Chronostratigraphic Chart

Michel-Levy Chart

As a crystal grows slowly in a magma chamber the magma from which it is crystallising may evolve, changing its composition. Thus later crystallising regions or “zones” of the crystal will have a slightly different composition from earlier crystallising regions. The crystal thus displays zonation.
Zonation is seen under cross-polarised light by variations in the birefringence colours of the different zones.

Volcanic rocks and textures are those associated with magmatic eruptions at the surface of the Earth. These eruptions can occur into air, water or ice. In each case cooling is very rapid, causing fine-grained textures to develop.

Minerals displaying a glass-like lustre are described as vitreous. These minerals are transparent or translucent and have relatively low refractive indices. Quartz and calcite, for example, have vitreous lustre.

Have a look at quartz (M410, M411, M412) and calcite (M540, M541, M542) in the Reference Series.

Igneous rocks with a vesicular texture contain air bubbles, called vesicles. These can be round or distorted. In thin-section, vesicles are filled with the mounting medium and consequently appear clear and colourless with low relief under plain polarised light. They are isotropic. Vesicles which are infilled with precipitated minerals, such as calcite or quartz are called amygdales.

Have a look at the rhyolitic glass (L113) and the porphyritic andesite (L130) in the 1A Reference Series.

If some areas of a mineral grain have different extinction positions from other areas of the same grain, the grain is said to display undulose extinction. Importantly, there is no sharp division between areas with different extinction positions; otherwise the phenomenon could be due to twinning. Undulose extinction is frequently observed in strained quartz, where the crystal lattice has been distorted by being squeezed.

Within a single mineral grain the crystal lattice may be oriented differently on either side of a plane, or twin plane. Thus the grain will go into extinction at different positions on either side of the twin plane. This is twinning.

Sedimentary rock structure characterised by a fining-up sequence. They are produced by turbidity flows.