl- valency 1, a melting point of 801°C, and a hardness of 2.5.
• Graphite has covalent bonds and Van der Waals bonds, while Diamond has covalent bonds alone
• Mineral properties are highly dependent on covalent bonding

Mineral Composition

• Atoms are arranged in an ordered framework and bond together
• Minerals can consist of one or several different atoms (e.g. Diamond is made of Carbon atoms and Pyrite is made of Iron and Sulphur (FeS2)
• Some have very complex makeup such as Tourmaline with the formula Na(Mg, Fe, Mn, Li,Al)3Al6Si6O18(BO3)3(OH,F)4

Identifying Minerals

• Minerals cannot be identified by looking at individual atoms
• Several properties are used to distinguish minerals
• The properties we can observe are key in the identification process.
• To identify minerals, look at properties in hand specimen, and consider optical properties and advanced methods (X-ray diffraction, electron microprobe analysis)

Colour in Minerals

• Colour results from the way the mineral interacts with light.
• The mineral's structure and composition determine which wavelengths of light are absorbed
• The colour we see are the wavelengths that are not absorbed.
• While colour is a useful characteristic, it is not a conclusive identification feature
• Some mineral colour is due to the chemical composition and this is called idiochromatic
• Idiochromatic minerals tend to have a constant colour, which can be used as an identifying feature; for example, Azurite (always blue), Cinnabar (always red) and Malachite (always green)
• Allochromtic minerals vary in colour because of trace elements
• Colour can also be caused by defects in the atomic structure
• The 'false' colour in minerals results from optical effects is known as Psudochromatic, opals and labradorite are examples of this.

Streak Colour

• Streak colour is the colour of a mineral in its powdered form
• A mineral is rubbed on a streak plate (white unglazed tile), and the colour of the mark is recorded.
• The powder's colour is less variable than the crystal's colour. For instance, calcite always produces a white streak, even if the crystal is pink, yellow, or uneven in colour.
• Distinctive colours of some minerals can help with identification, but this test is not always definitive.

Lustre

• Lustre describes how the surface of the mineral scatters light.
• Appearance of surface is compared with a common material.
• Terms used to describe lustre include earthy, soapy, waxy, resinous, metallic, glassy, pearly, silky, and vitreous.
• Quartz is often described as glassy, galena and pyrite as metallic, muscovite as pearly and hematite as earthy

Hardness

• Hardness measures a mineral's ability to resist scratching and is a surface feature
• Toughness is an internal feature to not be confused with hardness
• Diamonds are hard but not tough, corundum is tough but not as hard as a diamond
• The hardness of a mineral represents the resistance of bonds in the crystal structure to being broken
• The atoms and ions in a hard mineral are more strongly bonded than those in a soft mineral
• Mohs Hardness scale is used to help in mineral identification
• Mohs hardness scale helps in mineral identification when used with other tests
• The Mohs scale does not reflect the true difference in hardness between minerals since example talc has a hardness of 1 and quarts has a hardness of 7, but quartz is almost 100 times harder than talc
• Never test the hardness of a display specimen or a gemstone

Specific Gravity (SG)

• Specific gravity (SG) represents the density of the mineral expressed by ratio of the weight of a volume of mineral and the weight of an equal volume of water at 4°C

Crystal Habit

• Crystal habit is the general appearance of a mineral and the shape of a single crystal. A well formed crystal with good crystal faces helps to identify the mineral habit.
• The crystal habit is a direct result of the internal arrangement of atoms
• Atomic arrangement controls the geometry of the crystal faces.
• Some minerals form a single well formed crystal, other form clusters of little crystals.
• Some minerals form blocky crystals, others form long slender crystals
• Crystal growth rate that grows at that same rate in all directions result in blocky crystals
• Crystal growth rate that grows preferentially in one direction presents needle-like or sheet like results
• The dimensions depend on growth rate in different directions.

Crystal Systems

• Minerals can be classified according to the 7 crystal systems.
• These are describing the atomic arrangement of a unit cell (a single building block)
• The 7 crystal systems can also be classified according to their symmetry

Fractures and Cleavage

• Cleavage is a direct result of the atomic structure since it is a weakness in the structure. As a result, the mineral preferentially breaks along cleavage planes
• Fractures are random breaks (not linked to regularly repeating weakness planes) Some minerals fracture a certain way due to their atomic structure.
• All mineral fracture but not all minerals have cleavage.

Examples of crystal

• Pyrite Cubic (Isometric)
• Rutile- Tetragonal
• Quartz- Trigonal
• Natrolite- Orthorhombic

Mineral and Rock Definitions

• A rock is a solid formed by geological processes and is an aggregate of one or more minerals.
• A mineral is a naturally occurring solid formed by geological processes with a definable chemical composition and crystal structure.
• Minerals are building blocks of rocks

Mineral Descriptions

• Silicates are the most abundant group of minerals in Earths crust (Si4+ O2-)
• Carbonates contain CO32- (Calcite)
• Sulfates have sulfur anion in structure (Gypsum)
• Phosphates have tetrahedral PO43- anion in the structure (apatite)
• Oxides contain oxygen (magnetite, chromite, spinels, hematite)
• Halides contain halogen anion (CI, F, Br and I)
• Sulfides are a group of minerals (Galena, sphalerite, Pentlandite, Pyrite, Bornite, Arsenopyrite)
• Native elements are mainly metals, (diamond, gold, silver) but also include diamond, not common in Earths crust

Describing Minerals Tips

• Start with the most obvious features.
• Consider colour, lustre, crystal shape, hardness, cleavage, and specific gravity (SG).
• Never test the hardness of a display specimen or a gemstone.