Minerals in Cheviot igneous rocks

Rocks and minerals

Minerals are the individual components which make up a rock. As a magma cools and crystallises, simple chemical compounds group together to form more complex compounds. These are the minerals.

Mafic Minerals in Cheviot Rocks

The mafic (dark) minerals are iron and magnesium-rich silicates that occur as primary, secondary and accessory minerals in Cheviot rocks.
The primary minerals are those that crystallised out from the parent magma before it solidified. Orthopyroxene and its calcium-rich counterpart clinopyroxene are two minerals that were among the first to crystallise out in the cooling Cheviot pluton.
Lower temperatures and addition of alkali elements together with the hydroxyl group (OH) and fluorine gave rise to amphibole minerals such as hornblende and actinolite.Yet lower temperatures and the addition of potassium gave rise to biotite which is dark mica.
Magnetite and related iron-titanium oxides occurred here as both primary minerals and also as secondary minerals. These secondary minerals formed as a result of alteration processes acting on the primary minerals after the Cheviot pluton had completely solidified.
Alteration processes can give rise to all the minerals mentioned above as well new, different mafic minerals. In Cheviot rocks, the new secondary minerals included: tourmaline, a complex boron-rich silicate, secondary actinolite, chlorite which shares a similar structure to biotite, sericite, a fine-grained mica that is a common product of feldspar alteration, and epidote.

Click on a mineral below for photomicrographs and more information

Accessory Minerals in Cheviot Rocks

The accessory minerals are generally primary mafic minerals that make up only a very minor proportion of the mass of the rock. The following occur in the Cheviot Hills: Apatite which is a complex compound containing phosphorous that occurs as very small crystals throughout the Cheviot pluton. Zircon which is a compound of zirconium and silica that also occurs as small crystals throughout the Cheviot pluton. Under the microscope it stands out with very sharp surface relief caused by its high refractive index, and under crossed polars is particularly colourful.

Felsic Minerals in Cheviot Rocks

The felsic (pale) minerals combine the Al, K, Na and Ca with silica to form the feldspar family which is by far the most abundant group of minerals in the Cheviot rocks. The feldspars subdivide into two main groups, the alkali feldspars which contain K and Na, and the plagioclase feldspars which contain Na and Ca. The most common alkali feldspars in Cheviot rocks are orthoclase and perthite.
If there is any spare silica left over after the other compounds have been formed, it crystallises out as quartz. Establishing the relative % of quartz, alkali feldspar and plagioclase, is vital for the correct classification of the parent rock type but this is difficult to do without the specialist equipment that we lack and so, despite our best efforts, our own estimates of composition must be considered provisional.

Identifying minerals in thin section under a polarising microscope

Many minerals are colourless in plain polarised light (PPL). Some, however, are not, and this can be a valuable aid to identification.
Some minerals change colour in PPL light when the microscope stage is rotated. This is called pleochroism.
Minerals which have different refractive indices to each other show different surface relief. They appear to stand out against each other. The Becke test may be used to verify this. If you use a high powered magnification in PPL light with the aperture stopped right down to reduce light intensity, a whitish line may be observed at the edge of the mineral as you raise or lower the stage. This is known as the Becke line. The mineral into which the the Becke line travels as you lower the stage has the higher refractive index.
Many minerals have cleavage which may show clearly under the microscope. This is the regular line by which a mineral separates/breaks, and is determined by the arrangement of its atoms. For example, basal sections of pyroxenes show two cleavage lines which intersect almost at right angles, while amphiboles, also in basal section, show two cleavages which intersect at about 57o and 123o.
Under polarised light (XP) minerals show a range of colours. These are called interference colours and depend on the birefringence of the mineral. They are described as 1st order, 2nd order, etc. and provide an important key to identification as long as the section is the correct thickness at about 0.03mm. Blues and greens do not occur in the 1st order, and grey to grey-white only are found in the 1st order. Charts of interference colours are readily available in reference books or online.

No vestige of a beginning, – no prospect of an end