A later article or chapter will be devoted largely to a brief historical sketch of the use of limestone in building construction and its effect on design, and an outline of the development of architectural practice with this material.
The remainder of the present chapter will be devoted to the material and its formation, and the classes or subdivisions into which it is divided bv geologists.
It is thought that the consideration of any material should be prefixed by a clear and concise definition of the various classes into which it is divided, with a general statement of the reasons for this classification of the product, and in the case of natural stone, a brief outline of the geology and economic importance of the products under consideration, their composition, origin and differences in formation, and consequent properties, appearance and comparable features. For a more complete or thorough discussion, the reader is referred to the various textbooks on the Geology of American Building Stones and to the reports issued by both State and Federal Government Departmental Surveys.
Limestones in General
Limestone is a so-called “aqueous” or sedimentary rock, composed principally of the mineral Calcite or calcium carbonate, of which the chemical formula is CaCo3. It is formed by the accumulation of calcium sediment in a body of water, principally in the ocean or one of its seas, but also in certain bodies of fresh water.
Limestone differs from such other sedimentary rocks as sandstone, conglomerate and shale, in con
sisting almost entirely of calcium carbonate, which is popularly called carbonate of lime. While there is no essential difference in the technical terms, “calcite” and “calcium carbonate,” which are frequently used synonymously, and no difference in the chemical composition, there is an untechnical popular usage of these terms which is considered appropriate. This consists of using the term “calcium carbonate” to designate the matter in its original practically non-crystalline shell or only slightly crystalline form, and the term “calcite” for this same matter in crystalline form.
In all forms of carbonate rock, we therefore have the same mineral matter as primary carbonate (shell remains) and often as both secondary and tertiary calcite (crystalline accretions thereto), the predominance of either form, that is the extent of crystallization, determining the appropriate use of the two terms.
At this point it seems appropriate to interject the statement that marbles, that is most marbles which are high in calcite, are converted limestone, i.e.—metamorphosed and recrystallized limestone and when of a pure, or reasonably pure calcium carbonate rock, are quite similar in composition to limestone. The marble, originally limestone has undergone a metamorpliic change due to heat, moisture and pressure and in this manner has lost, either entirely or very largely, its original shell or other sediment grain formation, becoming either fully or very nearly crystalline in structure.
So-called marbles, which are only semi-crystal
Fig. 4. Corinthian Cap, carved from a single block of limestone
Fig. 5. Corinthian Cap and Column, carved from a
single block of limestone
The remainder of the present chapter will be devoted to the material and its formation, and the classes or subdivisions into which it is divided bv geologists.
It is thought that the consideration of any material should be prefixed by a clear and concise definition of the various classes into which it is divided, with a general statement of the reasons for this classification of the product, and in the case of natural stone, a brief outline of the geology and economic importance of the products under consideration, their composition, origin and differences in formation, and consequent properties, appearance and comparable features. For a more complete or thorough discussion, the reader is referred to the various textbooks on the Geology of American Building Stones and to the reports issued by both State and Federal Government Departmental Surveys.
Limestones in General
Limestone is a so-called “aqueous” or sedimentary rock, composed principally of the mineral Calcite or calcium carbonate, of which the chemical formula is CaCo3. It is formed by the accumulation of calcium sediment in a body of water, principally in the ocean or one of its seas, but also in certain bodies of fresh water.
Limestone differs from such other sedimentary rocks as sandstone, conglomerate and shale, in con
sisting almost entirely of calcium carbonate, which is popularly called carbonate of lime. While there is no essential difference in the technical terms, “calcite” and “calcium carbonate,” which are frequently used synonymously, and no difference in the chemical composition, there is an untechnical popular usage of these terms which is considered appropriate. This consists of using the term “calcium carbonate” to designate the matter in its original practically non-crystalline shell or only slightly crystalline form, and the term “calcite” for this same matter in crystalline form.
In all forms of carbonate rock, we therefore have the same mineral matter as primary carbonate (shell remains) and often as both secondary and tertiary calcite (crystalline accretions thereto), the predominance of either form, that is the extent of crystallization, determining the appropriate use of the two terms.
At this point it seems appropriate to interject the statement that marbles, that is most marbles which are high in calcite, are converted limestone, i.e.—metamorphosed and recrystallized limestone and when of a pure, or reasonably pure calcium carbonate rock, are quite similar in composition to limestone. The marble, originally limestone has undergone a metamorpliic change due to heat, moisture and pressure and in this manner has lost, either entirely or very largely, its original shell or other sediment grain formation, becoming either fully or very nearly crystalline in structure.
So-called marbles, which are only semi-crystal
Fig. 4. Corinthian Cap, carved from a single block of limestone
Fig. 5. Corinthian Cap and Column, carved from a
single block of limestone