1902 Encyclopedia > Serpentine

Serpentine




SERPENTINE, a compact crypto-crystalline or fibrous mineral substance, occurring in rock-masses which commonly present dark green colours, variously mottled and fancifully compared to the markings on certain serpents, whence the name "serpentine." For a like reason it is some-times called " ophite," while Italian sculptors have termed it "ranocchia," in allusion to its resemblance to the skin of a frog. In consequence of its variegated tints, the stone is frequently cut and polished for ornamental purposes, and is hence popularly called a marble. From true marble, however, it differs in chemical composition, being essen-tially a hydrated silicate of magnesium, usually associated with certain metallic oxides (such as those of iron, nickel, and chromium) which confer upon the stone its character-istic tints. In some localities serpentine is found in masses which are evidently intrusive among other rocks, while elsewhere it occurs interbedded, usually in lenticular masses, associated with gneiss and crystalline schists. It is noteworthy that the serpentine is frequently crushed and brecciated, exhibiting polished slip-faces which are sometimes striated. The surface of an exposed mass of serpentine is generally barren, whence bosses of the rock are known in the Alps as "monts morts." The origin of serpentine has been a subject of much dispute. It was pointed out by Sandberger and Tschermak that the alteration of olivine may give rise to this product, and pseudo-morphs of serpentine after chrysolite are well known to mineralogists. Professor Bonney and many other geologists regard serpentine as being generally an altered eruptive rock, due to the hydration of peridotites, such as lherzolite ; probably it may also result from the decomposition of olivine-gabbro and other rocks rich in magnesian silicates. Augite and hornblende may become altered to serpentine. On the contrary, Dr Sterry Hunt and certain other chemical geologists believe that serpentine has generally been formed as an aqueous sediment, probably precipitated by the reaction of sulphate or chloride of magnesium upon the silicate of lime or alkaline silicates derived from the disintegration of crystalline rocks and found in solution in many natural waters. Serpentine is a rock of rather limited occurrence. Its principal localities in England are Cornwall, especially in the Lizard district, where it occupies a considerable area. The famous scenery of Kynance Cove owes much of its beauty to the vivid colours and brilliant surface of the serpentine. The rock is worked into vases, columns, mantelpieces, &c, and of late years has been used to a limited extent for the decoration of shop-fronts in London. The beauty of the Lizard rock is heightened by the white veins of steatite which traverse it, and in some cases by disseminated crystals of bastite, which glisten with metallic lustre. Much of the Lizard serpentine is of rich red and brown colour. Green serpentine is found near Holyhead in Anglesea. A singularly beautiful variety of mottled red and green tints, with veins of steatite, occurs near Portsoy in Banffshire, Scotland. It is also found with chrome iron ore in the Shetland Islands. The green serpentine of Galway occurs in intimate association with crystalline limestone, forming the rock known as "ophicalcite" or "serpentinous marble." Such an association is by no means uncommon; but, though the beauty of the serpentine may thus be enhanced, its durability seems to be impaired. On exposure to the weather the carbonate of calcium decomposes more readily than the silicate of magnesium, and hence the stone soon presents a rough eroded surface. The Galway rock comes into the market under the name of "Irish green" or " Connemara marble." Ophicalcites also occur in Ayrshire, Scotland, and in various parts of the Scottish Highlands; and the green pebbles found in Iona belong to this type of rock.





On the Continent serpentines are largely worked at Zoblitz and at Waldheim in Saxony. The famous rock of Zoblitz, mentioned by Agricola, is known to have been wrought for between three and four centuries, and is still extensively explored by open quarries and by subterranean galleries. The rock usually presents various shades of green and brown, red being very rare; but its most interesting feature is the frequent presence of pyrope, or Bohemian garnet, which occurs scattered through the rock in dark red grains, that decompose on weathering to a green chloritic product. Very little of the Zoblitz serpentine comes to England, but it is common throughout Germany, and a good deal is sent to Bussia and even to the United States. It has been used in the construction of the mauso-leum of Prince Albert at Frogmore, and for Abraham Lincoln's monument at Springfield, Illinois. The best known of the Italian serpentines is the "verde Prato," which has been quarried for centuries at Monteferrato, near Prato in Tuscany. According to Capacci this serpentine is probably of Eocene age. It has been largely used as a decorative stone in ecclesiastical architecture in Prato, Pistoia, and Florence. A good deal of serpentine is found near Genoa and Levanto. The "verde di Pegli" is obtained from Pegli, not far from Genoa, while the "verde di Genova" is a brecciated serpentinous limestone from Pietra Lavezzara. Serpentine also occurs at various other points of the Apennines, in Elba, and in Corsica. The term "ophiolite " has been vaguely used to include not only serpentines but many of the rocks associated with the Italian serpentines. In like manner the term "gabbro," derived from a locality near Leghorn, was at one time used as a general name for serpentine and its associates, though now usually restricted to a rock composed essentially of plagioclase and diallage. It is notable that this true gab-bro is often found in company with serpentine.

Serpentine is found in numerous localities in the Alps and in France. An elegant variety is quarried at Epinal in the Vosges, and a beautiful ophicalcite is worked at St A^eran and Maurins, in the department of Hautes-Alpes. The serpentine of the Bonda Mountains in Spain has been described by Mr J. Macpherson. In North America serpentine is so extensively distributed that only a few localities can be mentioned. It is found at Syracuse in New York; on Manhattan and Staten Islands; at Hobo-ken in New Jersey; at Newport, Rhode Island ; at New-buryport, Massachusetts; at Westchester, Chester county, and at Texas, Lancaster county, in Pennsylvania. It also occurs between Clear Lake and New Idrea in California. A fine ophicalcite has been obtained from near Milford and New Haven in Connecticut, and a beautiful variety has been worked at Port Henry, Essex county, New York (Dana). The Canadian eozoon occurs in a serpentinous limestone.

See GEOLOGY, vol. x. pp. 228, 232; MARBLE, vol. xv. p. 528 ; and MINERALOGY, vol. xvi. p. 414. The literature of the Italian and Saxon serpentines is rather voluminous. Of recent English writings on serpentine reference may be made to Bonney, in Quart. Journ. Geol. Soc., London, xxxiii. p. 884, xxxiv. p. 769, xxxvii. p. 40, xxxix. p. 21, and in Geol. Mag., [2] vi. p. 362, [3] i. p. 406 ; and to Collins, Quart. Journ. Geol. Soc., xl. p. 458, and Geol. Mag., [3] ii. p. 298. Sterry Hunt has written an elaborate paper in Proc. Roy. Soc. Canada, 1883, sect. iv. pp. 165-215. See also Teall, British Petrography, 1886, and Becker, in Amer. Journ. of Science, May 1886. (F. W. B*.)






The above article was written by: F. W. Rudler.



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