Metamorphic
Serpentinite (Serpentine Rock)
Primarily composed of serpentine group minerals, such as antigorite, chrysotile, and lizardite. Chemical formula for serpentine minerals is (Mg,Fe)3Si2O5(OH)4.
Hardness: 2.5-6 on Mohs scale (variable depending on mineral composition). Color: Shades of green (light to dark, often mottled), yellow-green, brownish-green. Luster: Greasy, waxy, silky (for chrysotile), or dull. Crystal Structure: Typically massive, fibrous (like chrysotile asbestos), or platy.…
- Hardness
- 2
- Luster
- Greasy, waxy, silky (for chrysotile), or dull
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Physical properties
Hardness: 2.5-6 on Mohs scale (variable depending on mineral composition). Color: Shades of green (light to dark, often mottled), yellow-green, brownish-green. Luster: Greasy, waxy, silky (for chrysotile), or dull. Crystal Structure: Typically massive, fibrous (like chrysotile asbestos), or platy. Cleavage: Poor to none; can have a platy or fibrous habit. Specific Gravity: 2.5-2.6.
Formation & geological history
Formed by the metamorphism of ultramafic rocks (like peridotite and dunite) through a process called serpentinization. This process involves the alteration of olivine and pyroxene by hydrothermal fluids (water-rich fluids) at low to moderate temperatures and pressures. Geological age varies widely, often found in ancient oceanic crust and subduction zones, dating back billions of years to more recent tectonic events.
Uses & applications
Industry: Source of chrysotile asbestos (historically, though its use is now heavily restricted due to health concerns), used as decorative building stone ('verde antique'), and for some refractory applications. Jewelry/Decoration: Polished and carved into ornamental objects, beads, and cabochons. Construction: Used as a dimension stone, crushed stone, and sometimes as a minor aggregate. No significant current use in main construction materials due to variable physical properties and potential asbestos content.
Geological facts
Serpentinite is the state rock of California. The name 'serpentine' comes from the Latin word 'serpens', meaning snake, due to its resemblance to snake skin and its often mottled green color. Some varieties of serpentine contain chrysotile, which is a type of asbestos. While chrysotile itself is a mineral within the serpentine group, the term 'serpentinite' refers to the rock composed predominantly of these minerals, not all of which are fibrous asbestos.
Field identification & locations
In the field, serpentinite is often identified by its greasy or waxy feel, shades of green color, and sometimes a mottled or slickensided appearance. It can be relatively soft and easily scratched. It is commonly found in ancient oceanic crust that has been uplifted and exposed, often in association with fault zones and mountain ranges. Notable locations include California, the Appalachian Mountains, Oman, and the Alps. Collectors should be aware of potential asbestos content in fibrous varieties and handle with care.
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