Loomisite, Ba[Be2P2O8]•H2O, the first natural example with the zeolite ABW-type framework, from Keystone, Pennington County, South Dakota, USA

A new beryllophosphate mineral species, loomisite (IMA 2022-003), ideally Ba[Be2P2O8]•H2O, was found from the Big Chief mine near Keystone, Pennington County, South Dakota, USA. It occurs as divergent sprays of very thin bladed crystals with a tapered termination. Individual crystals are found up to 0.80 x 0.06 x 0.03 mm. Associated minerals include dondoellite, earlshannonite, mitridatite, rockbridgeite, jahnsite-(CaMnFe), and quartz. No twinning or parting is observed macroscopically. Loomisite is murky white in transmitted light, transparent with white streak and silky to vitreous luster. It is brittle and has a Mohs hardness of 3½-4, with perfect cleavage on (100) and (-110). The measured and calculated densities are 3.46(5) and 3.512 g/cm3, respectively. Optically, loomisite is biaxial (+), with α = 1.579(5), β = 1.591(5), γ = 1.606(5) (white light), 2V (meas.) = 82(2)º, 2V (calc.) = 85º. It is non-pleochroic under polarized light, with a very weak (r > v) dispersion. The mineral is insoluble in water or hydrochloric acid. An electron microprobe analysis, along with the BeO content measured with an ICP-MS, yields an empirical formula (based on 9 O apfu) (Ba0.96Ca0.06)Ʃ1.02[(Be1.96Fe0.06)Ʃ2.02P1.99O8]•H2O, which can be simplified to (Ba,Ca)[(Be,Fe)2P2O8]•H2O. Loomisite is monoclinic, with space group Pn and unit-cell parameters a = 7.6292(18), b = 9.429(2), c = 4.7621(11) Å, β = 91.272(5)◦, V= 342.47(14) Å3, and Z = 2. Its crystal structure is characterized by a framework of corner-sharing PO4 and BeO4 tetrahedra. The framework can be considered as built from the stacking of sheets consisting of 4- and 8-membered rings (4.82 nets) along [001] or hexagonal layers (63 nets) along [010]. The extra-framework Ba2+ and H2O are situated in the channels formed by the 8-membered rings. Topologically, loomisite represents the first natural example with the zeolite ABW-type framework, which is adopted by over 100 synthetic compounds with different chemical compositions. © 2022 Cambridge University Press. All rights reserved.12 month embargo; published online: 20 October 2022This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu