All Souls Church in Halifax, stands proud, its pale colour defining it from the rest of the sandstone and grit stone buildings.
So what makes it stand out so much?
The facing stone of the church is limestone, which is a sedimentary rock.
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Most limestone is composed of skeletal fragments of marine organisms such as coral, crinoids and molluscs.
The stone used here, is a different type of limestone. It is known as Magnesian Limestone CaMg(CO3)₂, often called Dolomite, was originally formed in the shallow tropical Zechstein sea some 250 million years ago. It has subtle colour variations from white to pale yellow, and is an oolitic limestone. Oolitic limestone is a carbonate rock made up mostly of ooliths (or ooids) which are sand-sized carbonate particles that have concentric rings of Calcium Carbonate. These rings are formed around grains of sand or shell fragments that were rolled around on the shallow sea floor, gathering layer after layer of limestone.This limestone formed in a shallow sea. The rock has an even structure rather like cod roe and it can therefore be cut or sculpted in any direction. This feature, coupled with hardness, colour and durability, gives the limestone its quality as a building stone. Its colour can be white, grey or yellow, with a fine grained texture.
Elements in the limestone contribute to colour :
Small amounts of iron in the structure give the crystals a yellow to brown tint.
A high manganese content gives the crystals a rosy pink colour.
Looking at the church, you will notice that the stone is eroded (there are many EarthCaches which describe this process, so here we just talk about Magnesian limestones). The effects of the environment on these type of limestones , was first studied in the 19th Century, when it was found that magnesian limestone used in the Houses of Parliament was affected by the damp, heavily polluted London atmosphere.
The magnesium-rich mineralogy does not appear to make the limestone more vulnerable to decay as there are many examples where such limestones are intact, though it has been found that magnesian limestones do not fare as well as some calcitic varieties in polluted urban settings.
In chemical terms, magnesian rich varieties of limestone are more soluble that other types of limestone, and these types also have greater volumetric expansion within the pore spaces when affected by acid rain. This leads to discolouration, efflorescence, blistering and ultimately severe surface exfoliation.
Lets take a walk. First the co-ords take you to the corner of the church. Have a look at the magnesian limestone, where it has suffered surface exfoliation.
(1) Please look at the rock, and describe the colour and texture of it.
(2) Some blocks have been replaced, how do they compare to the exfoliated blocks?
Now lets walk round to the front door of the church.
(3) One of the columns made out of magnesian limestone is coloured, what colour is it, and what element has caused it to be like this?
(4) Using the above information why do you think that perhaps the use of this type of rock was not ideal in 19th century industrlal Halifax?
This being an EarthCache, in order to log it, I ask that you answer the above questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.