A hydrous magnesium silicate (Mg3Si2O5(OH)4) occurring in altered basic and ultrabasic igneous rocks by the decomposition of olivine and pyroxene. There are two main forms: chrysotile (an asbestos variety) and antigorite. It is soft, green to black, and used in decorative carving.
Serpentine is a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)3Si2O5(OH)4) minerals; In mineralogy and gemology, serpentine may refer to any of 20 varieties belonging to the serpentine group. There are three important mineral polymorphs of serpentine: antigorite, chrysotile and lizardite.
Overview
Serpentine is said to owe its name either to its serpent-like colours and patterns or from an old belief that the stones were effective protection from snake bites. Serpentines may also pseudomorphously replace other magnesium silicates. Alterations may be incomplete, causing physical properties of serpentines to vary widely.
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. 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 ultramafic rocks there commonly contain abundant serpentine. Hence, antigorite may play an important role in the transport of water into the earth in subduction zones 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; these areas have been called "serpentine barrens".
Most serpentines are opaque to translucent, light (specific gravity between 2.2–2.9), soft (hardness 2.5–4), infusible and susceptible to acids.
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 more attractive and durable varieties (all of antigorite) are termed "noble" or "precious" serpentine and are used extensively as gems and in ornamental carvings. New Caledonian serpentine is particularly rich in nickel, and is the source of most of the world's nickel ore.
The Māori of New Zealand once carved beautiful objects from local serpentine, which they called tangiwai, meaning "tears".
The lapis atracius of the Romans, now known as verde antique or verde antico, is a serpentinite breccia popular as a decorative facing stone. 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.
Antigorite
Lamellated antigorite occurs in tough, pleated masses. Antigorite is named after its type locality, the Valle di Antigorio in Italy.
Two translucent varieties of antigorite, bowenite and 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 colour, often mottled with cloudy white patches and darker veining. It is the serpentine most frequently encountered in carving and jewellery.
Although not an official species, bowenite is the state mineral of Rhode Island: this is also the variety's type locality.
Williamsite is oil-green with black crystals of chromite or magnetite often included.
Chrysotile
The most common of the serpentines, chrysotile, is actually a group of polytypes: monoclinic clinochrysotile, and orthorhombic orthochrysotile and parachrysotile. These are the fibrous asbestiform serpentines, accounting for over 90% of all asbestos in use.
The opaque grey-white, yellow or green recurved fibres are quite flexible and may be separated.
Chatoyant varieties of fibrous serpentine, such as the so-called "California cat's eye" are called satellite.
Lizardite
Extremely fine-grained, scaly lizardite (also called orthoantigorite) comprises much of the serpentine present in serpentine marbles.
Lizardite is named after its type locality: Lizard Point, Cornwall, UK.
The California State Rock is Serpentine.
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