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A NEW ZN-BEARING HEMATOLITE-LIKE MINERAL FROM LANGBAN, VARMLAND, SWEDEN.

ABSTRACT

Arakiite, idealized as (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], is monoclinic, space group Cc, with unit-cell parameters refined from powder data: a = 14.248(8), b = 8.228(4), c = 24.23(1) [dot{A}], [beta] = 93.62(3)[degrees], V = 2843(2) [[dot{A}].sup.3], a:b:c = 1.7316:1:2.9445, Z = 4. The strongest seven reflections in the X-ray powder-diffraction pattern are [d([dot{A}])(I)(hkl)]: 12.07 (100) (002); 6.046 (100) (004); 4.040 (90) (006); 3.148 (30) ([bar{4}]04, [bar{1}]17); 3.030 (70) (224); 2.411 (40) (424, [bar{5}]15); 1.552 (70) (640, [bar{3}]51). The mineral was found on a museum specimen, previously labeled as dixenite, from L[dot{a}]ngban, V[ddot{a}]rmland, Sweden, and occurs on one surface as red-brown to orange-brown aggregates of micaceous plates. Associated minerals are calcite and very minor magnussonite, and the bulk of the specimen is specular hematite. Arakiite is megascopically indistinguishable from either hematolite or dixenite and possesses the following physical properties: steak is pale brown; lustre is resinous to submetallic; diaphaneity is opaque (masses) to translucent (thin edges); non-fluorescent; hardness is estimated at 3-4; cleavage is {001} perfect; tenacity is brittle; fracture is uneven, almost subconchoidal; calculated density is 3,41 g/[cm.sup.3] (for empirical formula and unit-cell parameters derived from crystal structure). Arakiite is biaxial negative, [alpha] 1.723(4), [beta] = 1.744(2), [gamma] = 1.750(2); 2 V(meas.) 44(3)[degrees] (extinction), = 40(10)[degrees] (direct), 2 V (calc.) = 56[degrees]; dispersion r [greater than] v medium; orientation is Y = b and X [wedge] c = +4[degrees] (in [beta] obtuse). There is no evidence of pleochroism. The crystal structure shows that manganese occurs as [Mn.sup.2+] iron occurs as [Fe.sup.3+] and arsenic occurs as both [As.sup.3+] and [As.sup.5+] in a 1:2 ratio. Averaged electron-microprobe analyses yielded ZnO = 4.48, MnO = 34.32, MgO = 12.76, [Fe.sub.2][O.sub.3] = 6.76, [Al.sub.2][O.sub.3] = 2.25, [As.sub.2][O.sub.3] = 6.56, [As.sub.2][O.sub.5] = 15.84, [H.sub.2]O (calculated assuming stoichiometry) = [13.74], total = 96.71 weight %, corresponding to [([Zn.sub.0.83][[M.sup.2+].sub.0.17]).sub.[Sigma]1.00][([[Mn.sup.2+]. sub.7.12][Mg.sub.4.77]).sub.[Sigma]11.89][([[Fe.sup.3+].sub.1.28][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], based on 34 (O+OH) anions. (OH) was confirmed by both infrared spectroscopy and crystal-structure analysis. The mineral name is for Dr. Takaharu Araki (1929-) for his numerous crystal-structure contributions to the science of mineralogy.

INTRODUCTION

Arakiite, ideally (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], is a newly recognized mineral species from L[dot{a}]ngban, Sweden. The specimen on which the new mineral occurs was acquired by one of us (M.N.F.) on exchange from the late John Fuller of The Natural History Museum, London, UK, in September 1985. It is a portion of a larger specimen (BM1921, 310) which was labeled as dixenite from L[dot{a}]ngban, V[ddot{a}]rmland, Sweden, and was initially acquired by the museum in 1921 from Mr. C. Wendler who had provided them with a number of L[dot{a}]ngban samples in the early 1920's. The misidentification is certainly legitimate; arakiite is megascopically indistinguishable from dixenite and hematolite. All three minerals occur at L[dot{a}]ngban as thin foliated reddishcolored masses on ore-bearing matrix. Routine X-ray powderdiffraction study indicated that the mineral is not identical to dixenite or hematolite. Further study using modern mineralogical techniques showed it to be a new mineral, the description of which is reported here. The crystal structure most closely resembles that of hematolite (Moore and Araki, 1978) rather than that of dixenite (Araki and Moore, 1981) and has been dealt with in a separate publication (Cooper and Hawthorne, 1999).

The mineral is named for Dr. Takaharu Araki (b. 1929), formerly of the Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, for his numerous crystal-structure contributions to the science of mineralogy. Dr. Araki, in close collaboration with Professor Paul Moore, solved the structures of many complex P-bearing and As-bearing phases, including those of hematolite and dixenite. The mineral and mineral name have been approved by the Commission on New Minerals and Mineral Names, I.M.A. The specimen used for this study now resides within the Harvard Mineralogical Museum, Cambridge, Massachusetts, and has been assigned the catalog number 134608. Additional material is preserved at The Natural History Museum, London, UK, as BM1921, 310.

SPECIMEN DESCRIPTION

The single sample used for this study measures 15 x 20 x 18 mm and consists principally of massive fine-grained specular hematite. Arakiite is only found on one surface where it occurs as aggregates of micaceous plates over an area of approximately 15 x 10 mm. A color photograph of essentially the complete arakiite aggregate is presented in Figure 1. Associated minerals are colorless anhedral calcite and tiny blue-green anhedral grains of magnussonite. We have no detailed information about the site of origin within the mine.

PHYSICAL and OPTICAL PROPERTIES

Arakiite is red-brown to orange-brown with a pale-brown streak. The lustre is resinous to submetallic, and masses are opaque whereas thin edges of grains are translucent. The mineral is predominantly anhedral and mica-like with no obvious forms; it has a perfect (001) cleavage. The tenacity is brittle, fracture uneven, almost subconchoidal, and the hardness is estimated at 3 to 4, considering that it is easily scratched by a needle. There is no evidence of fluorescence under either longwave or shortwave ultraviolet radiation. Density could not be measured because of the meager quantity of pure material; the calculated density is 3.41 g/[cm.sup.3] (based on the empirical formula and unit-cell parameters determined from the crystal-structure study). Twinning was neither observed megascopically nor in the subsequent X-ray single-crystal and crystal-structure studies.

Spindle-stage optical measurements at 590 nm indicate that the mineral is biaxial negative with [alpha] = 1.723(4), [beta] = 1.744(2), [gamma] = 1.750(2); 2 V (measured by extinction method) 44(3)[degrees], 2 V (measured directly) = 40(10)[degrees], 2 V (calculated) = 56[degrees]. The discrepancy between measured and calculated 2 V is attributable to uneven extinction related to the "curved" nature of the plates. The dispersion is r [greater than] v medium, and no pleochroism was observed. The optical orientation is Y = b, X [wedge] c = +4[degrees] (in [beta] obtuse).

X-RAY DIFFRACTION

Arakiite is monoclinic, space group Cc, with unit-cell parameters refined from powder data: a = 14.248(8), b = 8.228(4), c = 24.23(1) [dot{A}], [beta] = 93.62(3)[degrees], V = 2843(2) [[dot{A}].sup.3], and a:b:c = 1.7316:1:2.9445. Cell parameters derived from the crystal-structure study (Cooper and Hawthorne, 1999) are: a = 14.236(2), b = 8.206(1), c = 24.225(4) [dot{A}], [beta] = 93.52(1)[degrees], V = 2824.0(7) [[dot{A}].sup.3], and a:b:c = 1.7348:1:2.9521.

A fully indexed X-ray powder-diffraction pattern is presented in Table 1. If one considers the whole pattern, the data are unique. However, strong 00l reflections at 12.07, 6.046, 4.040 and the strong reflection at I.552 [dot{A}] are comparable, in both intensity and spacing, to those diffraction lines published for both hematolite and dixenite and, additionally, to the unnamed [Fe.sup.3+] analogue of hematolite (Dunn and Peacor, 1983) and to the unnamed L[dot{a}]ngban arsenate of Roberts and Dunn (1988). Studies are in progress on the latter two; suffice to say at this time that neither is crystallographically nor chemically identical to arakiite. All known members of this "family" of structures can be readily differentiated by routine X-ray powder-diffraction film methods, including 57.3 mm cameras.

CHEMISTRY

A thin cleavage plate of arakiite was analyzed with a Cameca SX-50 electron microprobe, using an operating voltage of 15 kV, a beam current of 20 nA, a beam 5 [mu]m in diameter, and a counting time of 20 s on a peak and 10 s on background. The plate was selected from an area on the sample adjacent to that from which the cleavage plate used for the crystal-structure analysis was picked; it was fixed to the surface of a plexiglass disk and then carbon coated prior to analysis. Probe standards are as follows: cobaltite (As); gahnite (Zn); spessartine (Mn); forsterite (Mg); fayalite (Fe); kyanite (Al). An energy-dispersion scan indicated the absence of any other elements with atomic number greater than 9 except those reported here. The following elements were sought but not detected: Na, Ca, K, Ti, V, Cr, Cu, Si, P, S, F. Data were corrected using the PAP procedure of Pouchou and Pichoir (1984, 1985). The valence states for Mn and Fe and the number of 0 atoms, were determined by crystal-structure analysis prior to the final interpretation of the electron-microprobe results. The paucity of pure material prevented quantitative determination of [H.sub.2]O. However, the presence of H as (OH) was confirmed both by crystal-structure analysis and powder infrared-absorption study; the formula was therefore calculated to give 23 (OH). The average of thirteen determinations and ranges and standard deviations are given in Table 2. Arsenic was initially quantified as [As.sub.2][O.sub.5], then partitioned as [[As.sup.5+].sub.2][O.sub.5] and [[As.sup.3+].sub.2][O.sub.3], in a 2:1 ratio as observed in the structural study. With 34 anions [O + (OH)], the empirical formula for arakiite is [([Zn.sub.0.83][[Mn.sup.2+].sub.0.17]).sub.[Sigma]1.00][([[Mn.sup.2+] .sub.7.12][Mg.sub.4.77]).sub.[Sigma]11.89][([[Fe.sup.3+].sub.1.28][Al .sub.0.67]).sub.[Sigma]1.95][([As.sup.3+]-[O.sub.3]).sub.1.00][([As.s up.5+][O.sub.4]).sub.2.08][(OH).sub.22.99]. The idealized formula is (Zn,[Mn.sup.2+])-[([Mn.sup.2+],Mg).sub.12][([Fe.sup.3+],Al).sub.2]([A s.sup.3+][O.sub.3])[([As.sup.5+][O.sub.4]).sub.2][(OH).sub.23] and the end-member formula is Zn[[Mn.sup.2+].sub.12][[Fe.sup.3+].sub.2]([As.sup.3+][O.sub.3])([As.s up.5+][O.sub.4])[(OH).sub.23]. Zinc is the key element which differentiates arakiite from hematolite. Both minerals have structures that are based on five close-packed repeat layers. Four of the five layers are topologically identical and only differ in cation ordering. The single distinctive layer involves tetrahedrally coordinated (Zn/[Mn.sup.2+]) in arakiite versus octahedrally coordinated ([Mn.sup.2+]) in hematolite. Full details of the structural similarities and differences between arakiite and hematolite have been published by Cooper and Hawthorne, 1999.

INFRARED-ABSORPTION STUDY

The equipment and procedures for acquiring the infraredabsorption spectrum for arakiite are identical to those used to obtain the spectrum of mcalpineite (Roberts et al., 1994) and are not repeated here. The spectrum (Fig. 2) clearly shows absorption bands for (OH). Strong to medium-strong bands at 3366 and 3589 [cm.sup.-1] are due to 0-H stretching in the hydroxyl groups.

ACKNOWLEDGMENTS

The authors thank E. Moffatt (Canadian Conservation Institute) for the infrared spectrum of arakiite and M. Clarke (GSC) for redrafting Figure 2. FCH was supported by Research and Major Equipment grants from the National Sciences and Engineering Research Council of Canada.

REFERENCES

ARAKI, T., and MOORE, P. B. (1981) Dixenite, [Cu.sup.1+][[Mn.sup.2+].sub.14]-[Fe.sup.3+][(OH).sub.6][([As.sup.3+][ O.sub.3]).sub.5][([Si.sup.4+][O.sub.4]).sub.2]([As.sup.5+][O.sub.4]): metallic [[[As.sup.3+].sub.4][Cu.sup.1+]] clusters in an oxide matrix. American Mineralogist, 66, 1263-1273.

COOPER, M.A., 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.

DUNN, P. J., and PEACOR, D. R. (1983) A ferric iron equivalent of hematolite from Sterling Hill, New Jersey and L[dot{a}]ngban, Sweden. Mineralogical Magazine, 47, 381-385.

MOORE, P. B., and ARAKI, T. (1978) Hematolite: a complex dense-packed sheet structure. American Mineralogist, 63, 150-159.

POUCHOU, J.-L., and PICHOIR, F. (1984) A new model for quantitative analysis. 1. Application to the analysis of homogeneous samples. La Recherche A[acute{e}]rospatiale, 3, 13-38.

POUCHOU, J.-L., and PICHOIR, F. (1985) "PAP" (phi-rho-z) procedure for improved quantitative microanalysis. In Micro-beam Analysis (J. T. Armstrong, ed.), San Francisco Press, San Francisco, California (104-106).

ROBERTS, A. C., and DUNN, P. J. (1988) Mineralogical data for a new, unnamed arsenate from the L[dot{a}]ngban mine, V[ddot{a}]rmland, Sweden. Geologiska F[ddot{o}]reningens i Stockholm F[ddot{o}]rhandlingar, 119, 181-182.

ROBERTS, A. C., ERCIT, T. S., CRIDDLE, A. J., JONES, G. C., WILLIAMS, R. S., CURETON, F. F. II, and JENSEN, M. C. (1994) Mcalpineite, [Cu.sub.3]Te[O.sub.6][cdotp][H.sub.2]O, a new mineral from the McAlpine mine, Tuolumne County, California, and from the Centennial Eureka mine, Juab County, Utah. Mineralogical Magazine, 58, 417-424.
                         X-ray powder-diffraction
                            data for arakiite.
[I.sub.est.] d[[dot{A}].sub.(meas.)] d[[dot{A}].sub.(calc.)]  hkl
    100              12.07                   12.089           002
    100               6.046                   6.045           004
     10               5.463                   5.422           113
     20               5.262                   5.255           113
     15               4.764                   4.756           204
     30               4.119                   4.114           020
     90               4.040                   4.030           006
     10               3.952                   3.943           115
      5               3.822                   3.826           312
     10               3.648                   3.664           023
      5               3.572                   3.561           220
      5               3.496                   3.487           314
     20               3.404                   3.415           206
                                              3.401           024
      5               3.317                   3.314           314
     10               3.223                   3.219           315
     30               3.148                   3.152           404
                                              3.147           117
     70               3.030                   3.026           224
     5b               2.862                   2.879           026
                                              2.847           208
      5               2.799                   2.801           316
      5               2.749                   2.753           406
     10               2.700                   2.693           130
     20               2.631                   2.635           132
      5               2.594                   2.599           422
                                              2.597           513
      5               2.546                   2.545           133
     20               2.498                   2.502           424
     40               2.411                   2.415           424
                                              2.411           515
     15               2.372                   2.370           331
     15               2.338                   2.341           228
                                              2.341           135
    10b               2.291                   2.292           515
                                              2.288           426
     5b               2.237                   2.234           408
                                              2.234           334
     5b               2.157                   2.161           604
                                              2.158           335
      5               2.106                   2.112           137
                                              2.105           335
                                              2.102           606
    20b               2.065                   2.062           517
     10               2.017                   2.017           623
      5               1.905                   1.907          2012
     20               1.859                   1.857           429
     15               1.810                   1.810           608
                                              1.810          1310
      5               1.777                   1.778          2212
     20               1.748                   1.747          5110
      5               1.661                   1.660          5112
      5               1.619                   1.621           538
     20               1.573                   1.574          2214
     70               1.552                   1.553           640
                                              1.553           351


114.6 mm Debye-Scherrer powder camera

Cu radiation, Ni filter ([lambda] CuK[alpha] = 1.54178 [dot{A}])

Intensities estimated visually; b = broad line

Not corrected for shrinkage and no internal standard

Indexed with a 14.248, b = 8.228, c = 24.23 [dot{A}], [beta] = 93.62[degrees]
                     Compositional data for arakiite.
                    Weight %        Range        Standard deviation
ZnO                   4.48        4.00-4.74             0.19
MnO                  34.32       33.84-34.95            0.34
MgO                  12.76       12.27-13.47            0.34
[Fe.sub.2][O.sub.3]   6.76        6.32-7.16             0.27
[Al.sub.2][O.sub.3]   2.25        2.02-2.47             0.14
[As.sub.2][O.sub.5]  15.84      {23.O0-24.2 [1]}        0.38
[As.sub.2][O.sub.3]   6.56
[H.sub.2][O.sup.2]  [13.74] [2]
Total                96.71


(1.)Total As calculated as [As.sub.2][O.sub.5], then partitioned as [[As.sup.5+].sub.2][O.sub.5] and [[As.sup.3+][O.sub.3] in a 2:1 ratio as determined from the crystal structure.

(2.)Calculated assuming the formula derived from crystal-structure analysis.
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Author:Roberts, Andrew C.; Grice, Joel D.; Cooper, Mark A.; Hawthorne, Frank C.; Feinglos, Mark N.
Publication:The Mineralogical Record
Geographic Code:4EUSW
Date:May 1, 2000
Words:2830
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