Slate

by Dr Tim Palmer

Compacted mud is an excellent building material in the right place, like a hot, dry desert, but it is quite unsuitable for the wet, northern European climate unless covered and kept dry. Rocks that are made of mud and silt are the Cinderellas of the useful sedimentary stones, overshadowed by their more glamorous relations, limestone and sandstone. Usually they are soft and weak, weathering easily and soon breaking down under the influence of ice and rain to their constituent particles of clay.

But mudrocks may be transformed from something soft, grey and dull into a versatile and attractive material that has the property of being much harder and easy to split into thin sheets. It can be used in flooring, walling, cladding and (literally above all) roofing. This splendid material is slate. The story of slate starts in the ancient seas that once covered much of the land around us. Rivers that drained the land carried with them a sediment load of silt that dropped out onto the sea floor, building deltas and barrier islands where the rivers met the sea. Close to the shore the coarser sediments (composed of the larger grains) formed sandstones, but the smaller silt and clay particles were carried further away from the shoreline and settled out more slowly into the deeper water.

Eventually these fine-grained sediments built up into thick piles of mud on ancient sea-beds, slowly compacting in thickness as the water was squeezed out of them by the increasing weight of the accumulating sediment pile. The grains in muds are of two main sorts. Some are minute particles of quartz, much smaller than the quartz sand-grains in a sandstone. Thin layers (laminations) enriched in quartz particles sometimes show up as slightly lighter-coloured layers in a slate and give away the original direction of the natural sedimentary bedding of a rock.

Most of the particles, however, are made up of clay minerals – a geological mixed-bag of chemically similar but distinct silicate minerals that are the end products of the weathering reactions of the minerals that composed the rocks of the land surface. Besides their small size (only a few thousandths of a millimetre across), clay mineral particles have one other important property. The minute individual crystals of which they are made are cut by planes of weakness in one direction, in the direction in the crystal structure along which they break easily, thus producing smaller grains that are flaky

Deposition of muddy sediments on the seafloor sets the stage for the formation of slate, but they then have to be altered into something stronger and harder by the effects of heat and pressure – the process known to geologists as metamorphism. Although slate represents the results of a fairly low level and gentle metamorphic alteration, the temperature and pressure increases involved still require something more than just burial of an increasingly thick sediment pile on the sea-bed.

The lateral forces provided by the sideways movement of continents and the tectonic plates that support them across the surface of the globe can provide the necessary changes. Squeezing and folding by compression of formerly quiet, muddy sea floors, has often happened in geological history. Not only is direct squeezing pressure applied, but the buckled sediments may become deeply buried to where pressures and temperatures are higher, until some such time as erosion brings them to the surface again. Often, such zones of squeezing mark the lines where ancient continents, formerly separated by an ocean, approached one another and collided.

The slate-belt of north Wales and southern Scotland, for example, marks the site of a former ocean, the Lapetus Ocean, that ceased to exist about 450 million years ago. In north Cornwall, as well as in Brittany and Spain, the same scenario happened, about 150 million years later. The increase in temperature and pressure that the muddy sediments are subjected to causes several changes, all of which combine to produce a harder rock with a well-developed rock cleavage, that is, a preferred direction of splitting.

The compression rotates some of the clay particles so that their flat, flaky faces are perpendicular to the direction of stress. Other clay particles start to re-crystallise and grow with a different orientation, again with their flat faces aligned in the same way. Some new clay minerals, such as chiorite, and other flaky minerals such as mica, may also form and grow with similar orientations. All these flaky minerals growing in an aligned direction also have their internal planes of crystal weakness minerals growing in an aligned direction and the effect is that the whole rock splits easily in a direction of 90 degrees to the original squeezing stress.

Of course, this direction need not have any relation to the direction in which the original bedding in the mud ran, so faint bedding laminations sometimes cross the planar surfaces where the rock has split along the cleavage. Slates may be quite varied in colour because of the different minerals that grew during the metamorphic alteration. Chlorite is green, so gives a greeny tint to the grey of the slate. Some slates contain quite a lot of iron, and are pink or purple.

Locally, gobstopper-sized blobs within purple slates have undergone alteration of the iron to a green form that is chemically reduced, giving the famous spotted slates of north Wales. The green blobs were there before the squeezing started, because they have also been stretched out and deformed. Rarely, fossils are found in slates that have been stretched on the geological rack in the same way – the Delabole Butterfly (actually a brachiopod that lived on the muddy sea floor) is an example.

A few slates started as segments other than simple muds. Some of the Lake District slates formed from the ash that was spewed out of the volcanoes that lined the shores of the Lapetus Ocean all those millions of years ago. The re crystallisation of minerals during the metamorphic alteration of slate can sometimes be a potential source of trouble. The original muds varied in the proportion of fine particles of organic matter that they contained. Organic-rich muds tended to develop iron sulphides (pyrites) as a waste product of the activities of bacteria that thrived in them on the seabed.

Metamorphism can cause the pyrites to grow as quite large crystals which are hard and shiny when the slate is freshly split. But beware! Pyrites are notoriously unstable when exposed to air and rain and will soon rot away to a rusty stain, leaving a hole. Some pyritic slates will rot away completely. Some of the slates imported into Britain in recent years have turned out to be expensive mistakes for just this reason.

Copyright © Dr Tim Palmer


Dr Tim Palmer (tjp@aber.ac.uk) is a Chartered Geologist and consultant on the petrography of building stones and limes with a particular interest in historic buildings and landscapes. He researches and teaches geology part-time in the Earth Sciences Institute at the University of Wales, Aberystwyth, SY23 3DB. Tel: 01970 627107.