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Hjalmarite, a new Na-Mn member of the amphibole supergroup, from Mn skarn in the Långban deposit, Värmland, Sweden.
Swedish Museum of Natural History, Department of Geology.ORCID iD: 0000-0003-2109-2277
Università degli Studi di Milano.
Swedish Museum of Natural History, Department of Geology.
Swedish Museum of Natural History, Department of Geology.
2019 (English)In: European journal of mineralogy, ISSN 0935-1221, E-ISSN 1617-4011, Vol. 31, p. 565-574Article in journal (Refereed) Published
Abstract [en]

Hjalmarite, ideally ANaB(NaMn)CMg5TSi8O22W(OH)2, is a new root-name member of the amphibole supergroup, discovered in skarn from the Långban Fe-Mn-(Ba-As-Pb-Sb-Be-B) deposit, Filipstad, Värmland, Sweden (IMA-CNMNC 2017-070). It occurs closely associated with mainly rhodonite and quartz. It is grayish white with vitreous luster and non-fluorescent. The crystals are up to 5 mm in length and display splintery fracture and perfect cleavage along {110}. Hjalmarite is colorless (non-pleochroic) in thin section and optically biaxial (-), with α = 1.620(5), β = 1.630(5), γ = 1.640(5). The calculated density is 3.12 Mg/m3. Average VHN100 is 782, corresponding to circa 5½ Mohs. An empirical formula, derived from EPMA analyses in combination with crystal structure refinements, is (Na0.84K0.16)Σ1(Na1.01Mn0.55Ca0.43Sr0.01) Σ2(Mg3.83Mn1.16Al0.01) Σ5(Si7.99Al0.01) Σ8O22(OH1.92F0.08)Σ2. An infra-red spectrum of hjalmarite shows distinct absorption bands at 3673 cm-1 and 3731 cm-1 polarized in the α direction. The eight strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 3.164, 100, (310); 2.837, 50, (330); 8.50, 44, (110); 3.302; 40, (240); 1.670, 34, (461); 1.448, 32, (-661); 2.727, 30, (151); 2.183, 18 (261).

Single-crystal X-ray diffraction data were collected at 298 K and 180 K. The crystal structure was refined in space group C2/m to R1=2.6% [I>2(I)], with observed unit-cell parameters a = 9.9113(3), b = 18.1361(4), c = 5.2831(5) Å, β=103.658(5)° and V = 922.80(9) Å3 at ambient temperature. The A and M(4) sites split into A(m) (K+), A(2) (Na+), and M(4’) (Mn2+) subsites, respectively. Among the octahedrally coordinated C group cations, Mn2+ orders strongly at the M(2) site. No significant violation of C2/m symmetry or change in the structure topology is detected at low temperature (R1=2.1%). The hjalmarite-bearing skarn formed at peak regional metamorphism, T  ≥ 600°C, at conditions of high SiO2 activity and relatively low oxygen fugacity. The mineral name honors the Swedish geologist and mineralogist S.A. Hjalmar Sjögren (1856–1922).

Place, publisher, year, edition, pages
2019. Vol. 31, p. 565-574
Keywords [en]
hjalmarite, amphibole supergroup, sodium–(magnesium–iron–manganese) subgroup, new mineral, crystal structure, Mn skarn, Långban
Keywords [sv]
hjalmarit, amfibolsupergruppen, natrium–(magnesium–järn–mangan)-undergruppen, nytt mineral, kristallstruktur, skarn, Långban
National Category
Geology
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-3603DOI: 10.1127/ejm/2019/0031-2822OAI: oai:DiVA.org:nrm-3603DiVA, id: diva2:1375842
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-06Bibliographically approved

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