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Arakiite.

Monoclinic

Chairman Emeritus of the Commission on New Minerals and Mineral Names of the International Mineralogical Association and Curator Emeritus

Department of Mineralogy

Royal Ontario Museum

100 Queen's Park

Toronto, Ontario, Canada M5S 2C6

(Zn,[Mn.sup.2+])[([Mn.sup.2+],Mg).sub.12][([Fe.sup.3+],Al).sub.2]([As .sup.3+][O.sub.3])[([As.sup.5+][O.sub.4]).sub.2][(OH).sub.23]

Locality: Langban Varmland Sweden.

Occurrence: Associated minerals are: hematite (specular), calcite and magnussonite.

General appearance: Aggregates of micaceous plates covering an area of 15 x 10 mm.

Physical, chemical and crystallographic properties: Luster: resinous to submetallic. Diaphaneity: opaque but translucent on thin edges. Color: red-brown to orange-brown. Streak: pale brown. Luminescence: nonfluorescent. Hardness: 3 to 4. Tenacity: brittle. Cleavage: {001} perfect. Fracture: uneven to almost subconchoidal. Density: could not be measured because of the small amount of material, 3.41 g/[cm.sup.3] (calc.). Crystallography: Monoclinic, Cc, a 14.248, b 8.228, c 24.23 A, [beta] 93.62[degrees], V 2843 [A.sup.3] Z 4, a:b:c = 1.7316:1:2.9448. Morphology: no forms were observed. Twinning: none observed. X-ray powder diffraction data: 12.07 (100) (002), 6.046 (100) (004), 4.119 (30) (020), 4.040 (90) (006), 3.148 (30) (404, 117), 3.030 (70) (224), 2.411 (40) (424, 515), 1.552 (70) (640, 351). Optical data: Biaxial (-), [alpha] 1.723, [beta] 1.744, [gamma] 1.750, 2V(meas.) 40[degrees], 2V(calc.) 56[degrees]; dispersion r [greater than] v, medium; nonpleochroic; Y = b, X ^ c = 4[degrees] (in obtuse angle [beta]) C hemical analytical data: Means of thirteen sets of electron microprobe data: MgO 12.76, MnO 34.32, ZnO 4.48, [Al.sub.2][O.sub.3] 2.25, [Fe.sub.2][O.sub.3] 6.76, [As.sub.2][O.sub.5] 15.84, [As.sub.2][O.sub.3] 6.56, [H.sub.2]O (13.74), Total (96.71) wt.%. [H.sub.2]O was calculated assuming stoichiometry. The crystal structure results showed that Mn is [Mn.sup.2+], Fe is [Fe.sup.3+] and As is present as [As.sup.3+] and [As.sup.5+] in a 1:2 ratio. Empirical formula: [([Zn.sub.0.83][Mn.sub.0.17]).sub.[sigma]1.00] [([Mn.sub.7.12][Mg.sub.4.77]).sub.[sigma]11.89][([[Fe.sup.3+].sub.1.2 8] - [Al.sub.0.67]).sub.[sigma]1.95][([As.sup.3+][O.sub.3]).sub.1.00][([As .sup.5+][O.sub.4]).sub.2.08][(OH).sub.22.99]. Relationship to other species: The crystal structure resembles that of hematolite.

Name: For Dr. Takaharu Araki (1929-), formerly of the University of Chicago, for his numerous crystal-structure contributions to mineralogy. Comments: IMA No. 1998-062. Note that the crystal structure has been solved.

ROBERTS, A. C., COOPER, M. A., HAWTHORNE, F. C., GRICE, J. D., and FEINGLOS, M. N. (2000) Arakiite a new As-bearing hematolite-like mineral from Langban Varmland Sweden. Mineralogical Record 31, 253-256. COOPER, M. C. and HAWTHORNE, F. C. (1999) The effect of differences in coordination on ordering of polyvalent cations in close-packed structures: the crystal structure of arakiite and comparison with hematolite. Canadian Mineralogist 37, 1471-1482.

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Author:Mandarino, J.A.
Publication:The Mineralogical Record
Geographic Code:1CANA
Date:Sep 1, 2001
Words:543
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