Publication Date:
2017-03-31
Description:
Mesaite (IMA2015-069), ideally Ca (V 2 O 7 ) 3 ·12H 2 O, is a new mineral from the Packrat mine, Gateway district, Mesa County, Colorado, USA. Crystals of mesaite occur as orangish red blades up to 0.1 mm long and ~10 μm thick. The streak is light pinkish orange and the lustre is vitreous, transparent. Mesaite has a brittle tenacity, {010} perfect cleavage; fracture is irregular, and no parting was observed. The mineral has a Mohs hardness 2. The measured density of mesaite is 2.74(1) g cm –3 . Mesaite is biaxial (–), α = 1.760(calc), β = 1.780(5), = 1.795(5) in white light; the measured 2V value = 81(2)°. Dispersion is strong, r 〈 v , and pleochroism is present in shades of brownish orange . Mesaite is monoclinic, P 2/ n , with a = 9.146(2), b = 10.424(3), c = 15.532(4) Å, β = 102.653(7)° and V = 1444.7(6) Å 3 . The strongest four diffraction lines in the powder diffraction pattern are [( d obs in Å, ( I obs ), ( hkl )]: 10.47 (100) (010), 2.881 (25) (132, 12, 033, 310), 3.568 (24) ( 14, 23, 13), 3.067 (17) ( 24, 32, 23). The composition of mesaite was determined by electron microprobe, and yielded an empirical formula of Mn 5.32 Ca 0.56 Zn 0.31 V 5.96 As 0.04 O 33 H 23.61 on the basis of 33 O atoms per formula unit (apfu). The atomic arrangement of mesaite was solved and refined to R 1 = 0.0600. The structure is formed of zigzag octahedral chains of edge-sharing Mn 2+ O 6 octahedra. Oxygen atoms of the octahedra are shared with V 2 O 7 groups, which link with adjacent octahedral chains to form {010} heteropolyhedral layers. The interlayer region contains Ca atoms and H 2 O groups. Each Ca bonds to two O6 atoms in the heteropolyhedral layer and to two fully occupied and six partially occupied O (H 2 O) sites in the interlayer, resulting in an effective Ca coordination of approximately seven. Similar zigzag chains of edge-sharing MnO 6 octahedra decorated with V 2 O 7 groups are also found in the mineral fianelite. Mesaite has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2015-069). The name mesaite is conferred for Mesa County, Colorado, USA.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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