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 KMg3 AlSi3O10 OH 2 phlogopite K Li Al 2 3 AlSi3O10 OH 2 lepidolite KAl2 AlSi3O10 OH 2 muscovite Amphiboles Ca2Mg5Si8O22 OH 2 tremolite Ca2 Mg Fe 5Si8O22 OH 2 actinolite Actinolite series minerals K Na 0 1 Ca Na Fe Mg 2 Mg Fe Al 5 Si Al 8O22 OH 2 Hornblende Compositional diagrams FeO wustite Fe3O4 magnetite Fe2O3 hematite A Fe O A1B1C1 A1B2C3 x x B C Si fayalite forsterite enstatite Fe Mg fayalite Fe ferrosilite forsterite Mg Pyroxene solid solution MgSiO3 FeSiO3 Olivine solid solution Mg2SiO4 Fe2SiO4 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 Ca0 004Mg1 859Fe0 158Mn0 003Al0 006Zn0 002Cu0 001Pb0 000 01Si0 0985Se0 002O4 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 Feldspar analysis Ca Na K 1 Fe Al Si 4 O8 Oxide wt in the of moles mineral of oxide in mole of 2 cations in of O determined the oxides in oxide in oxide by analysis mineral the mineral 2 oxide Atomic weight of oxide g mol SiO2 60 08 1 2 65 90 1 09687 73 83 Si Al2 O3 Fe 2O3 CaO Na2O K2O 101 96 159 68 56 08 61 96 94 20 2 2 1 2 2 3 3 1 1 1 19 45 1 03 0 61 7 12 6 20 0 19076 0 00645 0 01088 0 11491 0 06582 12 84 0 43 0 73 7 73 4 43 Al 1 48569 100 SUM Cation moles of moles of O Number of cations contributed moles of in by each ion in the sample cation mineral 4 73 83 147 66 2 95 3 25 68 0 87 0 73 15 47 8 86 38 52 1 30 0 73 7 73 4 43 1 03 0 03 0 03 0 62 0 35 125 44 200 38 Fe3 Ca2 Na K of moles Oxygen choosen 8 Ca0 73 Na15 47 K8 86 Fe 0 87Al25 68 Si73 83O200 38 Ca0 03 Na0 62K0 35 Fe 0 03 Al1 03Si2 95 O8 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 Figure 3 17 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