1911 Encyclopædia Britannica/Serpentine (mineral)

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SERPENTINE, a mineral which, in a massive and impure form, occurs on a large scale as a rock, and being commonly of variegated colour, is often cut and polished, like marble, for use as a decorative stone. It is generally held that the name was suggested by the fancied resemblance of the dark mottled green stone to the skin of a serpent, but it may possibly refer to some reputed virtue of the stone as a cure for snake-bite. Serpentine was probably, at least in part, the λίθος ὀφίτης of Dioscorides and the ophites of Pliny; and this name appears in a latinized form as the serpentaria of G. Agricola, writing in the 16th century, and as the lapis serpentinus and marmor serpentinum of other early writers. Italian sculptors have sometimes termed it ranochia in allusion to its resemblance to the skin of a frog.

Although popularly called a “marble,” serpentine is essentially different from any kind of limestone, in that it is a magnesium silicate, associated however, with more or less ferrous silicate. Analyses show that the mineral contains H4Mg3Si2O9, and if the water be regarded as constitutional the formula may be written Mg2(SiO4)2H3(MgOH). Serpentine occurs massive, fibrous, lamellar or granular, but never crystallized. Fine pseudomorphs having the form of olivine, but the composition of serpentine, are known from Snarum in Buskerud, Norway, the crystals revealing their character by containing an occasional kernel of the original mineral. The alteration of rocks rich in olivine has given rise to much of the serpentine occurring as rock-masses (see Peridotite). Studied microscopically, the change is seen to proceed from the surface and from the irregular cracks of the olivine, producing fibres of serpentine. The iron of the olivine passes more or less completely into the ferric state, giving rise to grains of magnetite, which form a black dust, and may ultimately yield scales of haematite or limonite. Considerable increase of volume generally accompanies serpentinization, and thus are produced fissures which afford passage for the agents of alteration, resulting in the formation of an irregular mesh-like structure, formed of strings of serpentine enclosing kernels of olivine in the meshes, and this olivine may itself ultimately become serpentinized. Serpentine may also be formed by the alteration of other non-aluminous ferro-magnesian silicates such as enstatite, augite or hornblende, and in such cases it may show microscopically a characteristic structure related to the cleavage of the original mineral, notably lozenge-shaped in the case of hornblende. Many interesting pseudomorphs of serpentine were described by Professor J. D. Dana from the Tilly Foster iron-mine, near Brewster, New York, U.S.A., including some remarkable specimens with cubic cleavage.

The purest kind of serpentine, known as “noble serpentine,” is generally of pale greenish or yellow colour, slightly translucent, and breaking with a rather bright conchoidal fracture. It occurs chiefly in granular limestone, and is often accompanied by forsterite, olivine or chondrodite. The hardness of serpentine is between 3 and 4, while the specific gravity varies from 2.5 to 2.65. A green serpentine of the exceptional hardness of 6, formerly regarded as jade, is known as bowenite, having been named by J. D. Dana after G. T. Bowen. The original bowenite came from Smithfield, Rhode Island, U.S.A., and a similar mineral was described by General C. A. McMahon as occurring in Afghanistan, where it is carved for ornamental purposes in the belief that it is jade (q.v.). Many common carvings regarded as jade are really serpentine, and therefore soft. Serpentine of columnar or coarsely fibrous form is termed picrolite, a name proposed by J. F. L. Hausmann from the Greek πικρός (bitter) in allusion to the presence of magnesia. The finely fibrous serpentine is called chrysotile from the lustrous yellowish colour which it usually presents (χρυσός, gold; τίλος, fibre) and this variety is extensively worked, especially in Canada, for use as asbestos (q.v.). In order to avoid confusion between the words chrysotile and chrysolite, it has been proposed by Dr J. W. Evans that the fibrous serpentine should be distinguished as karystiolite — a modification of the ancient name, taken from its occurrence near Karystos in Euboea. Foliated serpentine is usually termed marmolite — a name given by G. T. Nuttall, from μαρμαίρω (to glisten) in reference to its lustre. A thin lamellar or flaky serpentine supposed to occur in the Antigorio valley north of Domodossola in Piedmont is called antigorite, having been named in 1840 by M. E. Schweizer, after whom a somewhat similar mineral is termed schweizerite. Antigorite has been studied by Professor T. G. Bonney and Miss C. Raisin (Quart. Journ. Geol. Soc., lxi., 1905, p. 690; lxiv., 1908, p. 152). An apple-green translucent serpentine passes under the name of williamsite, having been so called by C. U. Shepard in honour of its discoverer L. White Williams, of West Chester, Pennsylvania, where this variety occurs.

“Common serpentine” is the impure massive kind which occurs in rock-masses and is extensively worked as “serpentine-marble.” It is sometimes veined with steatite, or magnesite, and may contain scattered crystals of diallage, bronzite or bastite (an altered rhombic pyroxene), which by schillerization may present a metallic lustre. In England the chief localities of serpentine are in Cornwall, especially in the Lizard district, where it is quarried and carved into mantelpieces, columns, vases and other ornaments. Much of it presents a rich red or brown colour, often mottled and sometimes veined. Professor Bonney has shown that it has been largely derived from olivine. Green serpentine occurs near Holyhead in Anglesey. A beautiful serpentine, generally mottled red and green, with veins of steatite, is found at Portsoy in Banffshire, Scotland, and was used for pillars in the great hall at Versailles. Serpentine containing chromite is found in the Shetland Islands.

The rock called “ophicalcite” consists of an intimate association of serpentine with limestone, often forming an ornamental stone which is beautifully clouded and zoned with various shades of green. It generally results from the metamorphism of an impure dolomitic limestone, the impurities having crystallized as new minerals which become altered to serpentine. Pseudomorphs of serpentine occur after forsterite. The best known serpentinous marble of the British Isles occurs in Connemara in Galway, Ireland, and passes in trade under the name of “Irish green.” Ophicalcites are developed also in various parts of Scotland, and the green pebbles found in Iona belong to this type of rock. The famous eozoonal marble of Canada is also of similar character.

In Saxony common serpentine is largely worked at Zöblitz near Marienberg and Waldheim. The rock of Zöblitz, mentioned by G. Agricola in the 16th century, is usually of dull green or brown colour, and frequently contains dark red Bohemian garnet or pyrope (q.v.). It was used in the mausoleum of Prince Albert at Frogmore, Windsor, and in Abraham Lincoln's monument at Springfield, Illinois, U.S.A. Italy is rich in serpentine, the best-known being the verde di Prato, which has been quarried for centuries at Monteferrato near Prato in Tuscany, and has been largely used in ecclesiastical architecture in Florence, Prato and Pistoja. Much serpentine is found near Genoa and Levanto. The verde di Pegli comes from Pegli not far from Genoa, while the verde di Genova is a brecciated serpentinous limestone from Pietra Lavezzara. Serpentine occurs also at many localities in the Apennines, in Elba and in Corsica. The term ophiolite has been vaguely used to include not only serpentines but many other rocks associated with the Italian serpentines. Verde antico is a brecciated serpentine with fragments of limestone, originally brought by the Romans from Atrax in Thessaly, and called lapis atracius. It is sometimes known as vert antique, or, following the old French, verd antique. The term serpentine is often improperly applied to the ancient green porphyry of Laconia in the Peloponnesus (porfido serpentino verde). True serpentine occurs at numerous localities in the Alps and in France, an elegant variety being quarried, at Épinal in the Vosges, whilst a fine ophicalcite is worked at St Véran and Maurins, dep. Hautes-Alpes. The Ronda Mountains in Spain also yield serpentine.

In North America serpentine is so widely distributed that only a few localities can be specified. It is found in St Lawrence county, Essex county and Warren county, New York, and also on Staten Island; at Montville and Hoboken in New Jersey; at Newport, Rhode Island; at Newbury and Newburyport, Massachusetts; Texas, Lancaster county, and West Chester, Chester county, Pennsylvania; at many localities in Vermont, and in California, Connecticut, Georgia, Maine, Maryland, Michigan, New Mexico, North Carolina and Washington.

For American serpentine see Stones for Building and Decoration, by George P. Merrill (New York, 1903); and for serpentine asbestos see the same author's Non-metallic Minerals (New York, 1904).

(F. W. R.*)