Quartz sandstone (orthoquartzite), Cambrian

From road-cuts near Skiag Bridge, Loch Assynt

Outcrop

  Quartz sandstone; Cambrian basal quartzite, Loch Assynt
This is a road-cut outcrop of hard, white Cambrian quartzite. The penknife is 9 cm long. Notice that there are two sets of bedding planes visible in the outcrop. There are prominent straight bedding planes spaced a few tens of centimetres apart, parallel to the knife and to the row of loose stones balanced on the ledge at the top. Between these are curved, more closely spaced, steeper planes, clearly seen in the unit above the penknife. The tops of these planes are cut off by the next thick layer. This structure is called cross bedding, and it is a common feature of many types of sandstones. In this case, the clean quartz sand was deposited as sand banks and shoals near the shore, and the cross beds represent the sloping sides of the sand bars against which the sand layers were laid down.

Hand specimen

  Quartz sandstone; Cambrian basal quartzite, Loch Assynt
This is a pure sandstone composed of rounded grains of quartz, cemented by further quartz between the grains. This makes the rock very hard and resistant to weathering and erosion, rather like a metamorphic quartzite formed by the action of heat and pressure. Even though both grains and matrix are made of the same material, you can see the rounded outlines of the sand grains in the hand specimen, as the original particles were thinly coated with iron oxide dust. There is a very small number of feldspar grains, showing up chalky white on the rock surface. In the pure white varieties of Cambrian quartzite these features are much more difficult to make out.

Thin section

  Quartz sandstone; Cambrian basal quartzite, Loch Assynt
In this pure quartz sandstone, the sand grains are cemented by quartz that has formed as outgrowths and overgrowths on the original sand grains. You can see the rounded outlines of the grains, marked by oxide dust. Compare this with the view between crossed polars, where you can see that the rock now consists of tightly interlocking quartz crystals.

Plane polarized light. Field of view 2 mm across.

  Quartz sandstone; Cambrian basal quartzite, Loch Assynt
This is a pure quartz sandstone, in which the sand grains are cemented by quartz that has formed as outgrowths and overgrowths on the original sand grains. The view under crossed polars reveals the irregular and interlocking shapes of the grains of quartz. This texture makes the rock very hard and strong, rather like a metamorphic quartzite formed by the action of heat and pressure. Sedimentary rocks like this, in which the quartz cement formed at low temperature, are commonly called orthoquartzite. Note that in places you can see the rounded outlines of the original sand grains, marked by oxide dust. Compare this with the view in plane polarized light.

Crossed polars. Field of view 2 mm across.

  Quartz sandstone; Cambrian basal quartzite, Loch Assynt
The bedding in the quartzite is commonly outlined by by bands of differing grain size. Here, alternating layers about 2 mm thick are composed of quartz-cemented sand grains averaging about 1 mm and 0.25 mm in diameter.

Crossed polars. Field of view 7 mm across.

D.J. Waters, Department of Earth Sciences, May 2003