Common Marbles and Compact Limestone

Few countries do not produce marble, although many of the commercial marbles are limestones.  The terms marble and limestone are wrongly and commonly confused.  Limestone is a sedimentary rock.  Marble is a metamorphic rock made up mostly of calcite (CaCO3).  In a marble the calcite is recrystalized to produce and interlocking granular mosaic of rougly equal-sized calcite crystals.  Calcite is the name given to calcium carbonate when it is in crystal form.  The recrystalization removes any of the original sedimentary structures and fossils.  No true marble will have fossils.  Non-calcareous mineral matter present in the original limestone will also be metamophosed and new kinds of mineral assemblages will be created.  Some marble is produced by thermal metamophism but by far the greater amount is the result of regional metamophism.  Considerable termperatures and pressures are involved.  As a result, the mosaic of calcite crystals shows a rough alignment which may not always be visible to the naked eye.  It can commonly be seen on the gross scale and marble sometimes shows a rough schistosity.  The length of time that the rock is subjected to metamophic processes determines the coarseness of the grain size that may be expected.  The process is aided by active pore fluids.  In marble it is aided by carbon dioxide.

The granular structure of marble is often called sugary and viewed from some aspects a cut surgace may look like sugar in the mass.  As marble weathers, the bonds between the grains of calcite are loosened and the surface assumes a sugary appearance known as saccharoidal weathering.  This feature may be used with care as one of the criteria used to decide whether a stone is a marble or a limestone.  A limestone and a marble will weather in differnt manner.  Because of the interlocking mosaic of calcite grains marbles have a very low porosity. 

A pure marble, which is entirely or nearly of calcium carbonate, is a monomineralic rock.  It is white in colour.  Because the calcite crystals which make up the rock are normally transparent, slabs of marble up to about 30mm (1.2 inches) thick may appear translucent.