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 RipeningLarger 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 fine-grained 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