Serpentine rocks are generally compact, of a dull to waxy luster, and of a smooth to splintery fracture. If tolerably pure they are soft and can be cut by the knife, but they are sometimes saturated by deposited silica, which makes them much harder. The general color is green, characteristically a yellowish-green; but sometimes yellow, yellow-brown, reddish-brown, and dark green to black. On smooth surfaces the rock has a somewhat greasy feel, recalling talc schist, from which it is, however, readily distinguished by its superior hardness. Talc leaves its mark on cloth, while serpentine does not. The yellow-green color resembles also that of epidote rocks, but here again the superior hardness of the epidote serves as a distinction.
The peridotites are extremely liable to alteration, so much so that unchanged occurrences are not at all usual. By alteration, under the influence of surface weathering agencies, the peridotites give rise to serpentine rocks; and there is no doubt that a great number of serpentines have originated in this way. The most common form of alteration is that in which the olivine and other magnesian silicates are changed to serpentine. On the other hand, some have been shown to be derived from augitic rocks, such as gabbro, diabase, etc., and others, from hornblendic rocks, as in the case of the Rauenthai serpentine, described by Weigand. Fragments of unaltered olivine, diallage, and bastite (the latter derived from a rhombic pyroxene) are often found embedded in serpentine, thus furnishing a proof of the mode of the origin of these rocks. Again, their mode of occurrence, in dykes and bosses, leads to the same conclusion. The chemical changes created by the conversion of olivine and augite to serpentine are illustrated by the following equations:
(1) Alteration of olivine
to serpentine.
2[2MgO.SiOJ + C02 + 2H2 = MgCO3 + 3MgO.2SiO2,2H2O
(2) Alteration of
enstatite to serpentine.
4[MgO-SiO2 + CO2 + 2 FT2 = MgCO3 + 2Si02 + 3MgO.2SiO2 + 2H20
Serpentines are dull-green and red, often mottled rocks. They are so soft that they can be easily scratched, or even cut with the pen-knife. Veins of fibrous chrysotile and steatite frequently traverse them. In some cases the serpentinous mineral is disseminated through limestone, as in the rocks known as ophioalcite. The chemical composition of serpentine rocks approaches that of the pure mineral, but generally differs somewhat on account of the other minerals present. Serpentine rocks are secondary by nature, forming from igneous rocks, such as peridotite, dunite, etc.; or when amphibolites or hornblende-schists, which have been made from sediments in the zone of constructive metamorphism, are brought by erosion into the zone of hydration, they may be converted into serpentines. Thus the origin of the material may be igneous or sedimentary, but, whereas the igneous rocks pass directly into serpentine, the sedimentary ones first pass through an intermediate metamorphic stage (hornblende-schists, etc.), and are then converted. This also explains in part at least the origin of the chromium and nickel. No formula can be given for the recognition of which origin a serpentine has had; the geologic mode of occurrence and relation to other rock masses is often a help, while the presence of nickel and chromium, substances to be expected in igneous, but not in sedimentary rocks, if it can be shown, is very significant.
Other magnesian minerals such as talc are also formed by the alteration of these rocks, but that to serpentine is the most important. All stages of transition to pure serpentine occur, and studies which have been made in recent years show that a large part, perhaps the greater part, of the occurrences of this mineral are to be assigned to the alteration of rocks of this group. Serpentine shows great resistance to the action of the weathering agencies at the surface, but eventually breaks down into a mixture of iron and magnesium carbonates, clay and silica, mixed with ferruginous matter. The peridotites ultimately weather down into brown ferrugineous soils which, on account of their lack of calcium and potassium, do not favor vegetable growth and are therefore often nearly barren of plant life on the surface.
Other minerals which may accompany serpentine, and which may at times be seen in it, are remains of the magnesia silicates from which it has been formed, olivine, pyroxene, and hornblende. Metallic looking specks or crystals of ores are common, magnetite, chromite, etc. In some varieties garnet occurs, chiefly pyrope, and that which is used for gems which for many years came in large part from a serpentine in Bohemia in the Czech Republic. In the Ural Mountains serpentine is the source of native platinum, and in other places of nickel ores. Serpentine is apt to be accompanied by other secondary minerals, by chlorite sometimes the purple red variety kammererite containing chromium, by talc, and by magnesium carbonates, magnesite, MgCO3, and breunnerite, MgFeCOs, etc. Serpentine rocks are usually massive but sometimes schistose, forming serpentine-schist. Not infrequently they are seamed by veins of the finely fibrous variety of the mineral called chrysotile, which has the structure of asbestos and is often so called. Serpentine has been mined in a number of places for its asbestos content. It is often assumed that the typical process of serpentinization of peridotite takes place a short distance below the surface. Serpentine is, however, also formed on a large scale at greater depths, where quantities of CO2 could not very well be assumed for the reason that such alteration would result in a mixture of serpentine and carbonates, whereas the large serpentine masses rarely contain admixed carbonates. The deep canons of the Sierra Nevada gold country, in California, show clearly that the serpentines of this range are not superficial, but descend to the depth of several thousand feet, and have been noted as such in the gold mines.
On account of its beautiful coloring, serpentine has been largely quarried for use as an ornamental stone, being used for interior purposes much as highly colored marbles are. Many locations, however have outlawed its use as a paving or decorative stone. Air testing along roads and driveways with paved with serpentine emit dust containing significant asbestos fibers, which are a hazard to those who breathe it. It is sometimes employed for the same objects for which soapstone is used; in many cases its softness is an objection to its employment.
Serpentine is a Source of Valuable Minerals. The magmas which form the peridotites usually carry small amounts of chromic oxide which often crystallizes with iron oxide to form the mineral chromite, FeCr2O4, a member of the spinel mineral group. It is often seen in dunite and usually forms small, black, pitchy-looking grains. Sometimes this mineral is concentrated in sufficient amount so that it becomes a useful ore, supplying the chromium ore used in the industrial arts.
In the formation of serpentine, fiberous minerals are often formed. The fiberous form of actinolite, chrysotile asbestos is mined from serpentine. Where the serpentine rock contains enough asbestos, it is quarried and the asbestos removed for use. Chrysotile asbestos has been mined in Vermont; New York; New Jersey; Idaho, Wyoming, California, and from the area around the Grand Canyon in Arizona.
The olivine of these rocks has been found by analysis to contain a minute amount of nickel oxide; when they change to serpentine and are then deeply weathered to remove much of the magnesium and calcium, it sometimes happens that this nickel is concentrated in the form of garnierite, a nickel silicate, sometimes in amounts sufficient to form deposits of value as a source of this metal, as in the nickel ores of Douglas County, Oregon, and in the Island of New Caledonia.
The peridotites, and to some extent their allies the gabbros, are also the source of platinum, which occurs in them by processes of magmatic segregation as the native metal or as sperrylite PtAs2. Through the process of decay of the rock it is washed down and, like gold, concentrated in alluvial deposits of platinum ore. The precious garnet, pyrope, used as a gem, also comes from a decayed and serpentinized peridotite from Bohemia, South Africa, etc. Lastly, the diamonds of South Africa have their source in decayed and greatly altered peridotite rocks. This altered rock, which was originally a mica peridotite, is known as kimberlite, by the miners as "blue ground." Some have held that the carbon forming the diamonds was derived from the shales through which the magma passed, others hold that it was original in the magma and that the diamond is a true crystalline constituent of the igneous rock like any other of its accessory minerals.
Serpentine is a common rock, and, while it rarely forms large masses or covers extensive areas, it is widely distributed over the world. In the form of layers, lenticular masses, etc., it is common in metamorphic regions from the alteration of both igneous and metamorphic rocks, and it thus occurs in eastern Canada, New England, New York, Pennsylvania, Maryland, California, Oregon, and other states; in southern England, Germany, the Alps and various other places. It also occurs in non-metamorphic sedimentary areas due to the conversion of igneous rocks which have penetrated the strata, as in places in Quebec, New Brunswick, New York State, etc. It is commonly associated with gold deposits in many areas, including California.
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