Mike Horne FGS

Describing Rocks

What is the general colour of the rock?
Does a hand specimen feel heavier than it looks?
What are the minerals in it?
What percentage is there of each major mineral?
What shapes are the crystals or grains?
What size are the crystals or grains?
Are they all the same size?

In Sedimentary rocks:-
Is the cement the same mineral as the clasts?
Are there any fossils or microfossils?

Textures:

In Igneous Rocks:-
Amygdaloidal - with filled gas bubbles
Aphanitic - grains too small to be distinguished with the naked eye.
Orbicular - rounded growths due to reheating as the crystal has grown. For example Rapakivi texture plagioclase in orthoclase as seen in Orbicular Granite (a.k.a. Baltic Brown) as seen on many TSB banks; or Ophitic texture 'augite' enclosing plagioclase.

orbicular granite

Phaneritic - grains can be seen with the naked eye.
Poikolitic - small crystals enclosed within a larger crystal
Porphyritic - larger crystals (=Phenocrysts) in a finer grained groundmass
Spherulitic - a mass of radiating crystals.
Vesicular - with gas bubbles
xenoliths - inclusions of 'country rock'

xenolith

In Metamorphic Rocks:-
Augens - lens shaped crystals

Augen Gneiss

Cleavage - a tendency for the rock to split in a particular direction, because of the alignment of the minerals [different from the cleavage within the crystals].
Gneissose - a layering of coarse grained minerals, often into light and dark bands.
Granoblasic - grains of roughly equal size
Idioblastic - well developed crystals
Maculose - a spotty texture = "spotted rock"
Poikoblastic - small crystals enclosed within a larger crystal
Porphyroblastic - containing larger crystals (=porphyroblasts) in a finer grained groundmass.
Schistose - parallel orientation of the mica grains
Relict structure- relics of an original texture enclosed in a metamorphic mineral as it has grown; may have an 'S' or 'Z' shape if the crystal has rotated during growth.
Xenoblastic - irregular shaped crystals

In Sededimentary rocks:
Arenaceous - Sandstones (1/16 to 2 mm grain size)
Argiallaceous - smaller grain size than sandstones - e.g. siltstones, mudrocks and clays.
Graded bedding - Clasts sorted by size within a bed
Grain size - size of the clasts, measured on a scale from Clay (under 4 microns) up to Boulders (greater than 256 mm)
Grain supported - clasts touching (the opposite of matrix supported)
Matrix supported - clasts not touching, supported in a groundmass of finer material.
Ooliths - rounded particles of limestone looking like fish eggs.
Pellets - fish poo.
Poorly sorted [= immature] - Clasts not the same shape and size
Rounding - the degree of rounding of the clasts from 'well rounded' to 'very angular'
Rudaceous - coarser grained rocks - e.g. conglomerates, breccias and tillites
Till = Boulder Clay - a mixture of different sized clasts in clay left behind by a glacier.
Well sorted [= mature] -Clasts the same shape and size

Pyroclastic or Volcaniclastic- formed from shards of volcanic ash = tuff

copyright Mike Horne - October 2016

Hull Geological Society Home Page                     Geology Courses Homepage