Stoichiometry • Some minerals contain varying amounts of 2+ elements which substitute for each other • Solid solution – elements substitute in the mineral structure on a sliding scale, defined in terms of the end members – species which contain 100% of one of the elements
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Stoichiometry Some minerals contain varying amounts of 2+ elements which substitute for each other Solid solution – elements substitute in the mineral.
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Stoichiometry• Some minerals contain varying amounts of
2+ elements which substitute for each other
• Solid solution – elements substitute in the mineral structure on a sliding scale, defined in terms of the end members – species which contain 100% of one of the elements
Chemical Formulas
• Subscripts represent relative numbers of elements present
• (Parentheses) separate complexes or substituted elements– Fe(OH)3 – Fe bonded to 3 separate OH
groups
– (Mg, Fe)SiO4 – Olivine group – mineral composed of 0-100 % of Mg, 100-Mg% Fe
• One of the ions is a determined integer, the other numbers are all reported relative to that one.
Normalization• Analyses of a mineral or rock can be reported in
different ways:– Element weight %- Analysis yields x grams element in
100 grams sample– Oxide weight % because most analyses of minerals and
rocks do not include oxygen, and because oxygen is usually the dominant anion - assume that charge imbalance from all known cations is balanced by some % of oxygen
– Number of atoms – need to establish in order to get to a mineral’s chemical formula
• Technique of relating all ions to one (often Oxygen) is called normalization
Normalization• Be able to convert between element weight
%, oxide weight %, and # of atoms• What do you need to know in order convert
these?– Element’s weight atomic mass (Si=28.09
g/mol; O=15.99 g/mol; SiO2=60.08 g/mol)– Original analysis– Convention for relative oxides (SiO2, Al2O3, Fe2O3
etc) based on charge neutrality of complex with oxygen (using dominant redox species)
Normalization example
• Start with data from quantitative analysis: weight percent of oxide in the mineral
• Convert this to moles of oxide per 100 g of sample by dividing oxide weight percent by the oxide’s molecular weight
• ‘Fudge factor’ is process called normalization – where we divide the number of moles of one thing by the total moles all species/oxides then are presented relative to one another
to get here from formula above, adjust by 8 / 200.38
Mineral assembly
• Most minerals will deal with ionic bonds between cations and anions (or anionic subunits which are themselves mostly covalent but do not dissociate)
• Assembly of minerals can be viewed as the assembly of individual ions/subunits into a repeatable framework
• This repeatable framework is a crystal or crystalline material
Mineral Assembly
• Isotropic – same properties in every direction
• Anisotropic- different properties in different directions most minerals are this type
• Assembly of ions from melts, water, or replacement reactions which form bonds
• The matrices the ions are in always contain many different ions – different conditions of formation for the same mineral creates differences…
Polymorphs• Two minerals with the same chemical formula but
different chemical structures• What can cause these transitions??
•sphalerite-wurtzite•pyrite-marcasite •calcite-aragonite•Quartz forms (10)•diamond-graphite
Complexes Minerals• Metals in solution are coordinated with ligands
(Such as H2O, Cl-, etc.)• Formation of a sulfide mineral requires direct
bonding between metals and sulfide – requires displacement of these ligands and
deprotonation of the sulfide
• Cluster development is the result of these requirements
Mineral growth
• Ions come together in a crystal – charge is balanced across the whole
• How do we get large crystals??– Different mechanisms for the growth of
particular minerals– All a balance of kinetics (how fast) and
thermodynamics (most stable)
Crystal Shapes
• Shape is determined by atomic arrangements
• Some directions grow faster than others
• Morphology can be distinct for the conditions and speed of mineral nucleation/growth (and growth along specific axes)
Ostwald Ripening
Larger crystals are more stable than smaller crystals – the energy of a system will naturally trend towards the formation of larger crystals at the expense of smaller ones
In a sense, the smaller crystals are ‘feeding’ the larger ones through a series of dissolution and precipitation reactions
• In the absence of ripening, get a lot of very small crystals forming and no larger crystals.
• This results in a more massive arrangement
• Microcrystalline examples (Chert)
• Massive deposits (common in ore deposits)
Topotactic Alignment•Alignment of smaller grains in space – due to magnetic attraction, alignment due to biological activity (some microbes make a compass with certain minerals), or chemical/ structural alignment – aka oriented attachment