Field book of common rocks and minerals

by Frederic Brewster Loomis

[Ophiolite]
Ophicalcite

This name is given to marbles which are streaked and spotted with serpentine. They are a mixture of green serpentine and a white or nearly white calcite, magnesite or dolomite in variable proportions.

Ophicalcite occurs in and with metamorphic rocks, and represents an impure limestone which has been metamorphised, the lime becoming marble, and the impurities becoming such silicates as pyroxene, amphibole, or olivine. This metamorphic rock has then come into the zone of ground-water and the silicate minerals have been changed by hydration to serpentine. Ophicalcite is then a metamorphic rock, in which secondary chemical changes have since taken place. It may have a wide range of accessory minerals present, such as magnetite, chromite, pyrope among the garnets, olivine, etc. Verde antique is a trade name for one of the ophiolites.

While not abundant, ophicalcite is in good demand as an ornamental stone for interior work; for it takes a high polish, and is beautiful; but, on the other hand, it will not stand exposure to the weather for the calcite is soluble, and there are numerous seams and cracks in it making it difficult to obtain large slabs.

It occurs in Quebec, Canada, in the Green Mountains of Vermont, and in the Adirondack Mountains.

CHAPTER V
MISCELLANEOUS ROCKS

There are a few rocks which do not fit into any of the three groups described, such as concretions, geodes, meteorites, etc., and they are gathered together here. There is also one type of rock, which really belongs among the minerals, but is likely not to be so recognized at first glance, and that is the material filling veins. These last are sometimes designated “vein rocks,” but are really massive deposits of one, two or more minerals, and should be referred to the minerals when found.

Concretions

In the sedimentary rocks there frequently occur inclusions of a nature different from the surrounding rock. In shape they are usually rounded, nodular, spherical, discoidal, ovate, flattened, elongated or ring-shaped, or combinations of the foregoing, making often curious and fantastic forms. In size they range from a fraction of an inch in diameter to several feet through. When broken, they may show a nucleus, around which more or less concentric layers have formed, or neither nucleus nor concentric structure may be visible. The layered structure of the surrounding rock in some cases continues right through the nodular mass. These structures are called concretions, and their formation in all cases is at least due to similar reactions.

In general the concretions differ from the surrounding rock in composition, but are usually composed of some one of its impurities, of lime in the clays or silica in limestones, of iron oxide in sandstone, etc. They seem to have originated as a result of the solution of the minor mineral, and then its redeposition around some center or nucleus. In many cases the nucleus is organic, such as a leaf, a shell, a bone, etc., so that when the concretion is split, in its center will be found the perfect imprint of the leaf, or the shell of a mollusk, or a bone of a higher animal, sometimes a whole skeleton. Again the nucleus may be inorganic like a grain of sand; and in still other cases no nucleus can be found, though there was probably one in the beginning. What has happened is somewhat like the case of accessory minerals in igneous and metamorphic rocks. A layer of sediment was laid down, including in it, here and there, something foreign to the run of the rock. Later when the water leaches through this rock, impregnated with lime for instance, it comes to the point where a leaf is decomposing. The products of the leaf decomposition are different from what is already present in solution, and may precipitate some of the lime in that neighborhood. As long as leaf decomposition continues the precipitation in that region will continue and increase the size of the concretion. This sort of action accounts for many of the concretions, especially those about organic remains. In some other cases where there is no nucleus, as the flint in chalk, what has taken place is that the small amounts of silica in the lime have been dissolved, and then around some center has constantly been added more and more non-crystalline silica until a mass of flint has accumulated. There may be a considerable variety of ways to account for different concretions, but in all cases solutions of one mineral have come in contact with solutions of a different kind, and precipitation about a center has resulted.