Dunite

WHAT IS IT?

  • An igneous rock of ultramafic composition (low silica) in the Peridotite group

  • Greater than 90% Olivine, with other minerals, such as Pyroxene, Chromite, Magnetite, & Pyrope

  • Olivine is an orthosilicate (contains (SiO4)4- anion)

  • Typically contains an Mg/Fe ratio of 9:1

  • Coarse grained

  • Color:  light yellowish green to dull brown

  • Soft, hardness of 3.5 to 4

  • Dunite & other Peridotite minerals are the major constituents of the upper mantle of the earth

  • Named after Dun Mountain near Nelson, New Zealand, where a deposit was discovered by the German geologist Ferdinand Con Hochstetter in 1859

  • Also called Olivinite (not to be confused with Olevenite)

  • Formed by:

    • Fractionation of Olivine from a mafic melt

    • Replacive formation by the reaction of pyroxene-bearing host rock with Olivine saturated magma dissolving the orthopyroxene in the host producing crystallized Olivine

COMPOSITION

  • The composition of the main component, Olivine, is (Mg2+,Fe2+)2SiO4

  • Typically contains an Mg/Fe ratio of 9:1

WHERE IS IT FOUND?

  • Usually found in ophiolite (uplifted ocean crust with some mantle material attached)

  • Commonly found with Platinum group metals, Chromite & Magnetite

  • Rarely found within continental crust materials

  • The largest deposits (by millions of tons) are:

    • Aaheim, Norway (2000 MT)

    • Twin Sisters Mountain, Washington State, near Mt Baker (1800 MT)

    • Arutats, Sweden (1800 MT)

    • Troodos Mountains, Cyprus

  • The Dunite in the Bushveld Igneous Complex of South Africa contains large reserves of Platinum

  • The Dunite dikes of the Fidalgo formation on Cypress Island (in the San Juan Islands) have had limited Chromite mining (200 tons)

WHAT IS IT USED FOR?

  • Dimension stone for construction

  • Cement manufacture

  • Construction aggregate

  • Polished countertops

  • Fertilizer manufacture

  • Carbon sequestration

    Mg2SiO4 (olivine) + 2CO2 -> 2MgCO3 (magnesite) + SiO2 (silica)

  • A mined source of Magnesium, Chromium, and Platinum

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

  • Metallurgy - Flux agent in blast furnaces