Marble and Other Stone
22 February 2007
Under Construction
Howard Roark – On a Definition of
Marble
“Generally speaking, there are three kinds [of marble]. The white marbles, which are derived from the recrystallization of limestone, the onyx marbles which are chemical deposits of calcium carbonate, and the green marbles which consist mainly of hydrous magnesium silicate or serpentine. This last must not be considered as true marble. True marble is a metamorphic form of limestone, produced by heat and pressure. Pressure is a powerful factor. It leads to consequences which, once started, cannot be controlled.”
“What consequences?”
“The recrystallization of the particles of limestone and the infiltration of foreign elements from the surrounding soil. These constitute the colored streaks which are to be found in most marbles. Pink marble is caused by the presence of manganese oxides, gray marble is due to carbonaceous matter, yellow marble is attributed to a hydrous oxide of iron. This piece here is, of course, white marble. There are a great many varieties of white marble.”
The Fountainhead. Ayn Rand. (1993:213 Signet)
Information about ANHYDRITE
Anhydrite, a mineral
(anhydrous calcium sulfate, CaSO4), does not form directly, but is the result
of the “dewatering” of the rock forming mineral Gypsum
(CaSO4-2H2O). That is, it has the same chemical formula as gypsum, a calcium sulphate, except that it is devoid of water; it has no
water of crystallization. Anhydrites formed through volcanism and the resulting
mineralization process (more than 20 million years ago in some instances),
creating calcium sulphate that infused cracks and
cavities in surrounding rock. Local temperature affects the formation of
crystals. If temperatures remain just below 58 degrees for a very long time a
few, very big crystals will develop. Lower temperatures (below about 58
degrees) cause many, smaller crystals to form. Gypsum can take a particular crystalline
form such as selenite which is known for its
translucency. Selenite, satin spar, desert rose, and
gypsum flower are the four crystalline varieties of gypsum. Gypsum is composed
of calcium sulfate dihydrate (meaning it has 2
molecules of water - CaSO4·2H2O). Gypsum and alabaster are the massive
form of gypsum - massive meaning that neither gypsum nor alabaster has crystal
habit obvious to the naked eye.
Anhydrite can be white,
sometimes with a greyish, bluish or purple cast. Fine
specimens of Anhydrite have been found in
Anhydrite is in the
orthorhombic crystal system, with three directions of perfect cleavage parallel
to the three planes of symmetry. It is not isomorphous
with the orthorhombic barium (barite) and strontium (celestine)
sulfates. Distinctly developed crystals are somewhat rare, the mineral usually
presenting the form of cleavage masses. On the best developed of the three
cleavages the lustre is pearly,
on other surfaces it is vitreous. When exposed to water, anhydrite readily
transforms to the more commonly occurring gypsum by water absorption. Anhydrite
is commonly associated with calcite, halite, and sulfides such as galena,
chalcopyrite, molybdenite, and pyrite in vein
deposits.
Alabaster
Alabaster
include diverse varieties of two distinct minerals: gypsum (a
hydrous sulfate of calcium) and calcite (a carbonate of calcium). Calcite
alabaster, being a carbonate, effervesces on being touched with hydrochloric
acid, whereas the gypsum alabaster remains practically unaffected.
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