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.
- Category: Selenide mineral
- Formula: FeSe2
- Crystal system: Orthorhombic/ Cubic
Fig: Orthorhombic ferroselite
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.