Introduction to describing thin-sections
Each description contains a list of the primary, minor and accessory minerals in each rock. Primary minerals are minerals that make up more than 5 % of the rock, minor minerals are between 1-5 % and accessory minerals are less than 1 %. Each primary and secondary mineral is described based on grain size, morphology, habitus and any special features present. Below is a more detailed descriptions of this criteria. Accessory minerals are listed but not described, as they are often so small in size or in such small amounts that a more detailed description is not possible.
Mode - given in %, describes the percentage of each mineral present, based on volume as seen on the surface of the thin section
Grain size - refers to the diameter of individual grains or particles and is classified as follows:
- very coarse grained (>5 mm)
- coarse grained (2-5 mm)
- medium grained (1-2 mm)
- fine grained (0,2-1 mm)
- microcrystalline (0,1-0,2 mm)
- cryptocrystalline (<0,1 mm)
Morphology - indicates the quality of the development of faces on crystals
- euhedral - crystals bounded on all sides by its characteristic faces
- subhedral - crystals bounded by only some of its characteristic faces
- anhedral - crystal lacks any of its characteristic faces
Habitus - describes the shape of crystals. Common examples include granular, prismatic, tabular, platy, fibrous, skeletal and any other suitable expressions.
Special features - describes any special features, potentially which are useful diagnostic characteristics. This can include common on cleavage, twinning, zoning, undulose extinction, kink banding, alteration and others.
In addition to the mineral descriptions, there is a general description of rock referring to both the plane and crossed polarised light pictures. This includes a description of the texture, including but not limited to crystallinity, granularity and the mutual arrangements of crystals. When appropriate, there is a description on how one can best identify specific minerals, an explanation of formation and more detailed descriptions of any special features which are of particular note. Only properties which can be seen in the photographs are discuss, thus descriptions of characteristics such as elongation and dispersion are omitted. The locality of the specimen is quoted, when known. Any additional observations are listed under "comments". For instance, when a thin section is too thin and show uncharacteristic interference colours.
Below is a list of the most important characteristics and textures used in the descriptions.
Crystallinity - describes the degree of crystallisation
- holocrystalline - composed entirely of crystals, has no glassy part
- hypocrystalline - some crystalline components and some glassy components
- holohyaline - composed almost entirely of glass components
- hyaline - completely glassy
Terms indicating relative size of crystals
- Equigranular - all crystals are approximately the same size
- Inequigranular - crystals differ substantially in size.
There are several common varieties:
- porphyritic - larger crystals (phenocrysts) embedded in a finer-grained groundmass. This texture can be seriate, when there is a continuous range in the sizes of crystals or hiatal, when the crystals show a broken series of sizes
- Seriate - crystals of the principle minerals show a continuous range of sizes
- glomeroporphyritic - eine Art porphyrischer Textur, bei der die Phänokristalle in Klumpen (Glomerokristalle) zusammengeschlossen sind
- poikilitic - larger crystals of one mineral (oikocrysts) enclose numerous small crystals of one or more other minerals (chadacrysts). These are generally, but not necessarily, randomly orientated, and uniformly distributed
- ophitic - a variety of poikilitic texture in which the chadacrysts are elongate and wholly or partly enclosed in the oikocryst
Other textures:
- trachytic/trachytoid texture - an orientation texture. subparallel arrangement of lath-shaped feldspars in the groundmass of a rock. Trachytoid texture is visible to the naked eye and trachytic is not
- granophyric - an intergrowth texture. Describes an intergrowth of intricate skeletal shapes of quartz and alkali feldspar. This can be crudely radiate
- myrmekitic - an intergrowth texture. Describes patches of plagioclase intergrown with vermicular quartz, often wart-like in shape
- perthitic - an intergrowth texture. Describes lamellae of sodium-rich feldspar in potassium-rich feldspar (the converse is known as antiperthitic)
- symplectite - an intergrowth texture. Describes an intimate intergrowth of two different minerals, one of which may assume a vermicular habitus
- spherulitic - a radial texture. Describes roughly spheroidal bodies composed of an aggregateof fibrous crystals of one or more minerals radiating from a nucelues
- variolitic - a radial texture. Describes a fan-like arrangement of divergent, branching fibres
- corona - an overgrowth texture. Describes a crystal of one mineral is surrounded by a rim of one of more crystals of another minerals
- vesicular - a cavity texture. Describes round, ovoid holes formed by expansion of gas in a magma
- amygdaloidal - a cavity texture. Describes vesicles are filled with later mineral growth