Chapter IV: The Rocks (3)

The color ranges from gray through red, green and purple to black. The grays and black are due to the presence of more or less carbonaceous material in the original rock, the carbon compounds having changed to graphite. The reds and purple are due to the presence of iron oxides, and the green to the presence of chlorite.

While the particles of slate are so small as to be indistinguishable to the unaided eye, the use of thin sections under the microscope shows that slate is composed mostly of quartz and mica, with a wide range of accessory minerals, like chlorite, feldspar, magnetite, hematite, pyrite, calcite, graphite, etc.

According to their chief constituents slates may be distinguished as argillaceous-slate or _argillite_, bituminous-slate, calcareous-slate, siliceous-slate, etc.

Slate will be found here and there in the metamorphic areas of New England, the Piedmont Plateau, the Lake Superior region, and in many places in the west.

Phyllite Pl. 66

Phyllite is a thinly cleavable, finely micaceous rock of uniform composition, which is intermediate between slate and mica schist. In this case the flakes of mica are large enough to be distinguishable to the eye, but most of the rest of the material can only be identified with the aid of a microscope. It is mostly quartz and sericite. Phyllite represents a degree of metamorphism greater than for slate, but less than for schist; and it may grade into either of these other rocks. Garnets, pyrite, etc., may be present as accessory minerals. The color ranges from nearly white to black, and it is likely to occur in the same places as do slates.

Marble Pl. 66

This is a broad term, and includes all those rocks composed essentially of calcium carbonate (limestones) or its mixture with magnesium carbonate (dolomite), which are crystalline, or of granular structure, as a result of metamorphism. It takes less heat to metamorphose a limestone, and for this reason the marbles have a more crystalline structure than most metamorphic rocks; and they do not have the tendency to split or cleave which is so characteristic of most metamorphic rocks. It is only when there is a large amount of mica present that the typical schistosity appears. Commercially the term marble is used to include true marble and also those limestones which will take a high polish; but in this book, and geologically speaking, no rock is a marble unless it has crystalline structure.

Marbles range widely in color according to their impurities. Pure marble is white. Carbonaceous material in the antecedent limestone is changed to graphite in the metamorphic process, and makes the marble black, but appears usually in streaks or spots, rather than in any uniform color. An all black “marble” is usually a limestone. The presence of iron colors the marble red or pink. Chlorite makes it green, etc.

Various accessory minerals are common in marbles, such as mica, pyroxene, amphibole, grossularite among the garnets, magnetite, spinel, pyrite, etc., through a long list.

Because it cuts readily in all directions and takes a high polish, marble is widely used as a building stone. In the moist climate of the United States it suffers in being soluble in rain water when used on the outside of a building: but for interior decoration it furnishes some of the finest effects.

The largest marble quarries are developed in Vermont, Massachusetts, New York, Pennsylvania, Georgia, Alabama, Colorado, California, and Washington.

Steatite _Soapstone_

Steatite is a rock composed essentially of talc, which is associated with more or less impurities, such as mica, tremolite, enstatite, quartz, magnetite, etc. It is found in and with metamorphic rocks, and is a rock which has been modified by hydration from a metamorphic predecessor. It was probably first a tremolite or enstatite schist, in which, after the metamorphic rock came into the zone where ground water exists, the tremolite or enstatite was altered to talc, the impurities remaining much as they were in the first place.

It is bluish-gray to green in color, often soft enough to cut with a knife, and has a greasy feel. It is very resistant to heat and acids; for which reasons it has proved very useful commercially in making hearthstones, laundry tubs, and fire backs; and, when powdered, in making certain lubricants. The Indians, in the days before Columbus, took advantage of the ease with which it is cut, to make from it large pots for holding liquids, which are today among the greatest treasures in collections of Indian relics. They also carved pipe-bowls and various ornaments and amulets from soapstone.

It is found in Vermont, Massachusetts, New York, New Jersey, Pennsylvania, Maryland, Virginia, North Carolina, Georgia and California.

Serpentine Pl. 67

Pure serpentine is the hydrated silicate of magnesium, as described among the minerals on page 138. Serpentine rock is serpentine with more or less impurities, such as pyroxene, amphibole, olivine, magnetite, chromite, calcite, magnesite, etc. It often also contains mica and such garnets as pyrope, as accessory minerals. Serpentine, like steatite, always occurs in and with metamorphic rocks, and was originally a metamorphic rock, but has since been changed by the hydration of its silicates, when it came into the zone in which ground water is present. In the first instance it was some sort of shale, clay and dolomite, which was metamorphosed to an amphibole or pyroxene schist. When this was exposed to the action of ground water, the amphibole or pyroxene minerals were changed to serpentine, resulting in a rock composed mostly of serpentine, but retaining the impurities which were in the metamorphic rock, and perhaps adding to them such amphiboles and pyroxenes as were not altered during the hydration process. The above is the commonest type of serpentine rock. It can and sometimes has been formed in a similar way from an igneous predecessor, by the hydration of its silicate minerals. In this latter case the serpentine would not be a modified metamorphic rock, but a modified igneous one. It is a case where such a rock as a diorite or a gabbro is exposed to ground water and the pyroxene present altered to serpentine. A serpentine formed in this way would be a very impure one.

Serpentine rock is used as an ornamental stone for interior decoration, because it takes a high polish and has pleasing colors, various shades of green. It is however decidedly soft and will stand very little exposure to weather, and it is also filled with seams which make it difficult to get out large slabs.

Serpentine rock occurs fairly commonly in the metamorphic belt of New England and the Piedmont Plateau, and in some of the western states, especially California, Oregon, and Washington.

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.