Hot Under the Collar (part III)Crystals & Textures

Rocks at last!!!!

How Does a Magma Crystallise?

Homogeneous Nuclei - crystal nuclei grown from the magma.Heterogeneous Nuclei - crystal nuclei that have survived in the melt.

Nucleation Textures - Spherulites

Usually in glassy rocks such as rhyolites, obsidian and pitchstone.

Nucleation Textures - Variolites

Variolitic Olivine Dolerite

Nucleation Textures - Overgrowths

Dendritic overgrowths on feldspar phenocrysts

Nucleation Texture - Corona (Overgrowths)

• Nucleation on the surface of a crystal often leads to preferred orientation

•Sometimes heterogeneous nucleation can cause metastable crystallisation of a phase that does not have the lowest gibbs free energy. •Melting generated from tridymite crystallises as tridymite rather than the stable form cristobalite.

Crystal Morphology (Shape)

• Crystal structure strongly influences crystal shape by the preferred growth of faces.• Those faces that grow fastest are the least developed.

Crystal Spatial Density

• Phenocrysts often have euhedral (perfect) faces.

Hornblende Andesite

Crystal Spatial Density

Minerals crystallised later in the crystallisation sequence are often intergranular (or intersertal if fine-grained with glass).

Dolerite

Crystal Spatial Density

Final crystallisation products are often equigranular

peridotite

Supercooling & Cooling Rate

• Supercooling is NOT the same as cooling rate.• Rapid cooling often causes to supercooling because it does give enough time for crystallisation to occur.• High magma viscosity also caused supercooling because diffusion is lower in viscous melts (e.g. Acid magmas)• High temperature magmas often have large supercooling when they cool since nuclei are destroyed at high temperature.• Unusual crystal shapes develop at large supercooling.

Hopper Instability

At large supercooling (low temperature) diffusion rates are low. Grow on the corners of crystals is then preferred because these are supplied by the largest volume of liquid. Such growth produces skeletal crystals (they contain holes).

Skeletal Olivines (Hoppers)

Picritic (Mg-rich) Basalt

Skeletal Plagioclase

Dolerite

Dendrite Instabilities

Dendrites

Curved Branching Pyroxenes in a Komatiite.

Comb-layering (Dendrites)

Comb layering in chilled margins of dykes.

Parallel Growth Crystals

Barred Olivine Chondrules

Peridotite

If All Else Fails: Glass Formation

Devitrification in a pitchstone

Cosmic Spherules

Magma droplets formed by the melting of extraterrestrial dust particles as they slow down in the atmosphere.

Cosmic Spherules

Knowing their maximum temperatures and cooling rates we can estimate the orbits of cosmic spherules.