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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