English: Serpentinite is a low- to high-grade metamorphic rock formed by alteration of olivine-rich peridotites (dunites - ultramafic, phaneritic, intrusive igneous rocks). Metamorphism of olivine in the presence of water results in the formation of the mineral serpentine (Mg3Si2O5(OH)4). A metamorphic rock composed principally of serpentine is thus a serpentinite.

Serpentinite has a mottled greenish to blackish color, often has the look & feel of hard candle wax, and ranges in texture from crystalline to “foliated”. Many serpentinites have a foliated look to them, but it’s really not due to an planar alignment of crystals. The appearance of “foliated” serpentinites is really the result of extensive development of slickenlined surfaces.

Many serpentinites also have a small component of magnetite that is usually significant enough to feel a slight tug when a magnet is placed next to the rock. Some serpentinites have "veins" of white asbestos, which is fibrous chrysotile serpentine.

Most Precambrian greenstone belts have significant occurrences of serpentinites. Slices of dunitic mantle caught up in orogenic belts by obduction (= ophiolites) are often serpentinized. Sometimes, mantle peridotite masses that were caught up in rising magmas have been serpentinized (for example, in kimberlites & lamproites).

The sample seen here is from an old decorative stone quarry in Vermont, where serpentinite was cut and polished into "green marble". This material is often nicknamed "Verde Antique" - "Antique Green".

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Info. from Dale (1912) - The commercial marbles of western Vermont. United States Geological Survey Bulletin 521: 49-50, 148-149:

SERPENTINE OF ROXBURY.

The serpentine of Roxbury is a marble only in a commercial sense. Roxbury is on the east side of the Green Mountain axis, in Washington County, 14 miles southwest of Montpelier. The quarry is about half a mile south of the station, on the east side of a north-south ridge. The serpentine there is 50 to 60 feet wide, dips 45º-65º roughly west, and is reported to continue for several miles in a north-northwest to south-southeast direction, with varying width but in places tapering out altogether.

. . .

Dark-colored serpentine is a hydrous silicate of magnesia and iron. Some serpentines are of metamorphic sedimentary origin; others are metamorphosed eruptive rocks, and to this class that of Roxbury probably belongs. The nature of the original dike rock is uncertain. It may easily have been peridotite, which has a large content of magnesia and iron oxides. The serpentine in thin section shows fibrous and radial texture with veinlets of magnesite and talc and with large particles of magnetite, which also appear on the polished face. The rock is of dark purplish or greenish color in the mass and is plentifully veined with white magnesite. Polished faces are almost black but crossed by a network of veins and veinlets of white and of light green wherever the magnesite is mingled with the serpentine. As many of these veins are faulted and sheared, the rock has evidently since its alteration to serpentine been subjected to compression. These sheared veins in thin sections show alternating blades of magnesite and foliated serpentine bent at right angles to the direction of the vein, forming "shear zones" like those in slates.

. . .

The serpentine of Roxbury takes a fairly good polish and its striking contrasts of shade and color and the irregularity of its veining make it a very attractive ornamental stone for interior work.

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SERPENTINE

ROXBURY

The serpentine quarry, opened before 1858, is about half a mile south of Roxbury station, and about 600 feet west of the railroad, in Washington County. It measures at the surface 100 feet north to south by 35 feet across, but at the bottom, 70 to 75 feet down, 120 feet north to south by 48 to 50 feet across. A new opening, 500 feet south of the old one, was begun in 1910. Operator, Barney Marble Company, Swanton, Vermont.

The serpentine, "Vermont verde antique" and its geologic relations have been described on page 49.

The serpentine is finished at Swanton and is used for columns, wainscoting, counter tops, base, and tiling.

Specimens: Wainscoting, post office, Danville, Illinois; the Delaware Hotel, Fort Worth, Texas; New St. Charles Hotel, New Orleans. Columns and panels (10 feet 6 inches by 4 feet 2 inches), Hall of Justice, San Francisco. Counter tops and base, Union Station, Washington, D.C. Base, First National Bank, Chicago. Tiling, city hall, Indianapolis, Indiana. The length of columns is limited to 15 feet.

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Locality: old quarry south of the town of Roxbury, Washington County, north-central Vermont, USA (vicinity of 44º 04' 43.49" North latitude, 72º 44' 33.22" West longitude)