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Crystalline Solution Series and Order-Disorder Within the Natrolite

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Crystalline Solution Series and Order-Disorder Within the Natrolite American Mineralogist, Volume 77, pages 685-703, 1992 Crystalline solution series and order-disorderwithin the natrolite mineral group* Mar,cor,vr Ross, Mlnu J. K. Fr-onn U.S. Geological Survey, MS 959, Reston, Virginia 22092,U.5.4. DaprrNr R. Ross Smithsonian Institution, Department of Mineral Sciences,Washington, DC 20560, U.S.A. Ansrru.cr The zeolite minerals of the natrolite g.roup(natrolite, tetranatrolite, paranatrolite, meso- lite, scolecite,thomsonite, gonnardite, edingtonite, and tetraedingtonite) have the general formula (Na,Ca,Ba)r-,6(Al,Si)ooO8..nHrO.Electron microprobe and X-ray analyseswere made of natrolite, tetranatrolite, gonnardite, and thomsonite from the Magnet Cove al- kaline igneous complex, Arkansas, and of selected specimens from the U.S. National Museum. This information and data from the literature indicate that natrolite, mesolite, scolecite,edingtonite, and tetraedingtonits show only small deviations from the ideal stoi- chiometry. In contrast, gonnardite, tetranatrolite, and thomsonite show large deviations from the ideal end-member compositions and composethree crystalline series.These are (l) the gonnardite seriesgiven by E"Na,u where x varies from ,,Car,Al,u*"Siro "O"o.nHrO, approximately 0.3 to 3.2 and tr denotesa cation vacancy in the intraframework channels, (2) the tetranatroliteseries given by EoNa,u where x varies "Ca,Al,u*"Sira-"Oro.l6HrO, from approximately 0.4 to 4, and (3) the thomsonite series given by EoNao*,Ca, ,A120-,Si20*"O8o.24H2O, where:c varies from approximately 0 to 2. The struc- tures of the natrolite minerals are composed of chains of AlOo and SiOo tetrahedra that link in one of three ways to form the natrolite-, thomsonite-, or edingtonite-type frame- works. The intraframework channels are occupied by NaOo(HrO)2, (Na,Ca)Oo(HrO)r, CaOo(H,O)r, CaOo(HrO)r, (Na,Ca)Oo(HrO)0,or BaOu(H,O)opolyhedra. The Na-bearing polyhedra are edge linked to form continuous chains, whereasthe Ca and Ba polyhedra are isolated from one another by channelvacancies. The structuresof the natrolite minerals are defined by combining each of the three types of framework structures with various combinations of channel-occupyingpolyhedra. Various polysomatic series can be con- structed by combining slices of two basic structures to form new hybrid structures. The compositional variation and complex crystallography of gonnardite can be explained if the gonnardite seriesis a pol'ysomaticseries composed of mixed domains of natrolite and thomsonite. A paranatrolite series (troNa,._"Ca*A1,6*"Si24_,O8o.24HrO)is predicted, and it is proposed that the structure of paranatrolite combines the natrolite framework with channel-filling NaOo(HrO), and (Na,Ca)Oo(HrO)opolyhedra. ft.mnonucrroN recrystallization processesoccurring during emplacement The minerals of the natrolite group are commonly found and cooling ofthe igneous rocks and to relate these pro- as later-stagecrystallization products within cavities in cessesto mineralization of the adjacent country rocks. basaltic rocks, in various hydrothermal deposits, and as Some of the replacementor overgrowth textures noted in alteration products of nepheline in nepheline syenites, the more mafic syenitesare sodic pyroxenesand amphi- phonolites, and related rocks. The latter type of occur- boles replacingdiopside, augite,and hedenberyite;F- and rence was noted in our studies of various syenitesfrom OH-rich grandite seriesgarnets overgrowing schorlomite the Magnet Cove alkaline igneous complex located near and melanite; phlogopite and biotite replacing clinopy- Hot Springs,Arkansas (Flohr and Ross, 1989, 1990).Since roxene; and titanite appearing as overgrowths on perov- many of the minerals in theserocks are extensivelymeta- skite. Nepheline is replacedby one or more of the follow- somatized, one of the purposesof our Magnet Cove in- ing minerals during middle-stage magmatic crystallization: vestigations was to characterizethe mineralization and feldspar, cancrinite, sodalite, and nosean. During late- stagecrystallization, natrolite, thomsonite, gonnardite, and tetranatrolite may replace nepheline. * Adapted from the Presidential Address of Malcolm Ross, given at the annual meeting of the Mineralogical Society of The present paper gives new data on the crystallogra- America on October 22, 1991, in San Diego, California. phy and composition of four members of the natrolite 0003-o04x/92l0708-0685$02.00 685 686 ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP mineral group found in the Magnet Cove rocks. To verify TABLE1. Names,chemical formulas, and unit-cellcontent of the the Magnet Cove studies, a reexamination was made of membersof the natrolitemineral group selectedspecimens from the U.S. National Museum rep- Mineralname Chemicalformula Z resentingsix of the nine members of this mineral group. Natrolite Na,6Al,6Sir4Oso.16HrO I The new chemical and crystallographicdata obtained on Tetranatrolite (Na,Ca),6(Al,Si)"oOso16H,O th the museum specimensare integrated with that obtained Paranatrolite (Na,Ca),"(Al,Si)noO6o24H"O*. 1 from the Magnet Cove samplesto give new insights into Scolecite Ca6Al,6Si,4Oe24H,O 1 Mesolite Nau$Cas$Al,6sirooe21.33H"O 3 the nature of the crystalline solution series within the Thomsonite (Na,Ca)1,(Al,Si)4oOe24H,O 1 natrolite mineral group. Gonnardite (Na,Ca)', 15(Al,Si)@Oe'nHrO ? Edingtonite Ba.Al,uSi"oO"o.32H.O V4 1/t ANal,vrtc,ll METHoDS Tetraedingtonite Ba"AlruSiroOuo.32H.O ' Zvalue based on unit-celldata given in Table 2. Light optical and electron microprobe analysesof the ** H,O content was calculated(Chao, 1980). zeolites replacing nepheline were made on polished thin sectionsof various alkaline rocks from the Magnet Cove igneousintrusion and on polished thin sectionsof single crystals of selectednatrolite group minerals obtained from listed in Table l. All formulas are normalized to Si + Al the U.S. National Museum (USNM). Mineral composi- : 40 (80 O atoms) so as to facilitate comparison of the tions were obtained using an automated ARL-SEMQ' mineral species.The numbers of formula units (Z) per electron microprobe. The method of chemical analysisis unit cell are based on the crystallographic data given in given by Flohr and Ross(1989, p. I l4), with the follow- Table 2. For a comprehensivediscussion of the previous ing modifications.To minimize Na loss,we generallyused studies of these minerals, the reader is referred to Got- a defocused,unrastered electron beam 30 pm in diameter tardi and Galli (1985). operating at 15 kV and with a 0.1-pA beam current. alkaline Analysis times were generally for 20 s. The accuracy of Natrolite rninerals from the Magnet Cove the analyseswas verified by reanalysis of feldspar stan- igneouscomplex dards, by comparison with analyses made by rastering In examining the various syenites from the Magnet over a large volume of the crystal, and by using longer Cove igreous complex, the following minerals, in addi- and shorter analysistimes. Our analysesshowed that Na tion to feldspar, were observed to replace nepheline: (l) loss could not be detectedduring the 20-s analyseswhen natrolite and cancrinite in ijolite l-1668, (2) tetranatro- the beam diameter was greater than 15-20 pm. Analysis lite and noseanin jacupirangite 86-53A, (3) analcite, gon- for HrO content in theseminerals was not made; thus the nardite, and thomsonite in jacupirangiteH-45-8 (Fig. lA), oxides sums are l2-l7o/o low, depending on the amount and (4) gonnardite and thomsonite in ijolite 85-l64' (Fig. of HrO in the crystal structure. When the wto/oHrO ex- lB). In nepheline syenite 2-DJ7, natrolite and sodalite pected for natrolite, tetranatrolite, mesolite, scolecite,or replace feldspar. The compositional trends of coexisting thomsonite is added to the oxide sums, totals tend to be gonnardite and thomsonite in ijolite 85-16A and in ja- less than l00o/0.Except for HrO, we do not believe that cupirangite H-45-8 are plotted in Figures 2 ar'd 3, te- any significant amounts of other chemical components spectively. Also plotted in Figure 2 are six analysesof remained undetected. We note that other reported elec- natrolite ruSNM 126671). These figures show the very tron probe analysesof the natrolite group minerals also large chemical variation in the gonnardite series,ranglng show low sums (e.g.,Kile and Modreski, 1988,Table 6; in composition from that very close to end-member Birch, 1989,Table 9; Nawaz, 1988,Tables 2, 3). natrolite (Na,uAl,.SiroO8o'l6HrO)to a composition ap- Some of our samples were examined with the JEOL- proaching that of end-member thomsonite (NaoCa.- 840 scanningelectron microscope.Crystallographic char- Al2oSi20O8o.24HrO). Relative to gonnardite,the thomson- acterization was made of the samples using the optical ite in these rocks shows a more restricted composition microscope, the single-crystal Buerger precessionX-ray range. Selected analyses of gonnardite and thomsonite camera, and the automated Scintag powder X-ray dif- that are plotted in Figure 2 are compiled in Tables 3 and fractometer. The crystal drawings presentedin this paper 4. Analyses of tetranatrolite from jacupirangite 86-534 were made using computer software written by Dowty are given in Table 5. (l e9l). The compositional trends for gonnardite, thomsonite, and tetranatrolite, so clearly delineated in the Magnet CHnnnc,c.L coMposrrroNs oF THE NATRoLTTE Cove rocks, have not been adequately described in the GROUP MINERALS literature (seeFoster, 1965a;Nawaz, 1988).Because of End-member and generalizedchemical formulas of the the poor understanding of
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