Serpentine

WHAT IS IT?

  • A medium to course grained ultrabasic (<45% silica) rock that is formed in dykes & small intrusions

  • Translucent transparency (diaphaneity)

  • Luster: Greasy, waxy, or silky

  • Easy to cut & accepts a polish

  • Serpentine is not the name of a single mineral - it is a group of minerals with similar characteristics

  • Classified as both an igneous and a metamorphic rock.  It is an igneous rock that has been altered (metamorphosed) by hydrothermal fluids.

  • Is found in areas where periodite, dunite/olivine, pyroxene, and other ultramafic minerals undergo hydrothermal metamorphism.  Usually, this occurs at convergent plate boundaries where an oceanic plate is pushing down into the mantle (such is the case below western Washington).  Seawater is the source of the water.

  • Found where the remnant of ancient ocean plates have been thrusted up to the surface.  These areas of exposed oceanic plate are known as ophiolites.  They are often the source of magnetite, chromite, chrysoprase, jade, and serpentine.

  • Colors:  green, white, black, gray, brown, yellow, purple, multicolored (esp. green & white)

  • Yellow serpentine fluoresces a cream-white color when exposed to shortwave uV

  • Hardness:  2.5 to 6 (softer than granite & jade, but harder than marble)

  • Serpentine is also called "False Jade" or "Teton Jade" (often dyed to deceive buyers)

COMPOSITION

  • Produced by the action of hot fluids on olivine, augite & hornblende

  • Chemical composition:  X2-3Si2O5(OH)4
    where X = Mg, Fe2+, Fe3+, Ni, Al, Zn, or Mn

    In rare cases, one of the two silicon atoms may be replaced with an Al or Fe atom, leading to the more general formula:

    (Mg,Fe,Ni,Mn,Zn)2-3(Si,Al,Fe)2O5(OH)4

    The subscript 2-3 indicates that 2 or 3 of the element in the first group are present

  • The primary serpentine minerals are antigorite & chrysotile.  There are other serpentine minerals, but most are rare.

    • Antigorite

      • Solid
      • (Mg,Fe)3Si2O5(OH)4

    • Chrysotile

      • Fiberous
      • A type of asbestos
      • Mg3Si2O5(OH)4
      • Not a single mineral, but a group of polymorphous minerals:
        (same chemical composition, different crystal lattice structures)

        • Clinochrysotile (monclinic) - most common form
        • Orthochrysotile (orthorhombic)
        • Parachrysotile (orthorhombic)
        • Lizardite (hexagonal)

  • Other members of the Serpentine group:

    • Amesite - Mg2Al(SiAl)O5(OH)4
    • Cronstedtite - Fe2+2Fe3+(SiFe3+)O5(OH)4
    • Fraipontite - (Zn,Al)3(Si,Al)2O5(OH)4
    • Nepouite - Ni3Si2O5(OH)4

      There are some other members not listed here, but they are extremely rare

WHAT IS IT USED FOR?

  • Architectural stone (facing stone, wall tiles, mantles, window sills)

  • As a building material, it is traditionally called "serpentine marble"

  • Sculptures

  • Ornamental use - bowls, vases, desk sets, clock bases, carved animals, fruit, flowers, legendary figures, deities, busts, statues, etc.

  • Gentlly worn jewelry (due to softness) - earrings, brooches, pendants.
    Beads can be made from harder serpentines

  • Source of asbestos

  • Heat insulator

  • CO2 sequestration - removing carbon dioxide from the atmosphere:

    Mg3Si2O5(OH)4 + 3CO2 + H2O -> 3MgCO3 + 2SiO2 + 3H2O