Geographic Minerals

Orthorhombic Ferroselite

Orthorhombic Ferroselite

Orthorhombic ferroselite and its isometric polymorph dzharkenite are iron selenides of general formula FeSe2 precipitated under reducing conditions in anoxic environments. It is an orthorhombic-dipyramidal mineral containing iron and selenium. They are a source of selenium in the Rocky Mountains where selenium occurrence is associated with Upper Cretaceous shale deposits.

Ferroselite was first reported in 1955 for an occurrence in the Ust’-Uyuk uranium deposit in Tuva, Siberia. Its name comes from the Latin ferro (iron) and “sel” for selenium.

General Information

  • Category: Selenide mineral
  • Formula: FeSe2
  • Crystal system: Orthorhombic/ Cubic

Fig: Orthorhombic ferroselite

Properties

In the frame of safety assessment calculations made for deep disposal of high-level radioactive waste, ferroselite and dzharkenite are also considered in geochemical calculations as one of the mineral phases limiting the solubility of Selenium-79.

  • Color: Steel-gray to tin-white with a rose tint, brass yellow
  • Crystal habit: Acicular prismatic
  • Twinning: Stellate and cruciform twins common
  • Mohs scale hardness: 6 – 6.5
  • Luster: Metallic
  • Streak: Black
  • Specific gravity: 7.20
  • Refractive index: Opaque
  • Solubility: insoluble in water

Ferroselite cannot be distinguished from rammelsbergite (FeAs2) by X-ray or in polished section; therefore, the exact identification of these two minerals can be made only by specific tests for As or Se.

Occurrence: In red-bed deposits of the Colorado Plateau-type, peripheral to pyrite and uranium concentrations (Utah, USA); in U–V ores in sandstone, with coalified wood (Colorado, USA); with other sulfides and selenides cementing sandstones and pelites (Tuva, Russia).

Dzharkenite’s type locale is in the Suluchekinskoye Se-U deposit in the Dzharkenskaya Depression of the Middle Ili River, Almaty Province, in southeastern Kazakhstan. It was discovered in 1995 and named after the depression.

Association: Uraninite, pyrite, marcasite, chalcopyrite, sphalerite, selenium, clausthalite, cadmoselite, bornite, cobaltomenite, laumontite, barite.

 

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