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Nontronite Is a Smectire Group Clay Mineral Which. Ff,T'lffi'"Fff"#:J""

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Nontronite Is a Smectire Group Clay Mineral Which. Ff,T'lffi' The American Mineralogist, Yolume60, pages 840448, 1975 Primary NontroniteFrom the VenezuelanGuayana W. C. ISpHoRpllrC Department of Geology and Geography, Uniuersity of South Alabama Mobile, Alabama 36688 Abstract Nontronite, the iron-rich end member of SmectiteGroup clays, has previouslybeen reported as a hydrothermalalteration product, as a constituentof soil clays formed by weatheringof a variety of rock types,and as an alterationproduct of volcanicrocks. This paper describesthe chemical,optical, and X-ray propertiesof a new occurrenceof the mineralwhere its origin is apparentlyrelated to directcrystallization from high temperature waters.The mineral is further unique in that it containsonly a very small percentageof aluminaand therebyclosely approaches the previouslyunreported wholly iron end-member. Significantdifferences were found in the optical propertiesof the Venezuelannontronit€ when comparedwith those of an earlier study, especiallywith regard to the variation in refractiveindex with Fe2Oscontent. It now appearsthat this variationhas been over-simpli- fied and may not be as orderlyas was previouslybelieved. The variouslines of evidencethat argue againstan origin by either hydrothermalalteration or chemicalweathering are dis- cussed,as well as thosesupporting an origin by direct crystallizationfrom high temperature watersof probablemagmatic or metamorphicorigin. lntroduction product in gneissesand slates.At the other sites,non- Nontroniteisa Smectire Group clay mineral which. ff,T'Lffi'"fff"#:j""1|# ,XXT::""."t1:j'# occurs as the iron-rich end-memberin the d.ioc- rr-;i",;-be an arterationproduct. At the type tahedral sub-group montmorillonite-beidellite- io.uri,v,Nontron, France, Berthier (1g27) descri6ed nontroniteseries' It haspreviously been *p":l:9^:t^1 ,tr" ri".r"r-: as occurringas "onion-like" massesas- constituentof sedimentaryrocks, as the productof -:-,--; -,,, alterationorvolcanic grasses andas a hydrother.mal alterationproduct. This paper describes the chemical, il:Tfl:iil;ilt"1T'ifi::"":'."!'j#fiit"fjo.ll: ;d; ;;ntronit. in alteredbasalts of the Columbia optical, and X-ray propertiesof a specimenfrom iii;; .;gi;" and attributed it to the weatheringof recently discovered new occurrencein southern" L"r"r,i. Ei"rs, palagonite,iddingsite, and augiteun- venezuela'Here the nontronite is apparently.un- orr.""olii"ns of poor drainage.similarly, Sherman related to weathering or alteration of previous ii describednontronite in weatheredbasalts minerals and has formed by crystallizationfrom "i(lgZal from Australia,New Zealand,Fiji, and waters of hydrothermal or possibly metamorphit H";;i;;Jre"na "na"ri,"s it wasreported to haveformed under a origin.The chemistryof the mineralfrom thisloca- varlety.. _,^:-.oI^;.,. cllmatlcconqluons' oy cnemlcalweatner- tion is also unusualin that it contains essentiallvno rng' alumina and approachesthe compositionoi ttr" previouslyunreportedtheoreticalendmember. venezuelanoccurrence The mineral describedin this paper was found 2 Geologicaloccurrence km southof the OrinocoRiver on FederalHighway Rossand Hendricks(1945) in their detailedtreat- 19, l5 kilometersnorth of the turnoffto Guri Dam. ment of montmorilloniteclays listed l3 locations Hereit occurredas an intrusionwithin granitic rocks wherenontronite had previouslybeen described. Of of the PrecambrianImataca Complex (see map, Fig. these,7 werein the United States,3 in the Soviet l). Themain intrusion, where exposed in a newroad- Union,2 in Germany,and I in Hungary.At 8 of the cut, is approximately2 metersthick and 372meters l3 locations, the mineral was associatedwith high, Numeroussmaller veinlets could be seenex- metamorphicrocks and occurred as an alteration tending outward into the surroundinggranites. The 840 PRIMARY NONTRONITE FROM VENEZUELA 841 dioctahedraliron end-memberof the beidellite- nontroniteseries was given as: (Si?.ssAlo.6zXFeo)Orr(OH)a but analysescorresponding to this chemicalformula had not been reported. The type mineral from Nontron,France, was included with analysesfrom an additionall3 samplesthat werejudged to containan "iron-bearing"impurity, based on the lact that the chemicalanalyses showed these to containmore iron thancould be accommodatedin the octahedralsites. 36pure" The 13 samplesare shownin TableI along with one analysisreported by Kerr and Pill (1950); sample115 is the Venezuelannontronite described in this paper.Analysis ll6 is the "type" mineralfrom Nontron, France,and is includedfor comparative purposes. The most striking difference between the Venezuelannontronite and the otherslisted in Table I is that of aluminacontent. The other published analysesshow a rangein AlzO, from 2.94to 17.65 percent,with a mean value of 9.55 percent.The Venezuelannontronite. with an aluminacontent of Ftc. t. Map showing Venezuelan nontronite locality and outcrop only 0.27 percent, contains one-tenththat of the of Precambrian and younger rocks in study area. lowestpreviously reported analysis. The significance of this low aluminacontent has bearing not only on the existenceof a completesolid solution series nontronitewas a uniform,yellow-green (l0Y 6/6), ex- tendingfrom beidellite,(Si? densemass containingoccasional thin (2-5 mm) ,'Alo.6?XAln)Oro(OH)n,to nontronite,but on theorigin of thisunique deposit stringersof specularhematite. Though not chemical- as well. ly analyzed,a similar, but smaller,exposure of the The existenceof a completesolid solutionseries samematerial was also seen 25 km to the southeastin in- cluding montmorilloniteand beidellitewas first the open pit iron mine of the BethlehemSteel Cor- proposedby (1877) porationat El Pao. Collins andhas since been largely confirmedin severallater reports (see Gruner, 1935; Laboratory Analysis Rossand Hendricks,1945). The existenceof a solid Chemistry solution seriesextending from beidelliteto the iron- Ross and Hendricks(1945) listed 13 reported rich end member,nontronite, is a later development analysesof samplesconsidered to representrelatively and wasproposed by Rossand Shannon(1925) and purenontronite (those containing down to two atoms Larsenand Steiger(1928) and, similarly,Ross and of iron per unit cell layer).The compositionof the Hendricksdid much to verify its existence.Thev dis- Tnsle l. Chemical Analyses (in percent) of Nontronitel Nwber: #I #2 #3 #lo #t1 *11 49,75 43.51 49.50 46.06 40.72 40,48 +o.c) 19.54 42.74 40.42 41.t8 41.88 40.O2 40.54 49.90 4A.A2 10.06 2,94 ).6.1A 12.22 4.96 6.oo Lt,o) 6.o4 9.76 ro.42 9.84 11.90 rO.55 5.r9 o.27 4.to 22.06 29,61 11.08 18.82 10.24 29.59 r4.39 10.32 26.7t 25.76 27.47 26.2o 27.53 31.61 o.25 0.05 0.61 ).,62 o.74 0. 1 tr tr 1.06 O.It tr 0.10 0.06 1 ?C n i6 2.6e 2.22 2.57 r.66 r.98 2.90 r,21 o.O4 \.94 o.3a tr 0.67 I.92 o.77 o.55 O.32 tr O.5? O.zt+ o.01 o.r5 u,z+ trr v,)z u.I4 a.L2 22.27 17.16 21.80 2r.35 l].45 2t,81 20,75 16.12 9 .66 t pilt Samples 1-13 from Rossand Hendicks (1945); Sample t4 from Kerr and (1950); Sample 15 lromVenezuelanlocality; Sample 16 from Nonton, France. W. C. ISPHORDING cussed,at length,the thenavailable information on probably discounted these, believing that the higher thechemical variation among nontronites, but admit- MgO percentages(1.45 percent averageversus 0.37 ted that wider variationmight exist.Evidence was percent in the uncontaminated samples)could be at- present,they believed,to supporttheir contention tributed to MgO in mixed-layerclays. Basedon the that a solid-solutionseries existed with limited-to- samples available for their analysis, they cannot be extensivesubstitution of aluminumfor siliconin the faulted for this logic. However, the Venezuelannon- tetrahedralsites, and of iron for aluminumin the oc- tronite does not support their conclusion. Though tahedralsites. The chemicalanalyses carried out on containing significantly lower octahedral and the Venezuelannontronite confirm this belief.Thus, tetrahedralalumina than any of the samplesused by unlikethe wholly iron end-membersof both the en- Ross and Hendricks, the MgO content is nearly five statite-ferrosiliteand phlogopite-anniteseries, the times higher than that found in their analyses.X-ray iron-rich end-memberof the beidellite-nontronitediffraction studies on the Venezuelan material failed seriesis stableand canform duringnatural geologic to disclose any reflections from mixed-layer clays; processes.The extremelylow aluminumsubstitution henceit is concluded that the magnesium is presentin in the octahedraland tetrahedralsites results in an octahedral coordination in the nontronite and that oxide chemicalformula for this mineral ol the correlation of low alumina content with low magnesiumwas more apparent than real. 4FerOr.MgO.lgsior.2lHrO (Alros),CaO, NazO or, in termsof octahedral-tetrahedralsite occupancy: Optical Analysis With regard to the optical properties of mont- Si? Fer.urrMgo.nr2Alo.oo6020(OH)1 erAlo.o6 morillonites, Ross and Hendricks (19a5)noted that The very limited substitution of aluminum for these can be lessdefinitely determined than those for, silicon creates only a slightly unbalanced charge in possibly,any other mineral group. This resultsfrom a the tetrahedral layer, Hence the presence of small number of factors, among which are: (l) the lack of amounts of sodium (0.02Vo)and calcium (0.77Vo)in observablecrystals, (2) the tendency of theseminerals the interlayer sites is largely due to excess charges to absorb immersion oils and, hence, undergo a arising in the octahedral layer caused by the undeter- change in refractiveindex, (3) the stacking order of mined amount of iron presentas
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