DOCSLIB.ORG

Mineralogy and Petrology of Very-Low-Metamorphic Grade

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Mineralogy and Petrology of Very-Low-Metamorphic Grade American Mineralogist, Volume 65, pages 8-25, 1980 Mineralogy and petrologyof very-low-metamorphicgrade Archaeanbanded iron-formations,Weld Range,Western Australia MaRUN J. Gore' Department of Geologt, University of llestern Australia Nedlands 6009. Australia Abstract There are two basicFe-rich lithologiesat Weld Range:(l) bandediron-formation contain- ing minnesotaite, siderite, quarlz,magnetite, greenalite, stilpnomelane, pyrite, and chamosite with trace amounts of pyrrhotite, arsenopyrite, chalcopyrite, apatite, and rockbridgeite; Q) Fe-shale.which occurs as laminated 2-30cm-thick bands within banded iron-formation, con- taining chamosite, stilpnomelane, siderite, greenalite, pyrite, magnetite, minnesotaite, and quartz with trace amounts of ilmenite, chalcopyrite, and apatite. Mineral assemblages,min- eral compositions, and the interpreted paragenetic sequencein both lithologies are essentially the sameas those reported from very-low-metamorphic grade Proterozoic iron-formations. In common with pubtshed greenalite analyses,those of greenalite from the Weld Range show a consistent excessof Si and a larger than corresponding deficiency in octahedral sites relative to the generallyaccepted formula of (Fe,Mg)"SiOro(OH)8.The earliestrecognizable mineral assemblagescontain Al-bearing greenalite(1.9-2.8 ttlVo AlzO), qu;artz,siderite, chamosite, magnetite, pyrite, and rarely pyrrhotite. These assemblagesare overprinted by stilpnomelane, which formed by reaction of a K-bearing fluid phasewith Al-bearing greenalite or chamosite or both. A secondaryAl- and Mg-poor greenalitedeveloped in someassemblages contempo- raneously with stilpnomelane. The presenceof quartz favored the growth of coarse-grained stilpnomelane. The relatively coarse grain size of quartz and siderite (up to 2.5mm) and the presenoeof angular fragments composedof early assemblagesin intraformational breccia in- dicates that the Fe-rich lithologies were well lithified prior to the development of stilpnome- lane. The incoming of minnesotaiteoccurred after that of stilpnomelane,and textures in- dicating reaction of earlier silicates and siderite to form minnesotaite are common. The assemblageminnesotaite-quartz+magnetite+pyrite represents the complete reaction of the earlier assemblages.The early assemblagesare preserved in bulk compositions which are Si- deficient or Al-rich or both, and under conditions of relatively lowPo, or high P.6, or both. The peak metamorphictemperature is estimatedto be 320150"C at low pressure' Introduction per amphibolite facies (Binns et al., 1976, p. 303- 313), and sirnilar variations in metamorphic grade Banded iron-formation is a widely distributed al- occur in the greenstonebelts in the northwest of the though relatively minor (<l-3 vol%o)fithology in the block. The mineralogy of bandediron-formations to- stratigraphic successionsof the 2.6-2.7 Gyr green- gether with that of juxtaposed tholeiitic metabasalts stone belts of the Yilgarn Block, Western Australia. and lesscomrnonly of pelitic metasedimentsindicates The Weld Range (Fig. l) is one of the few localities that most of the bandediron-formations in the green- in the Yilgarn Block from which samples of un- stone belts have been metamorphosedto gradesbe- weathered, very-low-metamorphic grade banded tween those of the upper greenschistand the upper iron-formation are available. In the eastern half of amphibolite facies. Banded iron-formation of lower the Yilgarn Block, the metamorphic grade in green- metanorphic grade is relatively uncorlmon, and few stone belts ranges from prehnite-pumpellyite to up- such banded iron-formations have been diamond- drilled to depths below the zone of weathering. I Present address:Department of Geology, Indiana University, The Weld Range is a marked physiographic fea- Bloomington,Indiana 472105. ture, 3-5km wide, 40km long, within which there is 0003-004x/80/0 l 02-0008$02.00 GOLE: BANDED IRON-FORMATIONS good exposureof metabasitesshowing mainly dol- Inclinotion 45o EXPLANATION Melers eritic and minor basalticand gabbroic textures.Such oo Somple g exposuresoccur between ridges - defined by weath- Loocotion BIF mognelite-rich ered, steeplydipping beds of banded iron-formation No. which form lessthan lD%oof the thicknessof the se- tl Fine- groined BIF - mognetite-poor quence. Poorly exposed, very fine-grained clastic Chl- Qtz tr r tl metasedimentsform a very minor part of the se- t-'_','-t - quence. In the Mt. Lulworth area, in the central part Fe shole f-1-'.--".i-r-r..fi of the Weld Range, variations in the mineralogy of tiriiii' metabasitesindicate a gradient in metamorphiccon- Ouortz- chlorite ditions from the lowermost greenschistfacies in the oxidized- Mog,Sit,Sid metosedrment 78 3_ north to approximately the greenschist-amphibolite faciestransition in the south. - Core samplesof banded iron-formation were se- Fine groined Chl- Qtz lected from diamond-drill Melers hole No. 3 (D.D.H. 3), 50 drilled by the Western Australia Department of Mines during iron ore exploration in the early 1960's t28.0- (Jones,1963). D.D.H. 3 is locatedon the northern Ltog,Minn, Qlz, Green, side of the Weld Range, 2km northeast of Mt. Lul- Sid,Stilp, Pyrlroce Po, worth (Fig. l), and was drilled below a prominent minorChom-Py bonds ridge of 84713 highly weathered banded iron-formation. o Minn, The hole intersected 847!5 t62.6 Qtz,Green, Py two bedsof bandediron-forma- 84716 Pv 19?'" -i !rc'M.lrJl].' tion within a sequenceof very fine-grained qaartz- Chom,Green,Stilp,Sid,Py,breccioted in ptoces - - - a4720 t789 chlorite-rich metasediments.The broad distribution 847 22- 44723 Minn,Mog,Qtz,Green, Sid, Stilp, Py, lroce Po of rock types in D.D.H. 3 and samplelocations are r932- given in Figure 2. Most samplesshow someevidence Sid,Minn! ofz, Mog,Py,fhin Chom-Py bonds j Sid,atz, Mi nn, ChomI Sti lp, Py ' '- iii of secondaryoxidation due to weathering,and parts a4732 Sid,Qfz, Minn, Mog, Py of the banded iron-formation, even in the deepest 847 2t5-2- sectionsofthe hole, are highly altered. In essentially OxidizedMog, Silicotes, Sid unweatheredsections of core. the banded iron-for- 23?.3- 246O E.OH. Fig. 2. Graphic log of D.D.H. 3, Weld Range. Minerals are listed in order of approximate decreasing relative abundance. Most contacts shown are gradational. Direction of the stratigraphic top is not known. Estimated thickness of the main banded iron-formation is 70-90m. mation contains well-preserved, arrested reaction textures showing a texturally early minslal assem- blage overprinted by several generations of later minerals. The minerals in the banded iron-forma- tion, in approxirnatedecreasing order of abundance, are minnesotaite,siderite, qtJartz,magnetite, green- alite, stilpnomelane, pyrite, and chamosite. Trace amounts of pyrrhotite, arsenopyrite, chalcopyrite, and apatite are widely distributed, and rockbridgeite, first reported by Jones (1963), occurs in trace amounts in two samples.Mineral proportions, how- ever, vary widely and over large sectionsof the core someof the mineralslisted aboveare absent(Fig. 2). Fig. l. Location and geological sketch map ofthe central part Much of the banded iron-formation is mesobanded, of the Weld Range, Western Australia (after Jones, 1963). and very different mineral assemblagescommonly t0 GOLE: BANDED IRON.FORMATIONS occur in adjacent mesobands. Within the banded Analytical methods iron-formation. 2-30cm-thick bands of massive to Bulk chemicalanalyses were obtained by XRF ex- laminated Fe-rich shalesoccur, having both mineral- cept for NarO (AAS), FeO (titration against potas- ogically sharp and gradational contacts with the sium dichromate), HrO and CO' (gravimetry), and S bandediron-formation. The mineralsin the Fe-shale, (Leco furnace). Electron microprobe analyseswere in approximate decreasingorder of abundance,are performed using an automated,3-spectrometer ARL- chamosite,stilpnomelane, siderite, greenalite, pyrite, snruq electronprobe microanalyzer.For silicatesand magnetite,minnesotaite, and quartz. Trace amounts carbonates.a seriesof natural mineralswere usedfor of ilmenite, chalcopyrite, and apatite are also present. prinary and secondary standards and the data re- This study is the first detailed description of the duced by the method of Benceand Albee (1968),us- mineralogy and petrology of very-low-metamorphic ing the alpha factors of A. A. Chodos(unpublished). grade banded iron-formation of the Archaean. It will Estimatesof HrO for silicatesand CO, for carbonates be shown that the mineral assemblages,the mineral were made for purposesof the reduction procedure. compositions, and many of the textural relations in Sulphide probe analyseswere performed using pyrite the banded iron-formation at Weld Range are essen- and gallium arsenide as standards and Magic IV tially identical to thoseof the well-studiedmajor Pro- (Colby, l97l) for data reduction. terozoic iron-formations. Electron microprobe analyses with the same sample number refer to a single mesobandwithin the sample.Samples are housedin the Rock Store of the Department of Geology, University of WesternAus- bulk samples of banded iron- Table L Chemical analyses of tralia. formation and Fe-shale from D.D.H. 3, Weld Range Bulk chemicalanalyses Bulk chemical analysesof banded iron-formation sio2 49.76 14.52 2L.20 21.05 and Fe-shale (Table l, anal. I to 3, and 4, respec- Tio 0.00 o.27 2 0.01- 0.r2 tively) illustrate three features that reflect important At2o3 0 .26 0.54 0.08 6.26 aspectsof their mineralogy. (a) The CaO content, Pe203 I'7.45 r.83 16.r-5 I7.09 even of the most COr-rich sample, is negligible, FeO 24.65 44,92 34.82 which indicatesthat the carbonateis siderite.(b) The ho 0 .17 r.86 1.43 0.92 relatively low Fe'*/Fe2* ratios reflect the presenceof Mgo 2 .50 2.!3 7.91 5.47 abundant sideriteand Fe'*-silicatesand the generally CaO 0.31 0 .55 0 .69 0.09 low proportion of magnetite.No hematitewas identi- Na2O 0.03 0.02 0.04 0.03 fied as part of the unaltered,low-metamorphic grade Kzo 0.02 0.20 0.0r 0.33 assemblages,and all hematite appearsto be due to Hzo 0.07 1.95 6.00 weathering.(c) Both the banded iron-formation and 0.I7 0.95 "2" 0.06 the Fe-shalebands are relatively S-rich.
Load more

Recommended publications
  • Devonian and Carboniferous Metamorphism in West-Central
    American Mineralogist, Volume 73, pages20-47, 1988 Devonian and Carboniferousmetamorphism in west-centralMaine: The muscovite-almandinegeobarometer and the staurolite problem revisited M. J. Hololwlv Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275, U.S.A. B. L. Durnow Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. R. W. HrNroN* The EnricoFermi Institute, The Universityof Chicago,Chicago, Illinois 60637,U.S.A. ABSTRAcT Important thermal metamorphic eventsin west-centralMaine occurred at 400 Ma (Mr), 394-379Ma(Mr), and325 Ma(Mr). EachiscloselyassociatedwithemplacementofS-type granites,such that the isogradpatterns produced in the surrounding pelitic schistsgenerally follow plutonic outlines. From north to south, gradeof metamorphism varies from chlorite to sillimanite-K-feldspar-muscovite. Mineral-chemistry studies on M, and M, indicate the following: (l) Much staurolite- grade chlorite is retrograde as demonstrated by the lack of a consistent relation between biotite composition and presenceor absenceofchlorite, given the restricted range ofX"ro allowed in these reduced graphitic rocks. Consequently,the first sillimanite-forming re- action for many pelitic schists in Maine does not involve chlorite. (2) Garnet zoning patterns are prograde in rocks of the staurolite zone and retrograde in rocks of higher grade. (3) Presenceof graphite and nearly pure ilmenite suggestslow Fe3* in micas and allows for end-membercalculations that include a "Ti biotite" (KTiD(Fe,Mg)AlSi3Oro(OH)r) and a "Ti muscovite" (KTi(Fe,Mg)AlSi3O,o(OH)r).(4) Staurolire contains about 3 H (48- oxygen basis) and shows subtle indications of nonideality in Fe-Mg Ko relationships. Garnet-biotite geothermometryindicates the following averagetemperatures for the first occurrenceof minerals:staurolite-510'C, sillimanite-580 "C, sillimanite-K-feldspar- 660 "C.
    [Show full text]
  • Modeling the Shape of Ions in Pyrite-Type Crystals
    Crystals 2014, 4, 390-403; doi:10.3390/cryst4030390 OPEN ACCESS crystals ISSN 2073-4352 www.mdpi.com/journal/crystals Article Modeling the Shape of Ions in Pyrite-Type Crystals Mario Birkholz IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany; E-Mail: [email protected]; Tel.: +49-335-56250 Received: 13 April 2014; in revised form: 22 August 2014 / Accepted: 26 August 2014 / Published: 3 September 2014 Abstract: The geometrical shape of ions in crystals and the concept of ionic radii are re-considered. The re-investigation is motivated by the fact that a spherical modelling is justified for p valence shell ions on cubic lattice sites only. For the majority of point groups, however, the ionic radius must be assumed to be an anisotropic quantity. An appropriate modelling of p valence ions then has to be performed by ellipsoids. The approach is tested for pyrite-structured dichalcogenides MX2, with chalcogen ions X = O, S, Se and Te. The latter are found to exhibit the shape of ellipsoids being compressed along the <111> symmetry axes, with two radii r|| and describing their spatial extension. Based on this ansatz, accurate interatomic M–X distances can be derived and a consistent geometrical model emerges for pyrite-structured compounds. Remarkably, the volumes of chalcogen ions are found to vary only little in different MX2 compounds, suggesting the ionic volume rather than the ionic radius to behave as a crystal-chemical constant. Keywords: ionic radius; ionic shape; bonding distance; ionic volume; pyrite-type compounds; di-chalcogenides; di-oxides; di-sulfides; di-selenides; di-tellurides 1.
    [Show full text]
  • Pyrite Roasting, an Alternative to Sulphur Burning
    The Southern African Institute of Mining and Metallurgy Sulphur and Sulphuric Acid Conference 2009 M Runkel and P Sturm PYRITE ROASTING, AN ALTERNATIVE TO SULPHUR BURNING M Runkel and P Sturm Outotec GmbH, Oberursel GERMANY Abstract The roasting of sulphide ores and concentrates is often the first step in the production of metals or chemicals. In many processes, the production of sulphuric acid is viewed as a by-product, while in some plants production is an important economic factor. Regardless of the purpose, a pyrite roasting plant consists of mainly three plant sections: roasting, gas cleaning and sulphuric acid. With the addition of air, the pyrite concentrates are transformed into solid oxides and gaseous sulphur dioxide at temperatures of 600 - 1000° C. After cleaning and cooling, the sulphur dioxide in the roasting gas is further processed to sulphuric acid. Two types of reactors are used depending on the application: stationary or circulating fluid bed . For over 60 years, Outotec has progressively been developing the principle of fluidised bed technology in several different reactor types for a multitude of process applications. The versatility of the fluidised bed reactor system has manifested itself in the treatment of minerals, including solid fuels, and for metallurgical processes both in the ferrous and non-ferrous fields. Process applications have included roasting, calcining, combustion and charring of coals, as well as off-gas treatment. This paper provides a summary of the pyrite roasting technology currently used along with a simple cost comparison of pyrite roasting and sulphur burning processes. Introduction Pyrite roasting and sulphur burning plants are built for the production of sulphuric acid.
    [Show full text]
  • Ultrafast Band-Gap Oscillations in Iron Pyrite
    Ultrafast band-gap oscillations in iron pyrite The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Kolb, Brian, and Alexie Kolpak. “Ultrafast Band-Gap Oscillations in Iron Pyrite.” Phys. Rev. B 88, no. 23 (December 2013). © 2013 American Physical Society As Published http://dx.doi.org/10.1103/PhysRevB.88.235208 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/88761 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. PHYSICAL REVIEW B 88, 235208 (2013) Ultrafast band-gap oscillations in iron pyrite Brian Kolb and Alexie M. Kolpak Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA (Received 7 August 2013; revised manuscript received 17 October 2013; published 20 December 2013) With its combination of favorable band gap, high absorption coefficient, material abundance, and low cost, iron pyrite, FeS2, has received a great deal of attention over the past decades as a promising material for photovoltaic applications such as solar cells and photoelectrochemical cells. Devices made from pyrite, however, exhibit open circuit voltages significantly lower than predicted, and despite a recent resurgence of interest in the material, there currently exists no widely accepted explanation for this disappointing behavior. In this paper, we show that phonons, which have been largely overlooked in previous efforts, may play a significant role. Using fully self-consistent GW calculations, we demonstrate that a phonon mode related to the oscillation of the sulfur-sulfur bond distance in the pyrite structure is strongly coupled to the energy of the conduction-band minimum, leading to an ultrafast (≈100 fs) oscillation in the band gap.
    [Show full text]
  • The Electrical Resistivity of Galena, Pyrite, and Chalcopyrite
    American Mineralogist, Volume61, pages248-259, 1976 The electricalresistivity of galena,pyrite, and chalcopyrite Doneln F. PnlorrronreNn RnlpH T. Suurv Departmentof Geologyand Geophysics,Uniuersity of Utah Salt Lake Cily, Utah 84112 Abstract. The sulfidesgalena, chalcopyrite, and pyrite are semiconductorswhose electrical resistivity and type are controlled by deviationsfrom stoichiometryand impurity content,and henceby their geochemicalenvironment. We measuredelectrical resistivity,type, and the impurity content (emissionspectrograph and microprobe) on small volumesof sample.Our results, together with those obtained from a comprehensiveliterature analysis, are usedto construct histogramsof the natural variability in carrier density and resistivity. Sulfur deficiency is the dominant defect in chalcopyrite and hence almost all natural samplesare n-type. lt appearsthat the copper/iron ratio is also important electrically,the copper-richsamples being the more resistive. lmportant donor deiectsin galena(z-type samples)are antimony and bismuth impurities, and sulfur vacancies;acceptor defects(p-type samples) include silver impurities and lead 'Mississippi vacancies.P-type samplesappear to be restrictedto Valley' and argentiferous deposits. In pyrite, electricallyactive impurities include cobalt, nickel, and copper as donors, and arsenicas an acceptor.Deviations from stoichiometry,in the same senseas galena,may be important. Pyritesfrom sedimentaryand epithermaldeposits are usuallyp-type if cupriferous sulfidesare not present.Samples from hypothermaldeposits
    [Show full text]
  • Banded Iron Formations
    Banded Iron Formations Cover Slide 1 What are Banded Iron Formations (BIFs)? • Large sedimentary structures Kalmina gorge banded iron (Gypsy Denise 2013, Creative Commons) BIFs were deposited in shallow marine troughs or basins. Deposits are tens of km long, several km wide and 150 – 600 m thick. Photo is of Kalmina gorge in the Pilbara (Karijini National Park, Hamersley Ranges) 2 What are Banded Iron Formations (BIFs)? • Large sedimentary structures • Bands of iron rich and iron poor rock Iron rich bands: hematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3) or pyrite (FeS2). Iron poor bands: chert (fine‐grained quartz) and low iron oxide levels Rock sample from a BIF (Woudloper 2009, Creative Commons 1.0) Iron rich bands are composed of hematitie (Fe2O3), magnetite (Fe3O4), siderite (FeCO3) or pyrite (FeS2). The iron poor bands contain chert (fine‐grained quartz) with lesser amounts of iron oxide. 3 What are Banded Iron Formations (BIFs)? • Large sedimentary structures • Bands of iron rich and iron poor rock • Archaean and Proterozoic in age BIF formation through time (KG Budge 2020, public domain) BIFs were deposited for 2 billion years during the Archaean and Proterozoic. There was another short time of deposition during a Snowball Earth event. 4 Why are BIFs important? • Iron ore exports are Australia’s top earner, worth $61 billion in 2017‐2018 • Iron ore comes from enriched BIF deposits Rio Tinto iron ore shiploader in the Pilbara (C Hargrave, CSIRO Science Image) Australia is consistently the leading iron ore exporter in the world. We have large deposits where the iron‐poor chert bands have been leached away, leaving 40%‐60% iron.
    [Show full text]
  • A Sulfur Isotope Study of Pyrite Genesis: the Mid-Proterozoic Newland Formation, Belt Supergroup, Montana
    A sulfur isotope study of pyrite genesis: The Mid-Proterozoic Newland Formation, Belt Supergroup, Montana HARALD STRAUSS' and JORGEN SCHIEBER 2 'Ruhr-Universitat Bochum, Institut fur Geologie, Postfach 102148, 4630 Bochum 1, FRG 2University of Texas at Arlington, Department of Geology, Arlington, TX 76019, USA Abstract-Different generations of sedimentary pyrite from the Mid-Proterozoic Newland Formation, USA, have been analysed for their sulfur isotopic compositions. The results indicate bacterial sulfate reduction as the pyrite forming process. The V'S values for early diagenetic pyrite, around -14%o, are in contrast to dominantly more positive values for many other Middle Proterozoic units. A progressive reduction of sulfate availability during diagenesis can be recognized by an increase in 34S content (Rayleigh Distillation) as well as through detailed petrographic observations. Contemporaneous seawater had a sulfur isotopic ratio between +14 and +18%o as measured from sedimentary barite within the unit. INTRODUCTION deposited between 1450 and 850 Ma ago (HARRISON, 1972), and the Newland Formation accumulated early in basin history. OBRADOVICH and PETERMAN THE MID-PROTEROZOIC Newland Formation (Belt Supergroup) contains (1968) defined a 1325 Ma Rb-Sr isochron in Belt rocks (including the Newland horizons of pyritic shale. These and surrounding sediments have been Formation) of the Big Belt and Little Belt Mountains. Because the whole Lower investigated in detail by SCHIEBER (1985) and a continuous spectrum of pyrite Beltian sequence shows the 1325 Ma age, it is likely that this age represents the types from early diagenetic fine crystalline to late diagenetic eudedral and approximate time of diagenetic smectite-illite trans concretionary pyrite has been distinguished.
    [Show full text]
  • Eg9601814 Supergene Alteration of Magnetite And
    EG9601814 SUPERGENE ALTERATION OF MAGNETITE AND PYRITE AND THE ROLE OF THEIR ALTERATION PRODUCTS IN THE FIXATION OF URANIUM FROM THE CIRCULATING MEDIA. BY MA EL GEMMIZI Nuclear Materials A uthority Cairo- Egypt. ABSTRACT. In most of the Egyptian altered radioactive granites, highly magnetic heavy particles were found to be radioactive. They are a mixture of several iron oxide minerals which are products of supergene alteration of the pre-existing hypogene iron-bearing minerals especially magnetite and pyrite. The end products of this supergene alteration are mainly hydrated iron oxide minerals limonite hematite and geothite. During the alteration, deformation and defects in the minerals structure took place , thereby promoting diffusion of the substitutional and interstitial ions (uranium) to words these sites The mechanism of the alteration of the hypogene iron-bearing minerals; magnetite and pyrite to form the secondary minerals hematite, limonite and geothite; the role of these minerals in fixing uranium from the ciculating media as well as the applicability of these minerals as indicators to the radioactivity of the host rocks were discussed. INTRODUCTION It is quite common that in all the altered rocks magnetite and to some extent pyrite which are present as accessories are suffering from some degrees of alteration. Magnetite and pyrite in the altered granite of Wadi Nugrus, El Missikat and £1 Aradyia, Eastern Desert was found by (1 & 2) to possess several degrees of alteration and crystal deformation. This means that both minerals are sensitive to postdepositional environmental changes. The granites of the present study were described as altered granites overlained by metasediments and underlained by magmatic bodies which invaded the granites itself by sills.
    [Show full text]
  • Pyrite's Evil Twin Marcasite and Pyrite Are Two Common Minerals. Both Fes2 Chemically, Making Them Polymorphs. Polym
    Marcasite - Pyrite's Evil Twin Marcasite and pyrite are two common minerals. Both FeS2 chemically, making them polymorphs. Polymorphs are minerals with the same chemical composition but different crystal structures. Diamond and graphite are polymorphs, both minerals being pure carbon. In diamond and graphite the different arrangement of carbon atoms gives these two minerals of very different physical properties. Pyrite and marcasite, on the other hand, have almost identical physical properties, making them tough to tell from each other. Let's go through their properties. Both are metallic and pale yellow to brassy yellow. Both can tarnish and be iridescent. Both are 6-6.5 on the Mohs' hardness scale. Neither have a particularly prominent cleavage, although marcasite does have one that occasionally shows up. Both have densities of about 5 grams per cubic centimeter (pyrite is a bit denser, but not enough to be detectible without delicate measurements). They can even be found together in the same rock. Fortunately these minerals often show good outer crystal shapes that are quite different. Pyrite crystals are generally equant, and dominated by cubes, octahedrons and 12-sided pyritohedrons. Marcasite crystals are usually rectangular (tabular) with wedge-shaped ends and tend to form in star shaped, radiating or cockscomb groups. Marcasite is also much more restricted in occurrence than pyrite, forming only in low temperature, near surface, very acidic environments. It is found in some ore deposits, in sediments formed under somewhat stagnant conditions and as ground water precipitates in rocks such as in limestone and shale. Although pyrite can also be found in many of these same environments, the crystal shapes are diagnostic.
    [Show full text]
  • Miissbauer Spectra of Minnesotaite and Ferroan Talc J.M.D. Corv, T
    American Mineralogist, Volume 76, pages 1905-1909, 1991 Miissbauer spectra of minnesotaite and ferroan talc J.M.D. Corv, T. Blxls* Department of Pure and Applied Physics,Trinity College,Dublin 2, Ireland S. GuccnNnprnr Department of Geological Sciences,University of Illinois at Chicago, Chicago,Illinois 60680, U.S.A, Arsrru.cr Mdssbauer spectra of five samples of minnesotaite and one of ferroan talc have been recordedat 296 K and 4.2 K. A singleferrous doublet with isomer shift 6 : 1.13 mm/s relative to aFe and quadrupolesplitting L:2J4 mm/s is observedin all minnesotaite samples at room temperature. The ferroan talc has a smaller quadrupole splitting, A : 2.55 mm/s. All minnesotaite samplesorder antiferromagnetically,with N6el points in the tange 25-37 K, but ferroan talc does not order magnetically down to 4.2 K. Two distinct ferrous populations in minnesotaite are inferred from the resolved magneticallysplit spec- tra at 4.2 K. The minority sites may be those coordinated by the excessOH in the min- nesotaitestructure. The P-cell and C-cell variants have distinctly different hyperfine fields (10.4 and 14.9T for the two sitesin the P cell but 8.9 and 14.4T in the C cell),although their N6el points are similar. INrnooucrroN taite, ferroan talc, and talc and found a lack of compo- Minnesotaite was originally considered by Gruner sitions having the octahedralFe fraction in the range0.45- (1944) to be an Fe analogueof talc having ideal formula 0.60, perhapsindicating the chemical limits of the struc- tures. {Fel*}[Sio]O,o(OH),,where { } and [ ] denotethe oc- tahedral and tetrahedral sites, respectively.More recent- Both minnesotaite and talc have a continuous octahe- ly, X-ray and electron diffraction studies (Guggenheim dral sheet(Fe in minnesotaite, Fe,Mg in ferroan talc) but and Bailey, 1982; Guggenheim and Eggleton, 1986) differ in the nature ofthe tetrahedral sheets.It is therefore showed that the structure is considerablymore complex.
    [Show full text]
  • Greenalite, Mg-Rich Minnesotaite and Stilpnomelane from the Osj6berg and Sirsj6berg Iron-Ore Mines, Hjulsj6, W
    MINERALOGICAL MAGAZINE, SEPTEMBER 1985, VOL. 49, PP. 611 613 Greenalite, Mg-rich minnesotaite and stilpnomelane from the Osj6berg and Sirsj6berg iron-ore mines, Hjulsj6, W. Bergslagen, Sweden GREENALITE, Mg-rich minnesotaite, and stilpno- subvertical body which strikes approximately E-W melane have been identified by electron micro- and is considered by Baker and de Groot (1983b) to probe analysis in samples taken from dumps of the be continuation of the Osj6berg ore horizon. The abandoned Qsj6berg and Sirsj6berg mines 5 km ore is separated from an underlying limestone NW of Hjulsj6, Bergslagen. This is the first horizon by a garnet-pyroxene-amphibole skarn. report of the occurrence of these minerals in the Magnetite is present in the skarn and the limestone. Proterozoic iron ores of Central Sweden. The two Epidote and calcite are later minerals filling tectonic mines, some 700 m apart, worked a large, 7 km long, fissures and cracks. meta-rhyotite-hosted, magnetite ore horizon--one Greenalite occurs in a sample from ()sj6berg of the many concordant skarn iron ores in the mine which comprises a fine grained aggregate of 1.8-1.9 Ga Proterozoic Bergslagen Supracrustal magnetite grains (+0.05 mm) with some larger Series (Oen et al., 1982). They were amongst the grains up to 0.5 mm in a matrix composed pre- largest iron producers of this area. dominantly of talc with minor green chlorite. Talc The subsurface geology of the mines has been also fills fractures in the larger magnetite grains. described by Geijer and Magnusson (1944); earlier Very minor pyrite and rare chalcopyrite are seen descriptions are given by Blomberg (1879) and enclosing magnetite.
    [Show full text]
  • Data of Geochemistry
    Data of Geochemistry ' * Chapter W. Chemistry of the Iron-rich Sedimentary Rocks GEOLOGICAL SURVEY PROFESSIONAL PAPER 440-W Data of Geochemistry MICHAEL FLEISCHER, Technical Editor Chapter W. Chemistry of the Iron-rich Sedimentary Rocks By HAROLD L. JAMES GEOLOGICAL SURVEY PROFESSIONAL PAPER 440-W Chemical composition and occurrence of iron-bearing minerals of sedimentary rocks, and composition, distribution, and geochemistry of ironstones and iron-formations UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 45 cents (paper cover) CONTENTS Page Face Abstract. _ _______________________________ Wl Chemistry of iron-rich rocks, etc. Continued Introduction. _________ ___________________ 1 Oxide facies Continued Iron minerals of sedimentary rocks __ ______ 2 Hematitic iron-formation of Precambrian age__ W18 Iron oxides __ _______________________ 2 Magnetite-rich rocks of Mesozoic and Paleozoic Goethite (a-FeO (OH) ) and limonite _ 2 age___________-__-._____________ 19 Lepidocrocite (y-FeO(OH) )________ 3 Magnetite-rich iron-formation of Precambrian Hematite (a-Fe2O3) _ _ _ __ ___. _ _ 3 age._____-__---____--_---_-------------_ 21 Maghemite (7-Fe203) __ __________ 3 Silicate facies_________________________________ 21 Magnetite (Fe3O4) ________ _ ___ 3 Chamositic ironstone____--_-_-__----_-_---_- 21 3 Silicate iron-formation of Precambrian age_____ 22 Iron silicates 4 Glauconitic rocks__-_-____--------__-------- 23 4 Carbonate facies______-_-_-___-------_---------- 23 Greenalite. ________________________________ 6 Sideritic rocks of post-Precambrian age._______ 24 Glauconite____ _____________________________ 6 Sideritic iron-formation of Precambrian age____ 24 Chlorite (excluding chamosite) _______________ 7 Sulfide facies___________________________ 25 Minnesotaite.
    [Show full text]