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 Aggregation of molecules builds larger and larger molecules becomes a nucleus at some point Nucleus size of this is either big enough to continue growth or will re dissolve Critical Size Overall rate of nucleus formation vs crystal growth determines crystal size distribution 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 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 Igneous Textures Figure 3 1 Idealized rates of crystal nucleation and growth as a function of temperature below the melting point Slow cooling results in only minor undercooling Ta so that rapid growth and slow nucleation produce fewer coarse grained crystals Rapid cooling permits more undercooling Tb so that slower growth and rapid nucleation produce many finegrained crystals Very rapid cooling involves little if any nucleation or growth Tc producing a glass 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 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
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