Nováčekite-I Mg(UO2)2(AsO4)2.12H2O
Nováčekite-II Mg(UO2)2(AsO4)2.10H2O
Arsenate, uranyl mineral, autunite group, forms a series with saléeite
Nováčekite-I is very unstable under ambient conditions and quickly dehydrates to nováčekite-II, in part also to metanováčekite. "Nováčekite" specimens in collections mostly seem to be either nováčekite-II or metanováčekite (Mindat).
Crystal System: Monoclinic
Specific gravity: 3.25 to 3.70
Hardness: 2 to 2½
Streak: Very pale yellow
Colour: Pale yellow, yellow
Luminescence: Fluoresces dull green to bright lemon yellow in both long wave and short wave UV

Sedimentary environments
Hydrothermal environments

Nováčekite occurs in the oxidised zone of uranium-bearing polymetallic hydrothermal mineral deposits (Webmin).


At the Michael Mine, Weiler, Seelbach, Seelbach, Ortenaukreis, Freiburg Region, Baden-Württemberg, Germany, infiltrating oxidising, U6+-bearing fluids reacted under ambient conditions with galena and native arsenic, forming a variety of U6+ (±Pb)-bearing arsenates such as hügelite, hallimondite, zeunerite, heinrichite or novacekite together with uranium-free minerals like mimetite or anglesite. Some parts of the vein were enriched to uranium concentrations of up to 1 wt% by this supergene process. Reduced (hypogene) uranium phases like uraninite were never observed (AM 105.727–744).

From Schneeberg, Erzgebirgskreis, Saxony, Germany, two specimens were found of nováčekite. The first specimen comprised a small piece of deeply altered, quartz and limonite vein material that was coated and veined by secondary uranium minerals. The nováčekite was present as a small area, apparently a filled cavity, as a platy, cleavable mineral of a straw-yellow colour. Small irregular flakes of zeunerite were present around the borders of the mass of nováčekite and represent the first deposition on the cavity walls. Small groups of crystals of zeunerite were present elsewhere on the specimen together with tufted crusts of uranophane.
The second specimen comprised a small, friable mass of vein material, light brown in colour and composed largely of limonite and quartz with cavities lined or filled completely by indistinct straw-yellow to lemon-yellow crystals of nováčekite. The veinlets are bordered by green crystals of zeunerite, which formed earlier than the nováčekite, and some of the nováčekite crystals show intergrown cores of zeunerite in parallel orientation (AM 36.680-686).

At Wheal Owles, Botallack, St Just, Cornwall, England, UK, nováčekite is associated with chalcopyrite, arsenopyrite and sphalerite (HOM).

At the Woodrow area, Laguna Reservation, Valencia County, New Mexico, nováčekite, straw yellow in colour, coats an iron-stained friable sandstone, and forms a series with saléeite (AM 39.675-676).

In the Wichita Mountains, Kiowa county, Oklahoma, USA , nováčekite occurs in recent sedimentary deposits in a friable red sandstone. Small cavities in the sandstone are filled with nováčekite crystals along with limonite, malachite, calcite and quartz grains (AM 41.152-153).

At the Cherkasar U Deposit, Pap, Chatkal-Kuraminskii Range, Namangan Region, Uzbekistan, nováčekite is associated with schoepite, paraschoepite, arsenuranylite, metazeunerite and uranospinite (HOM).

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