Lecture 5 - Crystallization.ppt
- 1. 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
- 2. 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…
- 3. 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
- 4. 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
- 8. 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)
- 9. 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
- 10. 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)
- 11. 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
- 12. 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 120o grain intersections (polygonal mosaic). © John Winter and Prentice Hall
- 13. 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)
- 14. 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
- 15. 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 fine- grained crystals. Very rapid cooling involves little if any nucleation or growth (Tc) producing a glass.