Serpentine is a group of common rock-forming hydrous magnesium iron silicate ((Mg,Fe)3Si2O5(OH)4) minerals. In mineralogy, serpentine may refer to any of 20 minerals belonging to the kaolinite-serpentine group. Owing to admixture, these minerals are not always easy to individualize, and distinctions are not usually made. There are three important aggregate structures of serpentine: antigorite, chrysotile and lizardite.
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. They have their origins in metamorphic alterations of peridotite, amphibolite and pyroxene: Serpentines may also pseudomorphously replace other magnesium silicates. Alterations may be incomplete and therefore physical properties of serpentines vary widely. Where they form a significant part of the land surface, the soil is unusually high in clay, and the flora is generally very distinctive. Areas of serpentine soil will show as strips of herb and brushland within otherwise forested areas.
Most are opaque to translucent, light (specific gravity between 2.2-2.9), soft (hardness 2.5-4), infusible and susceptible to acids. All are microcrystalline and massive in habit, never being found as single crystals. Lustre 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 calcite and dolomite. Occurrence is worldwide: Canada (Quebec), USA (California), Afghanistan, China, France, Norway and Italy are 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 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. Often dyed, they may imitate jade: Misleading synonyms for this material include "Korean jade," "Suzhou jade," "Styrian jade," and "New jade."
Overview
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Polished slab of bowenite serpentine, a variety of antigorite. Typical cloudy patches and veining are apparent. |
The Maori of New Zealand once 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 breccia popular 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 colour, but may also be yellowish, gray, brown or black. It has a hardness of 3.5-4 and its lustre is greasy. The monoclinic crystals show micaceous cleavage and fuse with difficuly. Antigorite is named after its type locality; Valle di Antigorio, 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. The name retinalite is sometimes applied to yellow bowenite. The New Zealand material is called tangawaite.
Although not an official species, bowenite is the state mineral of Rhode Island: This is also the variety's type locality. A bowenite cabochon featured as part of the "Our Mineral Heritage Brooch," presented to First Lady Mrs. Lady Bird Johnson in 1967.
Williamsite is oil-green with black crystals of chromite or magnetite often included. Somewhat resembling fine jade, williamsite is cut into cabochons and beads. It is found mainly in Maryland and Pennsylvania, USA.
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. Although it is safer than amphibole asbestos due to its greater heat resistance, alternatives are now widely used in order to avoid asbestosis.
The opaque grey-white, yellow or green recurved fibres are quite flexible and may be separated. They have a silky lustre and a hardness of 2.5-4 with no cleavage present. The name chrysotile is derived from the Greek: chrysos meaning "golden" and tilos meaning "fibre." An important source is Quebec, Canada.
A chatoyant variety, the so-called "California cat's eye" is called satelite. Locally polished cabochons make popular gems.
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, olivine or pyroxene, in which case the name bastite is sometimes applied. Bastite may have a silky lustre.
Lizardite is named after its type locality: Lizard Point, Cornwall, England. It is worked by local artisans into various trinkets which are sold to tourists.
Antigorite
Chrysotile
Lizardite
See also
