Getting to know rock-forming minerals

Geologists use the term ‘mineral' to mean any naturally-occurring substance which is an ingredient of a rock. Rocks are named and classified according to which ingredients they contain, as well as by their grain size and their mode of formation.

There are hundreds of different minerals which form the rocks that make up the Earth's crust. Many of these minerals are rare, but six common minerals make up the commonest of the rocks around us. These are illustrated on Plate 1 and Plate 2.

plate 1

PLATE 1   ROCK-FORMING MINERALS
A. Common quartz   B. White feldspar, with flat face glinting in light.  C. Milky white feldspar and glassy quartz  
D. Labradorite, an uncommon feldspar which has a blue-yellow sheen   E. White mica, also feldspar and citrine quartz

Quartz, which consists of silica (the common term for silicon dioxide), is the most abundant mineral of all. It occurs in most rocks, and is easily recognised particularly when it is white in colour. Sample 1A (Plate 1, sample A), was found on the beach at Strachur. Impurities can discolour quartz, particularly iron which gives it a brown colour. Quartz is quite hard and is usually milky white or glassy. Only rarely can individual crystals of quartz be distinguished - a piece of quartz such as picked up from the beach will consist of numerous interlocking, deformed crystals which had no room to grow into the symmetrical, well-formed crystals shown on Plate 3. In fact quartz is easily recognised in igneous rocks because it rarely shows crystal faces, and just looks glassy and shapeless.

Sand - such as beach sand - is mostly made of quartz grains. These are not crystals, but are fragments of crystals. Angular sand is an important constituent of cement and concrete. It is also used for sandpaper, and when very pure, quartz sand can be used for glass manufacture.

Feldspar occurs in many igneous rocks. It commonly has a milky white colour, or it can be pink due to a small content of iron. Usually there are crystal faces which glint when the rock sample is turned to catch the light (samples B and C, Plate 1 - notice also the glassy quartz). Feldspar occurs in rocks of the granite family as well as rocks of the gabbro family, and although the type of feldspar is different that need not concern us here. Feldspar does not survive erosion very well, so it is not common in sedimentary rocks. Samples 1B and 1C were found on Strachur beach.

An attractive but uncommon type of feldspar, known as labradorite (either pronounced la-BRAD-orite or LAB-ra-dorite) has a spectacular sheen and turns various colours including green, yellow and blue (Sample 1D). To get the best effect, rotate the specimen to catch the light. This is similar to the feldspars in the facing stone which decorates the front of Morrison's store in Dunoon. These slabs are made of a Norwegian stone called larvikite (named after the locality of Larvik in Norway) belonging to the granite family, and the large feldspar crystals have a bluish sheen as they reflect the light. Larvikite, which is also used for gravestones, floor tiles and kitchen bench tops, sometimes has the trade name "Blue Pearl".

Larvikite

Sample of larvikite, similar to the facing stone on Morrison’s store, Dunoon.
Kindly donated by Fraser Downie, Stokes Memorials

Mica is a platy mineral and may be black, white (sometimes colourless), or green. Different coloured micas have different compositions and tend to occur in different kinds of rock. For example, white mica is common in schists and black mica is common in granite. Usually mica occurs as thin plates or sheets, as in sample 1E, which was found on Strachur beach. Note the natural ‘citrine' quartz on the other side of the rock - this specimen does not have well-formed crystals but it has the same colour as the artificially-induced colour of sample D, Plate 3.

Pyroxene is a dark brown mineral which contains a lot of iron and magnesium. It is an important constituent of the darker, heavier rocks such as the gabbro family. It is not represented in this collection as good specimens are hard to find. Moreover pyroxene often alters to another dark mineral, hornblende, which is illustrated as Sample 2A (Plate 2, sample A). Hornblende is usually quite shiny and light is reflected from the crystal faces. This rock sample, which is a typical gabbro for this general area, comes from near Tyndrum. It is not important to distinguish pyroxene from hornblende at this stage, as either or both minerals are present in rocks of the gabbro family and contribute much of the density and dark colour that characterises these rocks.

plate 2

PLATE 2   ROCK-FORMING MINERALS (CONT’D)
A. Rock containing greenish-black crystals of hornblende  B. Crystals of pyrite or ‘fools gold’

Pyrite is iron sulphide, and it is a metallic brassy-looking mineral and is usually crystalline. It occurs in many rocks, such as slate, metal ores, and igneous rocks. Many of the rocks in this collection contain some pyrite. Pyrite bands occur within large rock outcrops on the shore near Creggans Inn, but Sample 2B was purchased at the Ardkinglas Woodland shop at Cairndow.

pyrite

A layer of pyrite crystals in schist on the shore just south of Creggans Inn
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