A microgabbro from the same intrusion as SW4, with prominent carbonate veins that impose a form of layering on the rock. This sample was collected from exposures near the bottom of Highbridge Road steps, at the same level as the canal, close to Brewin's Cutting.

This fine-grained igneous rock shows a fine igneous texture of interlocking crystals, including randomly-oriented plagioclase laths, some of which show lamellar twinning under XPL. The predominant greenish-brown colour of the thin section is caused by abundant serpentine minerals, notably replacing olivine and pyroxene crystals throughout the rock; you can still see the typical globular or oval shapes of the original olivine crystals. Opaque iron oxide grains are scattered across the whole section; these were likely magnetite in the original igneous assemblage, though in this sample much of the opaque material is very fine grained. At high magnifications, small interstitial patches of a colourless mineral occur, which under crossed polars can be seen to be composed of radiating fibrous crystals with first-order grey interference colours. This may be the zeolite mineral analcime, precipitated from diagenetic fluids in voids after the rock crystallized.

Irregular veins and smaller patches of pale crystalline material, with very high ('washed out') interference colours, are also common. These represent calcite precipitated from fluids passing through the rock, possibly the same pervasive fluids that caused the replacement of olivine by serpentine. Curiously, the diagonal veins seem to delineate portions of the rock that show different degrees of alteration. This could be due to several factors: perhaps the veins predated the fluid percolation and acted as a partially impermeable barrier to the fluids, reducing their effect in some layers; or perhaps the veins formed at the same time as fluid percolation and focused the fluid flow into narrower zones, leaving the rock nearby less heavily altered.

This microgabbro was intruded into the surrounding Carboniferous rocks about 307 million years ago, towards the end of the Carboniferous - perhaps not all that long after the surrounding sediments were laid down; there is evidence nearby that the magma interacted with still poorly consolidated, wet sediments. It is likely related to other examples of mafic intrusions and volcanic rocks scattered across central England, notably the volcanic tuffs and agglomerates at nearby Barrow Hill and Tansey Green quarries, less than 3 km to the north-west. Tansey Green is known for preserving trunks of conifer trees in growing position, which appear to have been rapidly buried by volcaniclastic deposits during eruptions at this time. The microgabbro at Brewin's Cutting was probably one of the subsurface conduits feeding these volcanoes.

This sample was collected as part of the 'Macro to Micro' project.