The colour index measures the proportion of ferromagnesian or mafic minerals present in the rock. Rocks with less than 30% of dark mineral are pale and described as leucocratic. Rocks with 30-60% are mesocratic and tend to be grey in colour. Those with over 60% mafic minerals are known as melanocratic.
TEXTURE
The texture of a rock describes the relationships between the grains. Grain size, shape and orientation, degree of crystallinity, and the ways in which grains come into contact with each other are all textural features. The texture of a rock depends upon the physical conditions in which the magma cools.
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Fine-grained igneous rocks have grain sizes of less than 0.25mm. (the crystals are not distinguishable with the naked eye and a hand lens is needed for identification). These cool quickly on the surface (extrusive) and are known as volcanic rocks. They commonly form in lava flows. An example of a fine grained igneous rock is basalt
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Medium grained rocks have a grain size of 0.25-2mm. (The crystals can be seen with the naked eye but a hand lens is needed for identification). They commonly form in dykes and sills. These rocks are known as hypabyssal. Eg. Dolerite
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Course grained igneous rocks have a grain size greater than 2mm. (The crystals are easily seen and identifiable with the naked eye). They cool slowly deep in the crust and are known as plutonic rocks – often associated with batholiths. Eg. Gabbro.
After grain size there are many terms to describe the physical texture of the rock which help us to tell where, and in what conditions the rock formed, and thus can be diagnostic. These terms include:
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Granular : separated into coarse, medium and fine depending on the grain size.
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Glassy: result of very rapid cooling in which there is too little time for crystals to form. These rocks are usually shiny and black.
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Porphyritic: when large, well formed crystals called phenocrysts are set in a fine-grained groundmass. Theses rocks are common in lavas.
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Vesicular: These rocks have little cavities or air bubbles in the groundmass produced by escaping gases in lava flows.
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Flow-banded: These rocks show a rough banding of lava constituents. The bands are highly contorted and continuous for several cms. Flow-banded rocks are commonly associated with viscous lavas.
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Amygdaloidal: when vesicles in the rock have been filled in by later generations of crystals.
MINERAL COMPOSITION
The main minerals in igneous rocks are: Olivene, pyroxene, amphibole, biotite and muscovite micas, quartz, plagioclase and orthoclase feldspars. Which minerals are present in a particular rock depend on temperature, viscosity and plate tectonic context.
Recurring mineral groupings are called mineral associations. Studying these enables us to predict which minerals may occur together in a rock and which are mutually exclusive. The main minerals present in an igneous rock are known as the essential minerals of that rock. These have a common temperature crystallisation range and so are commonly found together. Minerals present in small amounts are called accessory minerals.
