Geological field trip through Scotland: Basalts from the Isle of Skye

Robert Sturm

The Isle of Skye is a part of the Inner Hebrides in the north-west of Scotland. It has a total area of 174,000 hectares and has an irregularly shaped coastline that is typical of the British Isles. Since the early nineteenth century, the island has become a centre of geological research, because rocks of different geological periods are exposed there. For instance, the gneisses of the Lewisian complex were formed in the Proterizoicum, 2,800Ma and, therefore, are some of the oldest rocks in Europe.

On the other hand, intrusive and extrusive igneous rocks can be assigned to magmatic events that covered wide parts of the island during the Tertiary. This event, which took place about 60Ma, resulted in the development of the Atlantic Ocean in its present form. In more recent times, two ice ages, which affected the island 26,000 years ago (the Dimlington glacial) and 11,000 years ago (the Loch Lomond glacial), resulted in the formation of a partly spectacular glacigen landscape (a landscape formed by the ice) with sediments that are of high interest for geological research.

Impressive evidence for the Tertiary volcanism is provided by the plateau lava series (these are horizontally stacked layers of lava), mainly exposed in the north and west of the island. These extrusive rock formations probably reached a thickness of 1,200m before they were subject to significant erosion. The main part of the lavas can be classified as basalts, that is, dark igneous rocks with a low content of silicon dioxide (SiO₂₎ and enhanced concentrations of iron oxide (FeO) and magnesium oxide (MgO).

Only the uppermost layers of the lava series exhibit a more acidic (that is, granitic) chemistry with higher contents of SiO₂ and, therefore, may be categorised as trachytes. In the centre of the plateau lava series, massive extrusive rocks can be found. However, the upper and lower parts of this impressive lithology are characterised by amygdaloidal basalts (see Fig. 2). It is these that I will concentrate on in the rest of this article.

Amygdaloidal basalts from the Old Man of Storr

As shown in Fig. 2, amygdaloidal basalts are marked by bulbous cavities that were formed by the escape of gas during the fast crystallisation of the rock. These cavities were subsequently filled with minerals, which had crystallised from mineral solutions permeating through the extrusive igneous rocks.

A well-known sampling site, where such amygdaloidal basalts can be found, is in the area around the Old Man of Storr in the northeast of Skye. This is a basalt obelisk, 30m high, that was chiefly formed by wind erosion and is recognisable from afar as a remarkable element of the rough landscape. The geodes of the basalts found around the Old Man of Storr are filled with a wide range of minerals, including calcite (CaCO₃), chlorite, quartz (SiO₂), gyrolite, as well as a high number of zeolite species (for example, analcime, natrolite, chabasite, heulandites, laumontite, stilbite, thomsonite, mesolite and garronite).

Preparation of the basalts for light-microscopy

To give some insight into the amygdaloidal basalt geodes and to learn something about their formation and the arrangement of the different minerals, you need to produce petrographic thin sections and to study them under the light-microscope.

I produced thin sections of geodes filled with various minerals using a standard procedure. To start with, small blocks with a base area of 2 x 4cm and containing impressive bubbles and cavities were cut out of the sample rocks and mounted with their lower sides on glass slides using a 2-component-resin (for example, Köropax 439). Subsequently, the blocks were ground from the other side, using silicon-carbide (SiO) powders with different grain sizes.

The polishing was continued until the blocks reached a thickness of about 35µm, being the optimum thickness for petrographic thin sections used for bright and dark-field microscopy. After finishing the polishing procedure, the preparations were covered with a layer of Canada balsam and a thin cover of glass. The microscopic examination of the sections and photography of selected geodes were carried out using an appropriately equipped microscope (Nikon, model SE-S-B).

Single steps for the production of petrographic thin sections:

1. The sample block is fixed onto a glass slide.
2. The sample block is ground and polished.
3. The section is covered with Canada balsam and a glass cover.

What can be observed under the microscope?

Representative results obtained from the microscopic work are summarised in Figs 5 to 7. As exhibited by the different images, the shape of the geodes found in the amygdaloidal basalt is subject to remarkable variations, ranging from drop-like over lentiform to spherical. Those geodes being characterised by drop-like or spherical shapes were commonly formed in a non-flowing lava, while elongated, ellipsoidal cavities preferentially developed during lava flow, whereby the longer axes of the cavities are often oriented parallel to the original flow direction. Some geodes, above all those with numerous mineral components, represent valuable indicators for various geological problems (for example, determination of original orientations of rock formations).

Monomineralic geodes, such as that illustrated in Fig. 6, are chiefly filled with calcite, such that the visualisation of the grain boundaries is only possible in the dark-field using crossed polarisators.

Due to the different orientation of single crystals, a typical pattern of undulatory (that is, speckled) extinction may be recognised under the microscope. The development of such calcite geodes required an extensive permeation of the former gas bubbles by a fluid enriched in CO₂ and calcium. Both constituents remained in rather high amounts after crystallisation of the lava (note that the rock chemistry is mainly characterised by the elements magnesium and iron) and, therefore, could be dissolved in a fluid phase. Within the hydrothermal temperature range (300°C to 150°C) the crystallisation of calcite grains reached a maximum.

Silicatic geodes are furthermore characterised by a predominance of numerous kinds of minerals, with the number of contained mineral phases varying between 3 and 8. Under the microscope, preferentially occurring minerals are, besides several species of zeolite (analcime, stilbite, chabasite and natrolite), different varieties of quartz and, above all, chalcedon.

The crystallisation of silicates takes place in a similar way as that of carbonates: After crystallisation of the extrusive igneous rock, which may be classified as poor in silicon (see above), excess amounts of the element and other basalt-incompatible elements (calcium and sodium) were dissolved in a fluid. Mineral formation within available cavities was a rather complex process, because it took place according to a temperature-controlled temporary sequence, which becomes clearly visible by the mineral zonation (Image 1 in Fig. 7).

Zeolite crystallization starts at temperatures of about 220°C with the high-temperature phase laumontite (CaAl₂Si₄O₁₂ x 4 H₂O). Continuous decline of the temperature and silicate concentration in the fluid phase causes the development of the phyllo-zeolites heulandite ((Ca_0,5,Na,K)Al₃Si₉O₂₄ x 7-8 H₂O) and stilbite (NaCa₄Al₈Si₂₈O₇₂ x 30 H₂O). At the final stage of crystallization (<100 °C) the needle-zeolites (for example, skolezite; CaAl₂Si₃O₁₀ x 3 H₂O) and cubic zeolites (chabasite; (Ca,K_0,5)₂Al₂Si₄O₁₂ x 6 H₂O) are formed.

This was a small study aimed at giving a first and limited insight into the wonderful world of amygdaloidal basalt rocks that can be found on the Isle of Skye. In future investigations, I will try to find out a little more about the chemistry and the formation of those geodes I have discussed above. To do this, I will have to apply more expensive analytical methods such as X-ray diffractometry or electron-microprobe analysis.

Reference

Anderson, F.W., Dunham, K.C.: The geology of northern Skye. Memoir of the Geological Survey of Great Britain, Edinburgh 1966.