Mineral growth

1. 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)

2. 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

3. 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

4. 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)

5. 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

6. 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)

7. 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.

9. 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)

10. 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

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

12. 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

13. 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

16. 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