A magnesium analogue of chalcophanite-in manganese-rich

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					                                       AmericanMineralogisl, Volume64,pages1227_1229,

       A magnesiumanalogueof chalcophanite-in
                                                         concretionsfrom Baja
                                   RussErl    M. porrrn'       AND Geonce      R. RossrrlnN
                                       Division of Geological and Planetary Sciences2
                                              California Institute of Technolo gy
                                                Pasadena, Calfornia 9 l 125


                 Concretionswhich carry chemical remanentmagnetizationin sedimentsin Baja Califor-
               nia, Mexico, previously called "hydropsilomelane,"were found to consistof siltstonematrix
               minerals cementedby a manganese     oxide. The X-ray powder pattern, infrared spectra,anc
               chemical analysesindicate that the manganese   oxide has the chalcophanitestructure. The
               high concentrationof magnesium,presumedto occur in the interlayer position of this min-
               eral, suggeststhat it is the magnesiumanalogueof chalcophaniteand extendsthe known
               range of substitutionin chalcophanite-structureminerals.

    ln a study of the chemical remanent magnetization               Rossman, 1977). Its spectrum is dominated by
 of sediments in Baja California, Larson and Walker                 absorption due to the siltstone matrix minerals.
 (1975) reported the presence of manganese-rich con-                   In order to investigate the manganese oxide miner-
 cretions which were found to carry remanent mag-                   alogy, powdered concretions were washed with l.0M
 netization in numerous sedimentary horizons. They                  acetic acid to remove the calcite, and difference in-
 informally named the manganese oxide phase ..hy-                   frared techniques developed previously (Potter and
 dropsilomelane" due to similarities in both chemistry              Rossman, 1979a) were then applied to produce in-
 and X-ray diffraction pattern to the manganese oxide               frared spectra of the pure manganese oxide phase of
 romanechite (psilomelane). We studied these con-                  the concretions. In the 4000 cm-'to 1400cm-' region
 cretions as part of a survey concerned with the min-              of the spectrum, absorption due to the hydrous com-
 eralogy of manganese oxides in the terrestrial weath-             ponents of the manganese oxide can be seen without
 ering environment (Potter and Rossman, 1979b).                    interference from the contaminating silicates, and
    Samples for infrared and qualitative chemical                  strongly suggests a chalcophanite structure based on
 analysis are from a locality 56 km south of San Fe-               comparison to standard chalcophanite and aurorite
lipe, Baja California, Mexico, and consist of weakly               spectra (Fig. l). Absorption in the 1400-200 cm-' re-
cemented siltstone in which the concretions are dis-               gion confirms that the manganese oxide has the chal-
persed. The siltstone was disaggregated by immer-                  cophanite structure. The positions and number of ab-
sion in water and the concretions hand-picked from                 sorption bands in Figure I are diagnostic for
the coarse fraction of the sediment. Removal of the                minerall with the chalcophanite structure (Potter and
manganese by chemical extraction (Mehra and Jack-                  Rossman, 1979c). Two such minerals are known,
son, 1960) showed that the bulk of the concretions                 chalcophanite and aurorite. These differ in the nature
consist of the siltstone matrix minerals calcite.                  of the cations between the layers of manganese octa-
qaartz, clay, and mica. This is in agreement with the              hedra, but are indistinguishable on the basis of their
infrared spectra of whole concretions (Fig. l) ob-                 X-ray diffraction and infrared absorption patterns.
tained by methods previously reported (potter and                  Chemical analysis of manganese-rich areas of the
                                                                   concretions by energy-dispersive X-ray analysis un-
                                                                   der a scanning electron microscope indicated Mn as a
  rPresent address: Owens-Corning
                                    Fiberglas Corporation, Gran-   major component with lesser amounts of Si, K, Ca,
ville, Ohio 43023.                                                 Al, Mg, and Na. We could find no areas as rich in
   2Contribution No. 3 147.
                                                                   oxide as those analyzed by Larson and Walker.
0003-004){. 79/ | | 12- 1227
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                       N R V E L E N G T(H I c B o N s )
                                         H                                                                   H
                                                                                           N R V E L E N G T(H l c s o N s )
                        3r.156                                                     B          10             15              25

          UJ                                                                 z
          z                                                                  s
          cl                                                                 co
          a                                                                  CE

                                                                                    1200      i000 800  600  q00
                   3600       3000      2q00       1800
                                            <B t
                          h I R V E N U M B Ec n ->                                           NRVENUMB<cu-t>
                                                                                                                                    (A) and
   Fig. l. Infrared spectra of concretions compared to reference manganese oxide spectra. Reference spectra of chalcophanite
aurorite (B). The spectrum of the whole concretion (D) shows bands   attributable to calcite (c), mica (m), kaolinite (k)' and quartz (q)' in
                                                                                                                       cm-r region are free
addition to the manganese oxide. Extracted samples show that the overlapping pairs of bands in the 3200-3500
from interference from the other components. They indicate    a chalcophanite-group manganese oxide. The spectrum of the manganese
oxide phase in the 1300-300 cm-r region, obtained in a difference spectnrm (C), is similar only to chalcophanite-group

   The X-ray and chemical data of Larson and        probe analysisfrom 100percent.[f somemanganese
Walker (1975)are in agreement                       is presentas Mn2*, as seems
                                with our infrared re-                            probablefrom published
sults. Their X-ray data are a significantly better  chalcophanite analyses,   the agreement would im-
match to chalcophaniteand aurorite than to romane-  prove. This would increasethe interlayer-sitepopu-
chite (Table l). Larson and Walker report that heat-lation, decrease  the manganese  octahedra layer-site
ing at 550"C causedthe 6.92A peak to disappearand   population, and increase  the water content. It is pos-
a new one to developat 4.87 This is strongly remi-
                             A.                     sible that the iron is not present in the manganese
niscent of the behavior of chalcophaniteon heating  oxide, but occursinsteadas a minor phase,in view of
(Dasgupta, 1974),it which the 6.96A chalcophanite   its low abundancein Larson and Walker's analysis
line disappearsand an intense 4.764 line is pro-    and its absence our analysis.The chemicaldata do
duced. Larson and Walker's chemical data yield the  not yield a recognizableromanechiteformula when
following formula on the basis of 7 oxygens (ex-    calculatedon the basisof l0 non-wateroxygens.The
                                                    mutually supporting resultsof infrared spectroscopy,
cluding water) with all Mn in the +4 oxidation state:
                                                    X-ray diffraction, and chemicalanalysisindicate that
        (MgouBao,Feo') (Mn'"AL')O'' 2HrO
                                                     the manganeseoxide in the concretions has the chal-
This is in agreementwith the formula of a magne- cophanite structure.
sium analogue chalcophanite,
               of              MgMn'O''3HrO, if        The phasewhich we are describingdiffers from all
allowance is made for uncertainty in water content, other previously reported minerals with the chal-
which was determined bv difference of the micro- cophanite structure in the nature of the substitution
                                           POTTER AND ROSSMAN: MAGNESIT]M ANALOGUE OF CHALC)PHANITE                                                                                    1229

Table      l.        X-ray diffraction data for chalcophanite,                                             aurorite,      ization (Larson and Walke\ 1975). Larson and
                       "hydropsilomelane," and romanechite*
                                                                                                                         Walker suggestthat these manganeseconcretions
 chalcophanite**                 ,rto.ia.n*               "hydropsilomelane,'              romanechlte**                 may be a common feature of continental playa sedi-
   4(A)          r                4(A)         I                4(A)         f                d(A)         1
                                                                                                                         ments. Their report that the concretionscarry rema-
                                                                                              9.68         3             nent magnetizationraisesthe questionof whetherthe
 6.96           10                6.94         10               6.92      10                  6.95         6
 4.08            5                4.06          5                                                                        magnetismresidesin the manganese      oxide or in an-
 3.50            6                3.46         7                3.48         4                3 48
                                                                                                                         other component. If the magnetismoriginates from
                                  2 . 76
                                                                                                                         the manganese  oxide, the study of the efect of inter-
                                  2-45                                                        2.42          3
                                                                                                                         layer cation substitution on the magnetic properties
                                  2 -40                         240          r                2.4r         t0            of chalcophanite-group  mineralsbecomes matter of
 2.24                             2-23                          223          4                2.26                       importance to paleomagnetic   investigations.
                                  2.t3                                                                     4
 1.90            3                1.90                          r.89                          7 -82
                                  1,80                                                                                                         Acknowledgments
 1.60            5                1.56                          r-.59                         1.56         3
 r.43            3                1-.4
                                     3                          1.38                          7-42         3               We thank E. E. Larson (University of Colorado) for the samples
  Data fron
                                                                                                                         ofconcretions used in this study.
              the follordng   sources:  chalcophanite,    J C.P.D,S.     File                                    Card
  No. 15-807;    aurorite,  Rad!ke d   aL,7967;     "hydropsilonelane,',
  Larson  and Ualker,1975;     ronanechice,   J.C.p.D.S,   File  C€rd No.                                       14-627
  Lines   with       lntensity         lower       than     3 have    been       onitred

                                                                                                                          Dasgupta, D. R. (1974) Oriented transformation of chalcophanite
 for the interlayer cation. Our reformulation of the                                                                         during thermal treatment. Z. Kristallogr., 139, 116-128.
chemical data of Larson and Walker indicates that                                                                         Larson, E. E. and T. R. Walker (1975) Development of chemical
Mg'* is the cation predominantly substituting for                                                                            remanent magnetization during early stages of red-bed forma-
                                                                                                                            tion in late Cenozoic sediments, Baja California. Geol. Soc. Am.
Zn2*, althoughsomeMn'* is probably present.There
                                                                                                                            Bull., 86,639-650.
is no evidencefor any Zrt2*or Ag* in the structure.                                                                      Mehra, O. P. and M. L. Jackson (1960) Iron oxide removal from
This lends support to the idea that wide composi-                                                                           soils and clays by a dithionite-citrate system buffered with so-
tional variations exist for chalcophanite(Radtke et                                                                         dium bicarbonate. In A. Swineford, Ed., Proceedings of the Sev-
al., 1967).We suggest   that variation in the nature of                                                                     enth National Conference on Clays and Clay Minerals, p. 317-
                                                                                                                            327. Pergamon, Oxford.
interlayer cations in minerals of the chalcophanite
                                                                                                                         Potter, R. M. and G. R. Rossman (1977) Desert varnish: the im-
structureis sufficientlylarge to justify the designation                                                                    portance of clay minerals. Science, 196, 1446-1448.
of a chalcophanitegroup.                                                                                                 -        and -       (1979a) The manganes€ and iron oxide miner-
   The paragenesis this phaseis different from that
                    of                                                                                                      alogy of desert varnish. Chem. Geol.,25,'19-94.
of other occurrencesof manganese                                                                                         -         and -       (1979b) Mineralogy of manganese dendrites
                                        oxides with the
                                                                                                                            and coatings. Am Mineral.,64, l2l9-L226.
chalcophanitestructure, and may indicate a signifi-
                                                                                                                         -         and -        (1979c) The tetravalent manganese oxides:
cantly larger rangeofoccurrence than has previously                                                                         identification, hydration, and structural relationships by in-
been recognized.  Chalcophanite-group     mineralshave                                                                      frared spectrosapy. Am. Mineral., 64, 1199-1218.
been thought to be relatively rare and of secondary                                                                      Radtke, A. S., C. M. Taylor aqd D. F. Hewett (1967) Aurorite, ar-
origin, associated with extensive                                                                                           gentian todorokite, and hydrous silver-bearing lead manganese
                                   manganese   mineral-
                                                                                                                            oxide. Econ. Geol.,62, 186-206.
ization.In Baja California, this phaseoccursas a pri-
mary mineral formed authigenically in silt deposits                                                                                  Manuscript received, December 14, 1978;
with no relation to any other manganesemineral-                                                                                      acceptedfor publication, March 27, 1979.

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