- Serpentine group
The serpentine group describes a group of common rock-forming hydrous
magnesium ironphyllosilicate ((magnesium, iron)3silicon|2oxygen|5hydroxide|4) minerals; they may contain minor amounts of other elements including chromium, manganese, cobaltand nickel. In mineralogyand gemology, serpentine may refer to any of 20 varieties belonging to the serpentine group. Owing to admixture, these varieties are not always easy to individualize, and distinctions are not usually made. There are three important mineral polymorphs of serpentine: antigorite, chrysotileand lizardite.
"Their olive green color and smooth or scaly appearance is the basis of the name from the Latin "serpentinus", meaning serpent rock," according to Best (2003). They have their origins in metamorphic alterations of
peridotiteand pyroxene. Serpentines may also pseudomorphously replace other magnesium silicates. Alterations may be incomplete, causing physical properties of serpentines to vary widely. Where they form a significant part of the land surface, the soilis unusually high in clay.
Antigorite is the polymorph of serpentine that most commonly forms during metamorphism of wet ultramafic rocks and is stable at the highest temperatures -- to over 600°C at depths of 60 km or so. In contrast, lizardite and chrysotile typically form near the Earth's surface and break down at relatively low temperatures, probably well below 400°C. It has been suggested that chrysotile is never stable relative to either of the other two serpentine polymorphs.
Samples of the oceanic crust and uppermost mantle from ocean basins document that
ultramaficrocks there commonly contain abundant serpentine. Antigorite contains water in its structure, about 13 percent by weight. Hence, antigorite may play an important role in the transport of water into the earth in subductionzones and in the subsequent release of water to create magmas in island arcs, and some of the water may be carried to yet greater depths.
Soils derived from serpentine are toxic to many
plants, because of high levels of nickel, chromium, and cobalt; growth of many plants is also inhibited by low levels of potassiumand phosphorusand low calcium/ magnesium. The flora is generally very distinctive, with specialised, slow-growing species. Areas of serpentine-derived soil will show as strips of shrublandand open, scattered small trees (often conifers) within otherwise forested areas; these areas have been called "serpentine barrens".
Most serpentines are opaque to translucent, light (
specific gravitybetween 2.2–2.9), soft (hardness 2.5–4), infusible and susceptible to acids. All are microcrystallineand massive in habit, never being found as single crystals. Luster may be vitreous, greasy or silky. Colours range from white to grey, yellow to green, and brown to black, and are often splotchy or veined. Many are intergrown with other minerals, such as calciteand dolomite. Occurrence is worldwide; New Caledonia, Canada( Quebec), USA(northern California), Afghanistan, Cornwall, China, Asia, France, Norwayand Italyare notable localities.
Rock composed primarily of these minerals is called
serpentinite. Serpentines find use in industry for a number of purposes, such as railway ballasts, building materials, and the asbestiform types find use as thermal and electrical insulation (chrysotile asbestos). The asbestos content can be released to the air when serpentine is excavated and if it is used as a road surface, forming a long term health hazard by breathing. Asbestos from serpentine can also appear at low levels in water supplies through normal weathering processes, but there is as yet no identified health hazard associated with use or ingestion. In its natural state, some forms of serpentine react with carbon dioxide and re-release oxygen into the atmosphere.
The more attractive and durable varieties (all of antigorite) are termed "noble" or "precious" serpentine and are used extensively as gems and in ornamental carvings. Often dyed, they may imitate
jade. Misleading synonyms for this material include "Korean jade", "Suzhou jade", "Styrian jade", and "New jade". New Caledonian serpentine is particularly rich in nickel, and is the source of most of the world's nickel ore.
Māoriof New Zealandonce carved beautiful objects from local serpentine, which they called "tangiwai", meaning "tears". Material quarried in Afghanistan, known as "sang-i-yashm", has been used for generations. It is easily carved, taking a good polish, and is said to have a pleasingly greasy feel.
The "lapis atracius" of the Romans, now known as verde antique or verde antico, is a serpentinite
brecciapopular as a decorative facing stone. In classical times it was mined at Casambala, Thessaly, Greece. Serpentinite marbles are also widely used: Green Connemara marble (or Irish green marble) from Connemara, Ireland(and many other sources), and red Rosso di Levanto marble from Italy. Use is limited to indoor settings as serpentinites do not weather well.
Lamellated antigorite occurs in tough, pleated masses. It is usually dark green in color, but may also be yellowish, gray, brown or black. It has a hardness of 3.5–4 and its luster is greasy. The monoclinic crystals show micaceous cleavage and fuse with difficulty. Antigorite is named after its type locality, the Valle di Antigorio in
Two translucent varieties of antigorite,
boweniteand wendelliteand williamsite, are prized by artisans and collectors for their ornamental value; these are the "precious serpentines".
Bowenite is an especially hard serpentine (5.5) of a light to dark apple green color, often mottled with cloudy white patches and darker veining. It is the serpentine most frequently encountered in carving and jewellery. The name retinalite is sometimes applied to yellow bowenite. The New Zealand material is called tangiwai.
Although not an official species, bowenite is the state mineral of
Rhode Island: this is also the variety's type locality. A bowenite cabochonfeatured as part of the "Our Mineral Heritage Brooch", was presented to First Lady Mrs. Lady Bird Johnsonin 1967.
Williamsite is oil-green with black crystals of
chromiteor magnetiteoften included. Somewhat resembling fine jade, williamsite is cut into cabochons and beads. It is found mainly in Marylandand Pennsylvania, USA.
Extremely fine-grained, scaly lizardite (also called orthoantigorite) comprises much of the serpentine present in serpentine marbles. It is triclinic, has one direction of perfect cleavage, and may be white, yellow or green. Lizardite is translucent, soft (hardness 2.5) and has an average specific gravity of 2.57. It can be pseudomorphous after
enstatite, olivineor pyroxene, in which case the name bastite is sometimes applied. Bastite may have a silky lustre.
Lizardite is named after its type locality on the Lizard Peninsula,
Cornwall, UK. [ [http://www.mindat.org/min-2425.html Lizardite: Lizardite mineral information and data ] ] It is worked by local artisans into various trinkets which are sold to tourists.
CaliforniaState Rock is Serpentinite.
* [http://www.cst.cmich.edu/users/dietr1rv/serpentine.htm R. V. Dietrich - Gemrocks]
* [http://mineral.galleries.com/minerals/silicate/serpenti/serpenti.htm Mineral description from Mineral galleries]
* Myron G. Best, "Igneous and Metamorphic Petrology, 2nd edition". Blackwell Publishing (2003). ISBN 1-40510-588-7
* Arthur R. Kruckeberg, "Geology and Plant Life; The Effects of Landforms and Rock Types on Plants." University of Washington Press, Seattle (2002). ISBN 0-295-98452-X
* Bernard W. Evans, "The Serpentinite Multisystem Revisited: Chrysotile is Metastable." International Geology Review, v. 46, pages 479-506 (2004).
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