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)SiO 4 – Olivine group – mineral composed of % of Mg, 100-Mg% Fe

KMg 3 (AlSi 3 O 10 )(OH) 2 - phlogopite K(Li,Al) 2-3 (AlSi 3 O 10 )(OH) 2 – lepidolite KAl 2 (AlSi 3 O 10 )(OH) 2 – muscovite Amphiboles: Ca 2 Mg 5 Si 8 O 22 (OH) 2 – tremolite Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2 –actinolite (K,Na) 0-1 (Ca,Na,Fe,Mg) 2 (Mg,Fe,Al) 5 (Si,Al) 8 O 22 (OH) 2 - Hornblende Actinolite series minerals

Compositional diagrams FeO FeO wustite Fe 3 O 4 magnetite Fe 2 O 3 hematite A1B1C1A1B1C1 x A1B2C3A1B2C3 A CB x

Fe Mg Si fayaliteforsterite enstatite ferrosilite Pyroxene solid solution  MgSiO 3 – FeSiO 3 Olivine solid solution  Mg 2 SiO 4 – Fe 2 SiO 4 FeMg forsteritefayalite

Minor, trace elements Because a lot of different ions get into any mineral’s structure as minor or trace impurities, strictly speaking, a formula could look like: Ca Mg Fe Mn Al Zn Cu Pb Si Se O 4 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; SiO 2 =60.08 g/mol) –Original analysis –Convention for relative oxides (SiO 2, Al 2 O 3, Fe 2 O 3 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

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 H 2 O, 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

Figure “ Ostwald ripening ” in a monomineralic material. Grain boundaries with significant negative curvature (concave inward) migrate toward their center of curvature, thus eliminating smaller grains and establishing a uniformly coarse-grained equilibrium texture with 120 o grain intersections (polygonal mosaic). © John Winter and Prentice Hall

Small crystals… 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