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Limestone used to be alive. Sandstone did not. Limestone it's a sedimentary rock that derives from the decomposition of (formerly) living organism and, because of that is mostly made of Calcium Carbonate. That classifies it as a calcite rock Sandstone is still a sedimentary rock that was formed by layers of sand that, somehow, sunk deep into the belly of Mother Earth. Once there, high temperature partially melted the sand, and high pressure bonded the components together. Since it is of an inorganic origination, it's a silicate rock.
Limestones are calcareous rocks made up of at least 50% of carbonate mostly in the form of calcite, aragonite, and dolomite. There are other carbonate varieties but these are not significant in the context of building stone. As a group, these carbonate rocks represent about one-fifth of all sedimentary rocks, occur on all continents, and span all ages from recent to very old. Limestones are regarded as non-clastic sedimentary rocks that can be formed from many geological processes. They are texturally, structurally and mineralogically extremely diverse - characteristics which reflect their derivation, environment of deposition, and diagenetic history. Although a large proportion of limestones are biogenic (contain fossil material), there are also chemically deposited limestones which are important as building stone, namely travertine.
The principal factor which unifies these carbonate rocks is their high reactivity to acids. Acids are a fact of life and here to stay. There are many of them in our home and in our environment. Wine, carbonated drinks, fruits and fruit juices, vinegar, and even some natural waters will react with these carbonate rocks. And there is little that we can do about it!! Many a posh resort and nice residences with originally finely polished or honed limestone surfaces on their bar tops or kitchen-tops have discovered this.
Another key characteristic of limestones is their softness relative to other rocks such as granites. The softness is mainly a function of the mineral composition. Calcite typically has a hardness of 3 on the Mohs scale of hardness whereas dolomite and aragonite are a bit harder (3.5 to 4). Because of this softness, it can be easily scratched by most things in the house such as cutlery, coins, tools, keys, toys, etc., so there should never be the expectation that nicely polished or honed limestone surfaces are going to stay that way for ever.
A physical feature of limestones which is important geotechnically is its intrinsic porosity. Many limestones, particularly the biogenic ones, have a medium to high degree of porosity. Porosities of 10-30% are not unusual in the French limestones and the famous Australian limestone from Mt Gambier has a porosity of 50%. But although technically weak and very absorbent, certain construction techniques (both old and modern) allow them to be used successfully and effectively. In cold climates, however, a porous limestone can suffer rapid degradation due to freeze-thaw cycling and some protection might be required.
One important structural aspect inherent in most limestones is the presence of very fine, squiggly lines which are often brownish. These structures, called stylolites, are pressure-solution features formed during the compaction and lithification of limestones. Mostly, the brownish colour is due to hydrated iron oxide but there can also be concentrations of clays and/or sulphides. Because these stylolites are natural planes of weakness and can often transmit fluids because they are not fully closed, any expanding clays can react to fluids and physically weaken the limestone.
A final comment is on the observation that over the last few years or so, a number of "marbles" have reverted to "limestones". In the stone trade, it has been long accepted that any limestone that can be polished can also be called a marble. If a limestone has undergone some post-depositional (diagenetic) modifications some recrystallization will have occurred. The crystals might still be quite small but the crystalline minerals of the limestone will accept a polish. Because marbles are "old hat" and limestone has become the buzzword of many architects, there is now a strong trend to market mildly recrystallized limestones that were once called marbles. Indeed, it is this group that appears to be capturing a significant portion of the stone market. Incidentally, some limestones from Oman and Saudi Arabia were seriously considered for the external cladding as was the beige-coloured Jurastone from Germany. |
