Limestone, Sandstone & Slate
Limestones are sedimentary carbonate materials that accumulate in water as layers or beds through various chemical and biological processes.
Predominantly of calcium and magnesium the chemistry of these carbonates often include other elements such as iron that will play a part in the structure, colour and patterning found in these materials. A limestone will frequently exhibit structures and other relic features that give clues to the age and environment that existed when the deposit was laid down. The range of warm beige, golden and burnt sugar colours the result of disseminated iron oxide locally becoming concentrated (mineralisation) in this stone. This is a limestone composed largely from broken fossil shell fragments (bioclasic). The sweeping patterns were created as ocean currents reworked and sorted the loose mineral and fossil fragments on the sea bed.
Sandstones are another sedimentary rock type. The source of sandstones grains are those minerals and rock fragments that remain after weathering and erosion of previous rocks. Such fragments are transported, modified, sorted and deposited in layers and beds usually through the action of moving water and sometimes wind. Silica makes up almost 47% by weight of the earths crust. Quartz is a simplest mineral form of Silica. Quartz is also relatively stable, hard, very common and is generally the predominant constituent particle found in sandstone.
As with limestone the presence of other minerals, rock particles, fossil traces and depositional structures all combine to influence the overall appearance of sandstone. The cement that bonds these grains together to form a stone is wide ranging. It can be siliceous, calcareous, ferruginous, carbonaceous etc. or various combinations of some or all. The nature of this cement naturally will have an effect on the colour of this material and will be one of the main factors governing its physical qualities of strength and porosity.
All slates are created from argillaceous (muddy) sediments such as clay, mudstones and deep sea ooze. Over time the very fine grained clays and mineral particles in these mudstones are buried by younger sediments. As overburden pressures increase water is gradually squeezed out so that flat platey clay minerals and micas rotate and are realigned perpendicular to the direction of pressure so forming a rock known as shale. This dynamic metamorphism over time and with increasing pressure and temperature drives out the remaining free water. Stronger bonding between tighter packed and increasingly aligned particles in addition to the formation of some new minerals leads to the formation of a rock we can now call slate.