Limestone Classification


Limestone Classification

Limestone is a sedimentary stone with 50% by weight calcite or calcium carbonate (CaCO3) content. However, commercial limestone usually has more than 50% calcium carbonate. Limestone is a “clastic” sedimentary stone. Almost all limestone is composed of grains or fragments of biologic origin, ranging from fossils or organically derived grains that weigh less than an ounce. Most limestone is marine in origin, composed of micro-sized fossils of marine invertebrate organisms, for example, the sea shells found on most beaches. One of the rarest type of limestone is composed of inorganic, precipitated calcium carbonate. Even less common is limestone of igneous origin called carbonatites, which is found in diamond-bearing rock.

fossils in limestone rock

What is Limestone?

Limestone is a carbonate stone, in which, it has the -CO3 radical combined with the calcium atom. Other carbonate minerals seen in dimension stone are the carbonates siderite (FeCO3), magnesite (MgCO3), and dolomite Ca,Mg(CO3). Dolomite is both a mineral and a stone, and is used largely as a commercial limestone. Since limestone by definition must be at least 50% calcium carbonate, the other 50% can be one of various clasts or minerals of other kinds of stone. The various minerals listed above react similar to limestone; they all have the same hardness, three good cleavages (i.e., they easily break into parallelograms, indicating they have the same atomic geometry); and they all react in some manner to cold, dilute hydrochloric acid and other diluted acids.

Durability of Limestone

There are certain types of limestone that are more durable than others; some may crumble while others flourish in situations dealing with weight and/or exposure to the elements. Limestone diluted with too much clay, sand, or other non carbonate grains is not acceptable as dimension stone because these elements compromise how well the limestone is cemented together; the clays may wash out, or if sandy, the sands may wash or weather out too easily, or the stone will not take an acceptable finish. However, clay is the source of coloring in many limestones, because it contains the iron oxides that yield yellow through red stain. Thus, a very small component of clay may be acceptable in commercial limestone. Large inclusions or bands of clay severely weaken the stone.

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