Page 18 - Curriculum Visions Dynamic Book

Sandstone
Silicates, and in particular quartz, are among the
least reactive substances and so are most resistant
to weathering. The quartz grains formed in igneous rocks are released as the other minerals are weathered away. As they roll about in rivers or are tossed about by coastal currents, the edges of the quartz grains are smoothed down and become more rounded in shape. However, they do not change chemically and so remain to form the skeleton of many later rocks. This is why sandstones are among the most common rocks.
Sandstones are grains of quartz and other small, unweathered rock fragments cemented together
by another mineral. Geologists call such rocks sedimentary rocks because they are formed of layers of rocks fragments (sediments) that have settled
out onto the ocean floor or some other place where, over time, they become “cemented” together. In this process the gaps between the grains become filled in.
The main forms of natural sandstone “cement”
are iron oxide and calcite. Iron oxide cement is red, orange or yellow and stains the outside of the sand grains, as well as sometimes forming thin bands of highly concentrated material. The iron staining helps to highlight patterns in the sandstone, making it easier to see in what conditions the sand grains were laid down. For example, the sandstone shown here was formed from beach deposits and the ripples of small waves can still be seen.
Calcite (calcium carbonate) is a white cement
that does not stain the sand grains. As a result, sandstones cemented with calcite are light-coloured. Calcium carbonate is much more likely to become weathered than iron oxide, so many calcite-cemented rocks crumble relatively easily when exposed to
wet weather.
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