Serpentine

Description: Serpentine was named in 1564 because the mottled green appearance reminded the naturalist of snake skin. The term "serpentine" actually refers to a large group of minerals with a common chemical structure and origin. Serpentine minerals are formed by the hydrothermal metamorphism of ultramafic rocks (ultramafics are igneous rocks from the mantle that are dark, have low Si content, and are rich in Iron (Fe) and Magnesium (Mg). It is common for Olivine and Pyroxene minerals to be transformed into Serpentine minerals during this process. This metamorphic transformation is common at convergent plate boundaries where oceanic plates subduct below continental crust.

Distinguishing characteristics of Serpentine minerals are their green color, the patterned/mottled appearance and the feel that is often described as "slippery". Serpentine minerals are soft and easily polished and are often used as material that can be carved into intricate, artistic sculptures. Some varieties of Serpentine have a fibrous habit (see photo), and these fibers do not burn, resist the transfer of heat, and can be used as insulators. They were widely used in buildings and many industrial applications during the first half of the 20th century, until it was discovered that inhalation of the fibers could lead to lung and other cancers. Safe, secure removal programs of these asbestos fibers are still taking place today in old buildings and residences. An interesting nature observation: Most plants cannot grow on soils produced by decomposed serpentines because the high levels of nickel, chromium and cobalt are toxic. Only specialized, slow-growing species can survive in these landscapes which have been dubbed "serpentine barrens".

Chemical Formula: D3[Si2O5](OH)4 ; where D = Mg, Fe, Ni, Mn, Al, Zn

Crystal Form: Some varieties can form flat, tabular crystals; others form fibers - elongated, and parallel to crystallographic axis

Crystal System: most are Monoclinic

Color: Common are shades of green; but yellow, black, white, and other colors have been noted.

Streak: White

Luster: Greasy, waxy

Fracture: Conchoidal, where present

Cleavage: None in some varieties; Good, in one direction in the fibrous varieties

Hardness: variable, 3 - 6 on Moh’s scale, depending on chemical composition

Density/Specific Gravity: 2.5 - 2.6 g/cm3

Magnetism: N/A

Taste: N/A

Hydrochloric acid: N/A

Radioactivity: N/A

Fluorescence: the variety Antigorite can exhibit a whitish luminescence, in blue, green or yellow shades

Distinguishing Physical Properties:

  • Color: commonly a shade of green

  • Luster: looks like wax, or silk and feels greasy

  • Hardness: fairly soft, but harder than a fingernail

  • Appearance: fibrous varieties are very distinctive

Photograph Attribution: Serpentine specimen photograph by Sean C. Murphy, 2020.

Photograph 20a Attribution: Fibrous serpentine specimen photograph by Sean C. Murphy, 2020.