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Decomposition Oi' Pyritized Carbonaceous Shaie To 75+ NOTES AND NEWS THE AMERICAN MINERALOGIST, VOL. 16, MAY_JUNE, 1961 DECOMPOSITIONOI' PYRITIZED CARBONACEOUSSHAIE TO HALOTRICHITEAND MELANTERITE Cu.rnrrs B. Scren, BattelleMemorial Jnstilute, Columbus,Ohio In April, 1959,the writer examinedseveral polished sections of gold- and silver-bearing pyritic zinc ore from a locality in the Dominican Republic. Three sections,two composeddominantly of pyrite dissemi- nated through carbonaceousillitic shale and the third composeddomi- nantly of sphalerite with minor pyrite, were stored in a desk drawer. In August, 1959, the desk was moved into a centrally air conditioned area where the temperature is controlled during the summer months between 2lo and 24" C. except for infrequent interruptions of short duration ((48 hours). The local dehumidification equipment is some- what inefficient so that the relative humidity in the room is high and not unlike that in mines and caves.In August, 1960,the writer noticed a white effiorescenceof acicular crystals on both the polished and the rough surfacesof the pyritized shalespecimens; the sphalerite-richspec- imen was unaffected. The white effiorescenceis water solubleand is composeddominantly of radial-fibrousaggregates of halorrichite IFe+'(Fe+3,Al)z(SOa)a..22IfrC-1 and subordinately of lath-shaped to tabular-vermiform crystals of melanterite [FeSOr.7HzO](Fig. 1). These minerals are intimately asso- ciated and appear to be in equilibrium. X-ray powder diffraction data agree with those reported for halotrichite by Bauer and Sand (1957) and for melanterite on ASTM card 1-0255.A spectrographicanalysis showedthat the major cationsare Fe and Al;trace amounts (<O.50/6) of Ca, Mg, Zn and Cu are also present. The presenceof SOr was con- hrmed by the BaSOatest. The acicularcrystals of halotrichitehave 7:1.490* 0.0O2,2[,c:38", atrd weak birefringence. The individual crystals have a maximum length of 1.5mm. and rangein thicknessfrom about 1.5to 5 microns.An aqueoussolution of the effiorescencegave a positive reaction for iron with both potassiumferro- and ferricyanideand with ammonium hydrox- ide which suggeststhat this halotrichite containssome ferric iron proxy- ing for aluminum and representsan intermediate composition in the halotrichite - bilinite [Fe+2Alr(So+)+. 22HzO]- [Fe+2Fs+a1SO+)n. ZZltro] SCTIES. Abundant shale and unaitered brass-yellowpyrite fragments ranging in size from a few microns to 0.2 mm. in diameter are enclosedin the halotrichite aggregates.Many fragments have a specular facet which NOTES AND NEWS showsthat they wereonce part of the now disruptedand pitted polished surface. In immersion mounts the tabular melanterite crystals appear as straight, hook-shaped,semicircular to almost annular and S-shaped 'fhey forms. have B:1.478+0.002 and (f )2V Iarge (-Jio). Most of lhese crystals are oriented in immersion mounts and show slightly to moderately ofi center Bxo and Bx. interference figures' Such crystals have the optic plane transverse to the elongalion, small extinction Frc 1. (Left) Photomicrographof halotrichitecrystals. Two melanteritecrystals are Iabeled(m). Planepolarized light. X80. (Right) Curved tabular crystalsoi melanterite. Crossednicols. X80. angles (0o-12"), and positive and negative elongation for Bx* and Bx,, sections, respectively. Halotrichite and melanterite are fairly common weathering products of pyritic aluminous rocks and accumulate in protected cracks and re- entrants. They also coexist in efllorescences which form on the walls and timbers of underground workings, particularly those in pyritic coal seams and pyritic ore deposits. In a recent paper' Brant and Foster (1959) briefly described discrete occurrences of halotrichite and melan- terite on the surface of a basement-stored pyritic coal cole, and con- cluded that both minerals formed under similar conditions. This paper supports their viewpoint. Where the relative humidity is high, pyrite decomposes rapidly' and 756 NOTESAND NIiWS weakly acid sulfate solutionsare releasedwhich attack aluminous min- erals and bring aluminum into solution. Evaporation of films of these aqueous solutions results in crystallization of halotrichite andfor melanterite.The halotrichite/melanteriteratio at any point is probably determinedby the local concentrationof clay minerals or other soluble aluminousminerals in the pyritic rock. Occelshaw(1925, see Mellor, 1935,and Campbelland Smith, 1951) has shown that halotrichiteis a congruentlysaturating compoundwhich is in equilibrium at 25o c. with aqueoussolutions containing between4 and 10/6 of FeSOrand between 25 and 20/6 of Alr(SOa)r,respectively, and that melanteriteis in equilibrium at 2s" c. with aqueoussolutions containing higher concentrationsof FeSOr and correspondinglylower concentrationsof Alz(SOa)3.His equilibrium diagram also shows that when alunogen [Alz(SOrB.16HrO]is not present in the crystallized product, the correspondingaqueous solutions contained at least 10/6 Fesor and lessthan2016 Alr(so4)sduring crystallizationor a minimum Fe/AI weight ratio of about 1.15. To obtain a solution with this mini- mum ratio would require the dissolutionof about 3 grams of iron-free illite containing 2570Alzos for each gram of pyrite. These data support the hypothesis proposed above that local shifts in the Al+s and Fe+2 concentration levels of the aqueous solutions are responsiblefor the variable halotrichite/melanteriteratio of the effiorescenceand that the two mineralscrystallize under the samephysical conditions. The reduced state of the iron in theseminerals is probably due to the combinedefiect of the low acidity of the parent solutions and the carbonaceousmatter in the shale. RernnnNcss Beunn, G. S., and SlNo, L. B. (1957), x-ray powder data for uiexite and haiotrichite: Am. Minerol ,42, 67G678. Btewr, R. A., exo Fosrer, W. R. (1959), Magnesian haiotrichite from Vinton County: Ohio, Ohio Jour. Sci.rl9,187-192. Cmrunnrr., A. N., awn Surrn, N. O. (1951), The phase Rule and. Its Applications by A. Findlay, Ninth Edition, Dover, New york, p. 350. Mnllon, J' w. (1935), A comprehensive treatise on inorganic and theoretical chemistry: Vol. 14, Longmans Green, London, pp. 299-300. occtrsuew, v. J. (1925), The equilibrium in the systems aiuminum suifate-copper sulfate- water and aluminum sulfate-ferrous sulfate-water at 2s" C. Jour. Chem. soc., 127. 2598-2ffi2..
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