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) Nucleation of small particles is generally a kinetically favored process – form fast, but also redissolve…

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

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

Imperfections Further effects on minerals associated with formation: –Zonation – form concentric rings or shells in which the composition or T-P conditions change during crystallization –Twinning – same kind of mineral with different alignments – commonly start from one point or line and grow out in different directions

Zoning Can be minor or major differences reflected in zones containing different phases, colors, or trace element compositions

Twinning Albite twinning a.k.a. polysynthetic twinning – occasionally visible in hand specimen -characteristic of all feldspars (Albite is a kind of feldspar, this characteristic happens to be named after it) Usually visible in thin section

Albite twinning This type of twinning is governed by a ‘twin law’, stating that the twins form parallel to each other, aligned along an optic axis This alignment along an optic axis results in the twins being a measure of composition – how different types of twinned feldspars interact with light

Crystal Chem  Crystallography Chemistry behind minerals and how they are assembled –Bonding properties and ideas governing how atoms go together –Mineral assembly – precipitation/ crystallization and defects from that Now we will start to look at how to look at, and work with, the repeatable structures which define minerals. –This describes how the mineral is assembled on a larger scale