Many marbles contain other minerals that are usually silicates of lime or magnesia. Diopside is very frequent and may be white or pale green; white bladed tremolite and pale-green actinolite also occur; the feldspar encountered may be a potassium variety but is more commonly a plagioclase (sodium-rich to calcium-rich) such as albite, labradorite, or anorthite. Scapolite, various kinds of garnet, vesuvianite, spinel, forsterite, periclase, brucite, talc, zoisite, wollastonite, chlorite, tourmaline, epidote, chondrodite, biotite, titanite, and apatite are all possible accessory minerals. Pyrrhotite, sphalerite, and chalcopyrite also may be present in small amounts.
These minerals represent impurities in the original limestone, which reacted during metamorphism to form new compounds. The alumina represents an admixture of clay; the silicates derive their silica from quartz and from clay; the iron came from limonite, hematite, or pyrite in the original sedimentary rock. In some cases the original bedding of the calcareous sediments can be detected by mineral banding in the marble. The silicate minerals, if present in any considerable amount, may colour the marble; e.g., green in the case of green pyroxenes and amphiboles; brown in that of garnet and vesuvianite; and yellow in that of epidote, chondrodite, and titanite. Black and gray colours result from the presence of fine scales of graphite.
Bands of calc-silicate rock may alternate with bands of marble or form nodules and patches, sometimes producing interesting decorative effects, but these rocks are particularly difficult to finish because of the great difference in hardness between the silicates and carbonate minerals.
Later physical deformation and chemical decomposition of the metamorphic marbles often produces attractive coloured and variegated varieties. Decomposition yields hematite, brown limonite, pale-green talc, and, in particular, the green or yellow serpentine derived from forsterite and diopside, which is characteristic of the ophicalcites or verd antiques. Earth movements may shatter the rocks, producing fissures that are afterward filled with veins of calcite; in this way the beautiful brecciated, or veined, marbles are produced. Sometimes the broken fragments are rolled and rounded by the flow of marble under pressure.
The so-called onyx marbles consist of concentric zones of calcite or aragonite deposited from cold-water solutions in caves and crevices and around the exits of springs. They are, in the strict sense, neither marblemarble was the “alabaster” of the ancients, but alabaster is now defined as gypsum, a calcium sulfate rock. These marblesmarble”) of the Italian antiquaries, the reddish-mottled Siena marble from Tuscany, the large Mexican deposits at Tecali near Mexico City and at El Marmol, Calif., and the Algerian onyx marble used in the buildings of Carthage and Rome and rediscovered near Oued-Abdallah in 1849 nor onyx, for true onyx is a banded chalcedony composed largely of silicon dioxide. Onyx are usually brown or yellow because of the presence of iron oxide. Well-known examples include the giallo antico (“antique yellow
These minerals represent impurities in the original limestone, which reacted during metamorphism to form new compounds. The alumina represents an admixture of clay; the silicates derive their silica from quartz and from clay; the iron came from limonite, hematite, or pyrite in the original sedimentary rock. In some cases the original bedding of the calcareous sediments can be detected by mineral banding in the marble. The silicate minerals, if present in any considerable amount, may colour the marble; e.g., green in the case of green pyroxenes and amphiboles; brown in that of garnet and vesuvianite; and yellow in that of epidote, chondrodite, and titanite. Black and gray colours result from the presence of fine scales of graphite.
Bands of calc-silicate rock may alternate with bands of marble or form nodules and patches, sometimes producing interesting decorative effects, but these rocks are particularly difficult to finish because of the great difference in hardness between the silicates and carbonate minerals.
Later physical deformation and chemical decomposition of the metamorphic marbles often produces attractive coloured and variegated varieties. Decomposition yields hematite, brown limonite, pale-green talc, and, in particular, the green or yellow serpentine derived from forsterite and diopside, which is characteristic of the ophicalcites or verd antiques. Earth movements may shatter the rocks, producing fissures that are afterward filled with veins of calcite; in this way the beautiful brecciated, or veined, marbles are produced. Sometimes the broken fragments are rolled and rounded by the flow of marble under pressure.
The so-called onyx marbles consist of concentric zones of calcite or aragonite deposited from cold-water solutions in caves and crevices and around the exits of springs. They are, in the strict sense, neither marblemarble was the “alabaster” of the ancients, but alabaster is now defined as gypsum, a calcium sulfate rock. These marblesmarble”) of the Italian antiquaries, the reddish-mottled Siena marble from Tuscany, the large Mexican deposits at Tecali near Mexico City and at El Marmol, Calif., and the Algerian onyx marble used in the buildings of Carthage and Rome and rediscovered near Oued-Abdallah in 1849 nor onyx, for true onyx is a banded chalcedony composed largely of silicon dioxide. Onyx are usually brown or yellow because of the presence of iron oxide. Well-known examples include the giallo antico (“antique yellow
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