Limestone

Marine organism’s skeletal fragments, such as in coral, or foraminifera (forams), a marine organism with an external shell, make up the composition of many limestone deposits. Limestone being a sedimentary rock that is generally composed of the minerals calcite and aragonite.

Most cave systems throughout the world exist in a bedrock of limestone. The caves having been formed by water eroding the limestone over thousands, if not millions, of years. The solubility of limestone by weak acid solutions and water is also the cause of karst landscapes, a geological formation shaped by the dissolving of layers of limestone bedrock. Sedimentry rocks are composed of around 10 percent, in volume, of limestone.

Limestone has many uses, the main uses through the ages being:
- Building materials (in both block form and Portland cement)
- Chemical feedstock
- White pigment or filler in such things as paints and toothpaste
- Road making aggregate
- Food additive
- Soil conditioner
- Medicine and cosmetics
- Sculpture

Many limestone deposits are composed of grains, as are most sedimentary rocks, the grains in limestone coming from the skeletal remains of organisms that secrete shells composed of calcite or aragonite that are left behind when the organisms eventually die. Many limestones also contain varying amounts of silica in the form of jasper, flint, chalcedony etc. as well as siliceous skeletal fragments. Limestone also contains different amounts of sand, silt and clay that in times past had been carried to the area by ancient rivers. Other limestone deposits contain no grains at all. These are formed by chemical precipitation of calcite oraragonite, especially in mineral springs or hot springs. Secondary calcite is often deposited by groundwater, this is what forms stalactites and stalagmites in caves. A further form of limestone formed by calcite is that of oolitic (granular) limestone. It is easily recognised by its granular appearance.

Limestone can therefore be either crystalline, massive, granular or clastic, it depends on how it came to be formed initially. It is also usual for crystals of barite, dolomite, quartz and calcite to line the cavities in the limestone bedrock. Given the right conditions to cause precipitation, calcite will form a mineral coating that will act as a cement binding existing rock grains together as well as fill any fractures. The creation of mountain ranges caused regional metamorphism (the change in geologic texture or minerals), this phenomena recrystallised limestone deposits into marble.

Limestone usage reached its peak in the 19th and 20th centuries. It was a period that saw train stations built of limestone blocks, as were banks. It was also used as facades on skyscrapers (as was done by the ancient Egyptians when building the pyramids). Many famous British building have been constructed of Portland limestone blocks. It was also widely used in the Middle Ages throughout Europe with many churches and castles of that era being made of limestone.

Limestone quarries are widespread throughout Australia. One of the oldest is at Lilydale, east of Melbourne, in Victoria. This quarry was founded in 1878 and has been in continuous operation for more than a century.

About 85 percent of limestone produced in Australia goes to cement manufacturing. This is followed by construction and agricultural uses. Other modern day usage is its use in the production of lime, paper manufacture, manufacturing of glass and the steel making industry as well as chemical, paint and rubber industries.