THE AMERICAN MINERALOGIST, VOL. 47, MARCH-APRIL, 1962 NSUTITE-A WIDESPREAD MANGANESE OXIDE MINERAL W. K. Zwrcrnn, W. O.J. GnoBuovrr.l Mnr;nn ewo H. W. Jarlo, Union Carbid,eOre Company, Division of Union Carbid,eCorporation, Tured.o,New York. Ansrnacr Nsutite is a widespread, hydrated, manganese oxide mineral of the composition: Mn1-*4+Mn"2+Oz-2"(OH)r*; where x:0.0G0.07 Ior analyzed material from Ghana, Greece, and Mexico; and x:0.16 for a manganoan variety from Ghana. Nsutite occurs as opaque, dark gray to black, porous to dense aggregates; G is variable fuom 4.244.67; H:g.S (Moh's scale) and 1150 (vicker's scale). rt shows a strong, diagnostic r-ray difiraction line at 1.64 it.In reflected light, nsutite is anisotropic; reflectivity in air:407o (E),3g.Sya (D), and,34.3/6 (C). Manganoan nsutite occurs in black, opaque masses; G:3.86; H:6.5-7 (Moh's ^scale)and 900 (vicker's scale). rt shows a strong, diagnostic r-ruy diffraction rine atl.67 L.Itisanisotropicinreflectedlight; reflectivity tnair:29.7To(E),zO.Zyo(D),and 16%(c). Thermochemical and t-ray difiraction studies show that manganoan nsutite inverts to nsutite when heated to 150' c. in air; to a pyrolusite-type structure at 450" c. in oxygenl to bixbyite at 550-850'c1 to hausmannite at 950'-1100o c.; and to a spinel form at tem- peratures above 1100'C. Manganoan nustite commonly forms from the oxidation of rhodo- chrosite or manganoan calcite. rt is a relatively uncommon mineral probably because it is converted to nsutite by further oxidation. INrnooucrroN, HrsroRy ANDNoMENCLATURE Gamma-MnOz is a collective name which is used to designatea group of slightly difierent synthetic phasesof manganesedioxide, largely char- acterized by their x.-ray diffra"ctionpatterns. Nsutite (6n.sd6,tit) is the naturally-occurring equivalent of one specific form of gamma-Mno2. ft is named for the type locality at Nsuta, Ghana, where it occurs abund- antly. The term gamma-MnOz was first used by Glemser (1939) for a specific, artificially prepared manganese dioxide. Later this term was applied to a group of materials with similar, or only slightly different, r-ray diffraction patterns, in papers published by Dubois (1936), McMurdie (1944),and Feitknecht and Marti (1945). Cole et at. (1947) summarized these data, including their own work, and suggestedthe terms I and II gamma-Mno2 for additional distinct modifications.Sev- eral authors noted the abundance of nsutite (,,gamma-MnO2',) in manganeseore from Nsuta in Ghana, West Africa: McMurdie (1944), (1948), Vinal (1950), Delano (1950), Skewes,Wadsley and Walkley (1952). Schossberger(1940) recognized differencesin the diffraction patterns of nsutite ("gamma-MnO2") and assumedthat a natural series of transitions exist between gamma-MnO2 and pyrolusite. Sorem and Cameron (1960) have also describednsutite (,,Nsuta-MnOr',) from its type locality at Nsuta, Ghana. They suggestreserving the term gamma- 246 NSUTITE 247 MnO2 for artificial products only and trace the usageof the term gamma- MnOz from its inception by Glemser to the present. The reader is referred to their paper and later work performed by Brenet et at. (1959), de Wolff (1959), and Feitknecht, et at. (1960) for a detailed discussion of the implications of the term. Sorem and Cameron (1960) observed slight variations in the r-ray diffraction patterns of nsutite (,,Nsuta- MnO2") and differentiatedbetween a "1.64, 1.65and 1.67 type,,,on the basis of prominent d-spacings in diffraction patterns, but give no ex- planation for the shift of these diffraction lines. Samplesof the "1.64 and 1.67types" of nsutite from Ghana have been chemically analyzed. The results show that the "1.64 type" from Nsuta contains 90.5770manganese determined as MnOz and 2.60/6 manganese determined as MnO. The "l.67 type," however, contains only 74.64o/s MnOz and as much as 11.02/6 MnO. This indicates that the observed shift in the x-ray diffraction lines is apparently due to an expansion of the unit cell caused by the presence of an increased amount of lower valent (larger)manganese ions. The two varieties,"types I.64 and 1.67,,' may thereforebe referred to as nsutite and manganoannsutite, respec- tively. Traore and Brenet (1959) describehow a mild cathodic reduction of synthetic gamma-MnO2 in a Leclanch6-type cell produces shifts in dif- fraction spacings due to the larger size of Mn2+ ions which are formed during this process.Further, they describehow the expanded structures can be recollapsedby a mild thermal treatment at 120" C. in air. Feit- knecht et al. (1960) were able to reduce partly some electrolytically pre- pared gamma-MnO2 with hydrazine, without changing its lattice type. They state that the volume of the elementary cell increases approxi- mately in a linear function with decreasing Mna+ content. One of our experiments was to heat a sample of manganoan nsutite for several hours at 150o C. The produced material gave a diffraction pattern identical to that of nsutite. The ratio MnO/MnO2 changedduring this heat treatment from 1:5 to 1:20, respectively. The authors have observed that nsutite is far more abundant than manganoan nsutite. The latter was identified in only two of many samplesstudied over the past few years. Cnnurcal CoruposrrroN Three samples of nsutite and one of manganoan nsutite have been analyzed. Analyses are presented here (Table 1) on nsutite from Nsuta, Ghana; Tartana,l Greecel and the Molango Piedras Negras area, I The "Tartana" mine, owned and operated by Dimitry Scaiistiri, is located some 20 miles north of Drama, Macedonia, Greece. The authors are indebted to Mr. Sanford R Knapp who kindly supplied the sample from his private collection. 248 W. K. ZWICKER, W. O. J. GROENEVELD MEITER AND H. W. TAFFE €N 6@* € NO, ON 6o r ' bo loo oo L l4 RFg. dJ + s F.sX h EEEg @ dEe€r3 4 za A ' *N + + + +' z >EE: z oGYt) z 2ZOZ z N9 9zb<aEsrt ts ts za E 3. 8. 3. €. a 6 + z qF.e€. ts E a359 Fl s66O€ :tsOrFO o roo+- ' Noi-N€ -i^^X q o looooo6 (,a z ESEE oo N6€or! ''':S$46o a E ts oo toooeo toob ^/. .9sls o+4 $€ 2ZAZ U , h@€, r@ F () + z + >sd> Eg<a*uf I +s $N ro@bh€a €o o': t+N+oos€'I | +d' o\/\/+ -b |l z-o .f ON@Nr O+r\€b€rt Fi r.t??q::l I 4J ":11q: 6O tsO @N+NNN€N+OO horo*-o^-Qd€-o -:^ oN' sE -? F@ NN OO9bOOr NNtNNt$4$O::61 'l oe' A^ ^ i F ^ + f,az EE33=-EEEBEgEF"F o NSATITE 249 I{idalgo State, Mexico; and also on manganoan nsutite from Nsuta, Ghana. Samples were prepared for analysis after exhaustive r-ray work had been done on the hand specimensand polishedsections made there- from in order to establishtheir purity. Nsutite from Nsuta has a porous structure and contains small amounts of fine-grainedquaftz. Nsutite from Greece is massive with very minor chalcophanite which occurs as violet plateletson one side of the specimen.Chalcophanite was removed from the sample used for the analysis. The Mexican nsutite sample is also porous and it has no visible impurities. The manganoan nsutite was obtained by hand picking oI an "w-ray pure" specimenfrom Nsuta. It contains some fine-grained quartz which shows up in the r-ray diffrac- tion patterns. The chemicalanalyses, made by R. E. Dutton,l are given in Table 1, columns 1-4. Available oxygen, determined by titration (oxalate method) has beenused to form MnO2 from total Mn (not listed here) and remain- ing Mn is calculated as MnO. Alternatively, Mn2O3, containing triva- lent manganesecould be formed through use of the equation: MnO*MnO2:1,11r9t The atomic ratios and formulas obtained from the chemical analyses are presentedin Table 1. In the caseof both samplesfrom Nsuta, SiO2 was deducted from the analyses inasmuch as quartz was observed as an impurity. Analyses have not been corrected for HzO-rzooC., but IfrO-tro" C. was not included in the formulas, not was its share of oxygen atoms brought into the totals. This was done since it was found that drying at I20" C. doesnot producethe collapseof manganoannsutite to nsutite. Two sets of formulas may be calculated depending on whether it is assumed that the deficiency of available oxygen to form stoichiometric MnOz is calculatedas MnO or Mn2O3.It seemsimportant to point out here that the very closestoichiometry of these formulas indicates that nsutite doesnot have a defectiveMnOz lattice. There is no agreementin the literature on the type of "lower valent" manganesein the structure of gamma-Mno2 type compounds. Frondel (1953) prefers the use of trivalent manganesein the formula of hydrohausmannite; Traore and Brenet (1959) the use of divalent manganese,and Feitknecht et ol. (1945) and (1960) the use of trivalent manganesein the formula of gamma-MnO2.It is apparent that nsutite is richer in Mna+ and poorer in HzO* than the manganoannsutite which has lower Mna+ and higher HzO* contents.This is the reasonwhy substitutions of the type 1 R. E. Dutton, Union Carbide Nuclear Company, Research Center, Tuxedo, New York. 250 W. K. ZWICKER, W. O. J. GROENEVELD MEITER AND H, W. JAFFE | 2 4 6 B tO 20 40 @EO|OO * % z- "/o Mn'*.ara. $-.;eee6 '' ;;fip-.dc-roo% Mn4++Mf*.crc. Frcs. 1 (left) and 2 (right). Analyses of three nsutites (1.64) and one manganoannsutite (1.67) plotted against idealizedformula containing Mna+ and Mn2+ or Mna+ and Mn3+. Mn'*o'- - Mn+'(oH)rt or Mn4+O2_+ Mn+B(OH)_1 may be postulated(Feitknecht and Marti (1945)).