Minerals 2. talc. What is a Mineral ?. Defining characteristics of minerals : Naturally occurring Inorganic (or at least never alive) Solid Ordered, repetitive internal structure (crystalline) Definite chemical composition (allowing for small variation within set limits).
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Can ice be considered a mineral ?
So far, we have looked at :
Silicates (containing silica ion SiO4 4- )
Carbonates (containing carbonate ion CO3 2- )
Sulphates (containing sulphate ion SO4 2- )
We will continue looking at the remaining important mineral families:
Phosphates (containing phosphate ion PO4 3- )
Sulphides (containing phosphate ion PO4 3- )
Halides (containing halide ions, e.g. Cl – and F- )
Oxides/Hydroxides (containing oxide O 2– / hydroxyl OH -)
Don’t be intimidated by this – the main point is that each mineral group is named after its constituent anion (no big whoop).
The form of phosphate called hydroxyapatite is a very important constituent of bones and teeth in vertebrate animals. Hydroxyapatite gives bones and teeth their great strength and provides animals phosphate for other purposes (we will discuss this later).
Hydroxyapatite (from a rock sample)
Hydroxyapatite crystals in tooth enamel
Sulphides are not only formed by inorganic processes – they are also readily formed by organisms through their metabolic activities.
For example, in a complex series of reactions, sulphur-reducing bacteria use dissolved sulphate (SO42- ) in water for energy and produce the poisonous, smelly, gas called hydrogen sulphide (H2S).
This hydrogen sulphide can then react with dissolved iron to form the mineral pyrite (FeS2). Pyrite is an important mineral in fossil preservation (as we will see later on).
Pyritized ammonite fossil
Halides contain members of the halide family of ions (e.g. Cl-, F-)
Halides can be important indicator minerals for climatic conditions. For example most halite (NaCl- salt) is deposited as a result of seawater becoming too salty to hold all its salt in solution. Thus salt deposits are good indicators of seas located in hot, dry climates.
Fluorite, is an important halide in that it is a good source of fluoride. Fluoride ions are incorporated in minor amount in bones and teeth, providing extra strength (yes, fluoride in toothpaste is commonly derived from this mineral)
Hematite (Fe2O3) is a major component of rust (this is what gives rust its red colour) and is a common weathering product of other iron minerals and is a good indicator of abundant oxygen.
Interestingly, bacteria are important mediators of rust formation.
Magnetite (Fe3O4) has the unique property of magnetism.
Some bacteria use this property to their advantage. They secrete chains of magnetite crystals – these are thought to help in a crude sort of navigation.
The yellowish orange-coloured mineral goethite is an iron oxide with a hydroxyl ion (OH-) attached to it. It is commonly found in association with hematite.
Not surprisingly, this mineral is the other important component of rust and is a common weathering product of iron-bearing minerals.
The appearance and physical behaviour of minerals are directly related to their chemical composition and bonding characteristics of their components.
So…the physical properties of minerals are very handy in the accurate identification of minerals. A few examples of important properties include:
Manner of Breakage (Cleavage/Fracture)
Reaction to Acid
Note: for this particular course, I will not expect you to remember chemical formulae, but anything else is fair game unless otherwise noted (including the basic components of minerals where given).
One of the most useful diagnostic properties is hardness (a measure of the resistance of minerals to abrasion or scratching).
Hardness is measured in units of Mohs scale of hardness (a relative scale developed by geologist Frederick Mohs based on the ability of harder minerals to scratch softer minerals). Hardness reflects, to some extent, the strength of bonds within a mineral.
So, for example if a mineral can be scratched by apatite but not fluorite, we can say that the mineral must have a hardness between 4 and 5.
No, I don’t want you to memorize this table – but at least know the concept of the scale.
For some minerals, it is hardness that makes them useful to organisms. For example, our bones are made of hydroxyapatite (similar to apatite), which has a hardness of about 5.
Thus, hydroxyapatite is a good material to make bones and teeth out of !
Arrangement patterns of atoms within minerals are revealed in their external crystal form.
The packing arrangement of atoms within any given mineral results in a characteristic geometry of crystal faces. Each mineral has its own set of crystal shapes that relate to the stacking patterns of unit cells.
crystals of quartz
Packing arrangements of differently sized atoms (smallest possible unit is called a unit cell)
crystals of halite
Some minerals can have the same composition, but have different crystal structures. For example, both diamond and graphite are pure carbon. But they are obviously very different minerals ! Minerals with the same composition but different crystal stucture are called polymorphs.
Diamond: a rigid framework of covalently bonded carbon atoms
Graphite: covalently bonded carbon atoms forms sheets that are weakly bonded
Arrangement patterns of atoms within minerals also result in characteristic patterns of breakage.
In some minerals, bonds between some atoms are weaker than others. The weaker bonds are often follow the boundaries of unit cells.
When a mineral breaks along well-defined planes, it is said to possess cleavage.
Broken sample of halite showing 3 cleavages at 90o
A cleavage plane in halite
(not at 90O)
(not at 90)
Some minerals can be identified based on the numbers and angles of cleavage (note, this doesn’t have to be memorized either).
Even in closely-related minerals, slight differences in the shapes and packing arrangements of unit cells can result in differences in cleavage.
2 cleavages at 90o
Stacking of single-chain units
2 cleavages (60o and 120o)
Stacking of double-chain units
Some minerals do not have a distinct cleavage (i.e do not break along nice flat planes). This is due to more uniform bond strengths throughout the crystal structure (so no preferred direction of breakage).
Quartz has a distinctive type of fracture called conchoidal fraction (names referring to the shape of a seashell). Glass breaks in the same manner.
Scoop-shaped fracture surfaces
Conchoidal fracture in quartz
Lustre is the appearance or quality of light reflected from the surface of a mineral. For some forms of minerals, lustre can be very diagnostic and can be directly attributed to the form and arrangement of its crystals.
For example, the nacre made by molluscs (pearls and mother of pearl) has a soft, pearly lustre that results from aragonite crystals acting as little prisms within the material.
Large pyrite crystals have a metallic lustre
Quartz has a vitreous (glass-like) lustre
In the form of nacre, aragonite has a pearly lustre
Colour primarily manifests the chemical content of a mineral.
In a few minerals, colour can be a very diagnostic property.
For example, the common sulphide mineral pyrite (iron sulphide- “fools gold”) has a characteristic brassy colour, whereas galena (lead sulphide) has a silvery grey colour.
In other minerals, colour can be very misleading.
Minor impurities or crystal defects can impart different colours
Reaction with acid: some minerals will effervesce (fizz) when reacted with acid. Calcite (CaCO3) and a similar-looking carbonate mineral, dolomite (CaMg(CO3)2) react with acid, but only calcite fizzes violently (dolomite has to be powdered before it fizzes)
Reaction of limestone (made of calcite) with dilute hydrochloric acid.
Optical properties: some minerals, such as calcite (CaCO3), will produce a double image when an object is viewed through its crystals.
This is due to the splitting of light rays as they pass through the calcite crystal.
Note: extinct creepy crawlies called trilobites actually made their eye lenses out of calcite (not exactly a choice material for this purpose, but trilobites modifies lens shape to correct for this optical effect) !
Various other unique properties, alone, or in combination, can also be useful in the identification of minerals.
When rubbed, sphalerite (ZnS) has the smell of rotten eggs. It also has a resinous lustre.
Graphite (pure carbon) has a greasy lustre, is opaque, and is very soft (this is what makes it a good drawing tool)
Halite (NaCl) tastes salty.