Magselite, [Mg(H ₂ O) ₆ ](SeO ₃ ), a New Selenite Mineral from the Pickett Corral Mine, Colorado, USA

The new mineral magselite, [Mg(H 2 O) 6 ](SeO 3 ), was encountered in post-mining mineral assemblages at the Pickett Corral Mine in western Colorado, USA.Magselite corresponds to natural magnesium selenite hexahydrate, a known synthetic compound which is easily synthesized by combining aqueous sodium or potassium selenite with a solution saturated in Mg(OH) 2 .At the type locality, magselite likely formed from mixing of Mgbearing groundwater with decomposing ferroselite, FeSe 2 .In the assemblage that produced the holotype, magselite occurs as radiating clusters of colorless trigonalcrystals to nearly 1 mm associated with andersonite.In a separate association with szilagyiite, NaCa 3 (UO 2 )(CO 3 ) 3 (SeO 3 )F(H 2 O) 6 , magselite forms mostly inconspicuous colorless crusts.Magselite is slowly soluble in room-temperature H 2 O and rapidly soluble in dilute HCl, potentially explaining its rarity in supergene environments.Magselite is trigonal, space group R3 [a = 8.9663(2), c = 8.9400(3) , V = 622.44 3 , Z = 3] and has a simple structure built from Mg(H 2 O) 6 aquo complexes hydrogen-bonded to SeO 3 trigonal pyramids in columns staggered along [001].The strongest powder X-ray diffraction lines (d given in and intensity given in parentheses relative to strongest line) for magselite are: 5.862 (27), 4.483 (100), 3.874 (75), 2.931 (40), 2.788 (52), 2.453 (38), 1.940 (33), and 1.778 (32).Magselite is nonpleochroic and uniaxial (-) with = 1.553(2) and = 1.493(2).Raman spectroscopy data are also provided. IntroductionPost-mining environments have been a prolific source of novel materials and additionally serve as excellent laboratories to study the transport and sequestration of heavy metals and other environmental contaminants found in mine runoff.In a post-mining setting, rare and exotic minerals can form in stabilized micro-environments that are typically transient during weathering and exhumation.Here, we describe the occurrence and properties of the new mineral magselite, a component of an unusual post-mining mineral assemblage in western Colorado, USA.Magselite is named for its chemical composition.Both the mineral and its name have been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature, and Classification (IMA2024-084).One holotype specimen is deposited in the collections of the Carnegie Museum of Natural History, catalog number CM34763. OccurrenceMagselite was found as part of a post-mining secondary mineral assemblage underground at the Pickett Corral mine (Pickett Corral #4), Montrose County, Colorado, USA (38.197N, 108.842W).These workings connect underground to the Blue Streak mine and are part of the Blue Streak mining complex.The Pickett Corral mine is an inactive uranium mine accessing sandstone-hosted uranium(-vanadium) mineralization in the Uravan