Crystalline Solution Series and Order-Disorder Within the Natrolite

Home , Edingtonite, Gonnardite, Mesolite, Natrolite, Nepheline, Scolecite

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, mesolite, 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 alkaline 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 structures 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 frameworks. 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 constructed 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 pyroxenes and 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 the compositional ranges of nine known members of the natrolite mineral group are several of the natrolite minerals, we decided to examine a number of well-crystallized USNM samples of six of the nine known natrolite minerals to seeif compositional t Any use of trade names in this report is for descriptive pur- posesonly and does not imply endorsementby the U.S. Geo- trends noted in the Magnet Cove samplescould be found logical Survey. in samplesfrom other localities. ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP 68'1

TABLE2. Crystallographicand locality data for the natrolite minerals Mineralname Spacegroup a (A) D(A) c(A) / Locality (sample no.)" Reference Natrolite Fdd2 18.31 18.67 6.60 1 MontSaint-Hilaire, Quebec presentstudy (orthorhombic) (usNMR18930) Fdd2 18.29 18.64 o.co 1 BoundBrook, New Jersey presentstudy (usNM126671) Fdcl2 18.31 18.60 6.59 1 clear crystals, Puy de Dome, present study France(USNM R16517) 'I Fde 18.32 18.62 6.59 cloudy cracked crystals, PUY present stucly de Dome,France (USNM R16517) FctcP 18.272 18.613 6.593 I Dutoitspan, South Alrica (CU Artioliet al. (1984) 193466) Tetranatrolite t42d 13.141 6.617 V2 MontSaint-Hilaire, Quebec presentstudy (tetragonal) (usNMR18930) t42d 13.08 6.624 Y2 Kloech,Austriaf (USNM presentstudy 1s891 3) t42d 13.074 6.620 v2 Marianberg,Bohemia Pechar(1989) r42d 13.21 6.622 Y2 Tvedalen,Norway Mazziet al. (1986)* Paranatrolite 19.07 19.13 6.580 1 MontSaint-Hilaire, Quebec Chao(1980) (orthorhombic) Scolecite EA 18.49 18.98 6.523 1 India(USNM 149846) presentstucly (monoclinic) (p:90.72) Fd 18.508 18.981 6.527 1 Berufjord, lceland Joswiget al.(1984) (B : 90.641 Mesolite Fcld2 18.44 56.85 6.58 3 Poona,India (USNM 155046) presentstudy (orthorhombic) Fdd2 18.4049 56.655 6.5443 a Poona,lndia Artioli et al. (1986) Thomsonite Pncn 13 15 13.06 13.26 1 Honshu,Japan (USNM present study (orthorhombic) 147446) Pmel ot ncm 13.08 13.04 13.20 1 Drain, Oregon (USNM 140406) present study or Pmcm Pncn? 13 08 13.09 13.22 1 Table Mt., Colorado (USNM present study c3615) (1990) Pncn 13.1043 13.0569 13.2463 I unknown(CU 2882) StAhlet al. Gonnardite 18.56 18.64 6.62 1 Styria, Austria (USNM present study (orthorhombic?) (pseudocell) R10022) Eddingtonite n,2,2 9.550 9.665 6.523 th Westergotland,Sweden (no. Galli (1976) (orthorhombic) 1505) Tetraldingtonite P42,m 9.584 6.524 V4 KilpatrickHills, scotland Mazziet al. (1984) (tetragonal) - Value of z based on chemicallormula units given-France-gonnardite-thomsonite in Table 1. .. Other samptesstudied: from Puy de Dome, (157727),gonnardite-thomsonite (15273), gonnardite (138378);from Maluti Mts., Lesotho-scolecite (1431 36). t Crystal of tetranatrolitefound in gonnardite sample 158913 from Kloech,Austria. * Crystat structure was reported to be of "gonnardite"; the structure is actually of tetranatrolite(see text).

Natrolite minerals from the U.S. National Museum not detected,and only small amounts of SrO (0.050/o)and collections K"O (0.020/0)were observed. It appears from our work in natrolite contain es- The museum samples, including three natrolite, two that the intraframework channels HrO molecules with only very small tetranatrolite, two scolecite,one mesolite, six thomsonite, sentially Na and Na. not and five gonnardite samples,were examined; their local- amounts, if any, of Sr and K replacing Ca does ities and unit-cell and spacegroup data, plus information appear to enter the natrolite structure. There is some in- on other studies in the literatur€, are given in Table 2. dication that the Na content is somewhat less than the One of the samples(RI8930) contains coexisting natro- ideal 16 atoms per formula unit; Na averages15.42 at- lite and tetranatrolite, one (158913) contains coexisting oms for the 44 natrolite analysessummarized in Table 6. tetranatrolite and gonnardite, one contains two types of This apparent deficiency of Na could be attributed to Na natrolites(R16517), and three (R10022, 157727,15273) loss during electron probe analysis, however, the older contain coexisting gonnardite and thomsonite. wet chemical analysesalso show such a deficiency; for Natrolite. The compositions of natrolite samples example,see Nawaz (1988),Foster (1965b), and Deer et 126671and R18930 are plotted in Figures2, 4, and 5; al. (1963). the average compositions of natrolite samples 126671, Tetranatrolite and paranatrolite. Selected chemical RI8930, and R16517are given in Table6. Natroliteshows analysesof Mont Saint-Hilaire tetranatrolite Rl8930 and very little compositional variation. Of the 44 analyses Magnet Cove tetranatrolite 86-53A are presentedin Ta- summarizedin Table 6, CaO, MgO, FerOr, and BaO were ble 5; their compositions are plotted in Figure 5 and lie 688 ROSS ET AL.: CRYSTALLINE SERIESIN THE NATROLITE GROUP

o Os er

Fig. l. (A) Photomicrograph (plain light) of a polished second phase,both of which probably formed by decomposition of tion of Magnet Cove jacupirangite H-45-8 showing fibrous gon- paranatrolite. (E) Grain-mount polished section photomicro- nardite (G) and platy thomsonite (T). (B) Polished section pho- graph (plain light) of a segmentof a spherulefrom sample 157727 tomicrograph (polarized light) of Magnet Cove ijolite 85-164 from Puy de Dome, France. The dark core area is composed of showing fibrous gonnardite (G) and platy thomsonite (T). (C) acicular gonnardite (G); the light rim area is composedof thom- Grain-mount polished section photomicrograph (plain light) of sonite (T). (F) Backscatteredelectron (BSE) image from a grain- a natrolite crystal (N) and a tetranatrolite crystal (TI.0 from sam- mount polished section of sample R10022 from Stlria, Austria. ple R18930 from Mont Saint-Hilaire, Quebec.(D) Grain-mount The acicular gonnardite (darker areas) interfingers with thom- polished section photomicrograph (plain light) of a composite sonite Qighterareas) that composesthe rim ofthe spherule.Elec- crystal from sample R16517 from Puy de Dome, France. The tron microprobe analyseswere made of all the natrolite minerals clear area is composed of natrolite (N), and the area with nu- in thesepolished sections. merous cracks (CK) is composedof twinned natrolite and a sec- ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP 689

6 t THOM85.164 a I GONN H-45-8 ! 1.. GONN85-1 6A ! l'.. THOMH-45-8 I + 1 NATR12667'I A 7 o fl l\. o o l- tL DO t-L tE[l r- 6 ffim " + T Ea E trA\ 3 + !n + @ 4 @ 4 T ntr T 2 th {r#\ o, "R + 2 + 2 (U qR. o () %\ o 1 m\tr 1 F 11 0 0 6 8 10 12 14 to o24 6 8 10 12 14 to Na+K Na+K

8 8 THOM + 7 GONN 7 o NATR' o [L LL 6 6 + T

5 F + T a 4 a 4 T + 3 o) 3 + + (g 2 (tt 2 (J _tr 1 ! 1 -trru " ! E/ o o o 15 to 17 18 19 20 21 15 16 17 18 19 20 21

AI AI 21 21 GONN85-16A ! NATR 126671 A 20 ! l' !m. -tl 19 n- 19 .tr-

11 17 Er qe. 16 x- 16

15 o246810121416 15 o246810121416 Na+K Na+K Fig. 2. Compositional trends in terms of atoms per formula unit (Table 1) of coexistinggonnardite and thomsonite in ijolite Fig. 3. Compositional trends of coexisting gonnardite and 85-16A from the Magnet Cove igneouscomplex, Arkansas.Also thomsonite in jacupirangite H-45-8 from the Magnet Cove ig- plotted are compositions of natrolite 126671. Small closed cir- neous complex. Small closed circles as in Figure 2. cles show the compositions of end-member natrolite (Na,rAl,u- Si2nOso.l6HrO) and end-member thomsonite (NaoCarAlroSiro- oso.24H,O). 690 ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP

TreLE 3. Selected electron microprobe analyses of three gonnardite samples Gonnardite USNMno. Rl0022 R10022 R10022 158913 15891 3 158913 85-16A 85-16A 85-16A 85-16A 85-16A 85-16A 2327353 78 79 38 39 44 45 46 51 wlch sio, 40.93 40.44 42.57 42.20 43.94 45.29 40.63 42.33 44.68 45.47 39.18 43.69 Al203 29.13 29.69 27.93 27.46 27.40 27.14 26.84 28.00 26.42 25.80 28.28 26.64 FerOs. 0.04 nd nd nd nd nd 0.02 0.13 0.11 o.47 0.08 0.19 Mgo 0.05 0.08 0.05 0.06 0.05 0.02 nd 0.22 0.19 0.00 0.04 0.17 CaO 6.94 6.11 4.86 5.09 2.71 0.82 7.48 5.44 1.89 1.46 9.63 3.48 BaO 0.02 nd 0.05 ncl nd nd 0.08 nd o.02 0.07 0.05 0.04 SrO 0.3it 0.29 0.07 0.10 0.08 0.04 na na na na na na Na.O 8.98 10.23 11.2s 9.98 12.97 14.93 8.60 10.60 13.73 14.69 6.69 12.44 KrO 0.03 0.02 nd 0.03 0.02 nd 0.03 0.04 0.02 0.04 0.02 0.02 Sum** 86.45 86.86 86.78 84.92 87.17 88.24 83.68 86.76 87.06 88.00 83.97 86.67 Cations based on formula unit with Si + Al : 40 Si 21.753 21.445 22.557 22.639 23.0s6 23.443 22.490 22.477 23.572 23.970 21.614 23.274 AI 18.247 18.555 17.443 17.361 16.944 16.557 17.510 7.523 16.428 16.030 18.386 16.726 40 40 40 40 40 40 40 40 40 40 40 40 Fe# 0.018 0.009 0.053 0.044 0 185 0.032 0.076 Mg 0.040 0.063 0.039 0.045 0.039 0.015 0.174 0.149 0.000 0.033 0.135 Ca 3.952 3.471 2.759 2.926 1.524 0.455 4.436 3 095 1.068 0.825 5.692 1.986 Ba 0.004 nd 0.010 0.017 0.000 0.004 0.014 0.011 0.008 Sr 0.102 0.089 0.022 0.031 0.024 0.012 4.115 3.624 2.830 3.002 1.587 0 482 4.462 3.322 1.265 't.024 5.768 2.205 Na 9.254 10.518 11.558 10.383 13.19s 14984 9.230 10.913 14.044 15.015 7.155 12.849 K 0.020 0.014 0.020 0.013 0.000 0.021 0.027 0.013 0.027 0.014 0.014 9.274 10.s32 11.558 10.403 13.208 14.984 9.251 10.940 14.057 15.042 7.169 12.863 Cationsum 53.389 54.156 54.388 53.403 54.795 55.466 53.713 54.262 55.322 56.066 52.937 55.068 /Vote:nd : not detected, na : not analyzed. . Total Fe reported as Fep+. .* HrO content of gonnardite is unknown.

TABLE4. Selected electron microprobe analyses of five thomsonite samples Thomsonite No. R10022 R10022 R10022 147446 147446 140406 140406 C3615 c3615 C3615 85-16A 85-16A 8937 22 70 86 90 13 63 66 32 35

Wlc/o sio. 38.64 38.57 38.32 36.33 35 10 40.15 39.06 40.78 40.39 39.67 36.81 37.04 Alro3 29.14 29.47 30.21 29.05 30.16 28.96 30.17 28.14 29.45 30.16 30.51 29.18 FerO3* nd nd 0.06 0.03 0.02 0.03 nd nd 0.02 0.o2 0.07 0.06 Mgo 0.14 0.14 0.15 0.02 0.14 0.15 0.15 nd 0.13 0.16 0.04 0.02 CaO 11.71 10.70 1'1.13 9.75 8.93 11 .57 11.57 '10.47 10.50 11 .23 12.73 12.55 BaO 0.02 0.08 no nd nd nd nd nd nd 0.06 0.16 0.03 SrO 1.35 1.59 1.88 6.53 6.76 0.62 0.83 0.20 0.23 0.27 na na Na.O 4.13 4.81 4.69 3.88 3.69 4.61 4.83 5.35 5.38 4.75 3.69 3.84 KrO 0.08 0.06 0.07 nd 0.02 nd nd nd nd nd nd nd Sum 85.21 85.42 86.51 85.59 84.82 86.09 86.61 84.94 86.10 86.32 84.01 82.72 H.O" 13.40 13.40 13.40 13.40 13.40 13.40 13.40 13.40 13.40 13.40 13.40 13.40 Sum 98.61 98.82 99.91 98.99 98.22 99.49 100.01 98.34 99.50 99.72 97.41 96.12 Cations based on formula unit with Si + Al = 4O Si 21.177 21.047 20.735 20.593 19.874 21.621 20.939 22.060 21.513 21.097 20.234 20.742 AI 18.823 18.953 19.265 19.407 20 126 18.379 19.061 17.940 18.487 18.903 19.766 19.258 40 40 40 40 40 40 40 40 40 40 40 40 F€F- 0.023 0.014 0.009 0.014 0.009 0.009 0.028 0.023 Mg 0.114 0.114 4.121 0.017 0.118 0.120 0.120 0.103 0.127 0.033 0.017 Ca 6.876 6.256 6.452 5.921 5.417 6.675 6.645 6.068 5.992 6.399 7.497 7.530 Ba 0.004 0.017 0.013 0.034 0.007 Sr 0.429 0.503 0.590 2.146 2.219 0.194 0.258 0.063 0.071 0.083 7.423 6.890 7.186 8.098 7.763 7.003 7.023 6.131 6.175 6.631 7.592 7 577 Na 4.389 5.089 4.920 4.264 4.051 4.813 5.020 5.611 5.556 4.898 3.933 4.169 K 0.056 a.042 0.048 0.014 4.445 5.131 4.968 4.264 4.065 4.813 5.020 5.611 5.556 4.898 3.S13 4.169 Cation sum 51.868 52.021 52.154 52.362 51.828 51.816 52.043 51.742 51.731 51.529 51.525 51.746 Notei nd : not detected, na : not analvzed. - Total Fe reported as Fes'. -' Weight percent HrO calculatedon the basis ot 24H2Oper formula unit (Table 1) 69r ROSS ET AL,: CRYSTALLINE SERIESIN THE NATROLITE GROUP

TABLE5. Selectedelectron microprobe analyses of cloudycrys- tals from sampleR16517 and tetranatroliteR18930 and 86-53A o c) Mineral Nat- Tetra Tetra Tetra Tetra Tetra ts USNM no. R16517 R18930 R18930 86-53A 86-53A 86-53A T (14)-. 77 74 12 18 23

Wto/o T sio, 45.19 38.21 39.07 39.17 40'59 37.85 64 Al,o3 26,50 29.61 28.55 30.34 29'32 31.26 T Fe,O"t nd nd 0.04 0'04 0'12 0.11 o3 MgO nd nd nd 0.04 0.04 0.10 z. CaO 0.35 5.00 3.61 5.26 4.11 6.68 BaO nd nd nd nd 0.05 no I2 SrO 0.04 0.34 1.06 nd nd nd o Na,O 15.42 11.78 12.18 11.94 12.67 11 4'l K,O 0.02 0.02 0.02 0.02 0.07 nd Sum 87.52 84.96 84'53 86.81 86.97 87.40 14 16 H.Ot 9.48 9.48 9.48 9.48 9.48 9.48 8 10 12 Sum 97.00 95.44 94.01 96.29 96.45 96.88 Cations based on lolmula unit with Si + Al : 40 Na+K si 23.651 20.906 21.491 20.911 21.606 20.270 Ar 16.349 19.094 18.509 19.089 18.394 19.730 * _./ 40 40 40 40 40 40 -+. Fe3t 0.018 0.018 0.049 0.045 ,i r' Mg 0.032 0.032 0.080 0) / 0.194 2.931 2.128 3.009 2344 3.833 lll// Ca +6T ,' Ba 0.010 w .t ,'l ar 0.012 0.108 0.338 2.435 3.958 0.206 3.039 2 484 3.059 T '11.847 Na 15.651 12.496 12.990 12 359 13.076 64 K 0.011 0.014 0.014 0.014 0.048 + 15.662 12.510 13 004 12.373 13'124 11.847 or3 a z. Cationsum 55.868 55.549 55 488 55.432 55'559 55.805 GONNR1OO22 tr coNN 158913 o : t2 Note: nd not detected. O NATR126671 A - natrolite and a Cloudy cracked crystals are composed of twinned 1 scoL 149846 V secondphise. Sample also contains clear uncrackedcrystal of untwinned MESO1 55046 V natrolite (Table 6). 0 '. Average of 14 analyses. 21 I 5 16 17 18 19 20 f Total Fe reported as F€F+. on the basis of 16HrOper formula unit + Weight percent HrO calculated AI (Table1). 21 THOMR10022 I GONNRlOO22 C 20 GONN158913 0 closeto trend line II. The natrolite and tetranatrolite por- NATR126671 A \-'.. scoL 149846 Y tions of the crystalsin our Mont Saint-Hilaire sample are MESO155046 v morphologically distinctive-the natrolite portions are 19 il'.. clear and colorlesswithout inclusions, whereasthe areas r\ of tetranatrolite overgrowth are cloudy white and contain (Fig. lC). The clear and cloudy numerous dark inclusions ilr\, phasesare easily separatedfrom one another and were 17 identified, using single crystal and powder XRD methods (Table 2), as natrolite and tetranatrolite, respectively. Chen g and Chao (1930) have previously reported on the occur- 16 v rence of thick overgrowths of tetranatrolite on natrolite in specimensfrom Mont Saint-Hilaire, Quebec,the same 15 locality from which sample R18930 was obtained. The o246810121416 composition of the tetranatrolite in their Mont Saint-Hi- Na+K laire sample is (Na,K)rorrCao 83 (Fe,Ti). 07A1r,uuSiro 3oOro r'' Fig. 4. Compositional trends for USNM samples of thom- l6.58HrO. Although the tetranatrolite describedby Chen soniti, gonnardite, natrolite, scolecite, and mesolite' Small closed Chao contains much less Ca than our specimen, its and circles ihow the compositions of end-member natrolite, scole- plots line as ours (Fig. 5, composition on the same trend cite, and mesolite (Table 1) and end-member thomsonite on the trend II). join NaoCarAlrosiroOro-NarCaoAl,uSiroOro.Dashed lines I and III SampleRl6517, from Puy de Dome, France,is mostly show the gonnardite and thomsonite seriestrends' respectively' composed of large, clear, colorlesscrystals of natrolite as Smalt closedcircles at the ends oftrend III define the ideal end- shown by single-crystal and powder XRD. However, member thomsonite, NaoCarAlroSi;oOEo'24HrO,and a hypo- 24HrO' overgrowths ofa secondphase appear on the prism faces thetical end-member, NarCaoAl,uSiroOso 692 ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP

TABLe6. Averages of electron microprobeanalyses of U.S. Na- tionalMuseum samples of natrolite,mesolite, and scolecite J7c) Mineral Nat Nat"' Nat Meso Scol Scol fo USNMno. 126671R16517 R18930 155046 143136 149846 M (7) (16) (21) (5) (7) (5) D T Wlo/c $+ o- sio, 46.12 45.80 46.37 45.31 45.14 44.50 T g Al,o3 26.25 26.28 25.44 25.36 25.33 24.99 Fe,O"f nd nd nd -o3 - .ifiinI nd 0.06 nd z. THOM 147446 v MgO nd nd nd nd nd nd THOM 140406 0 CaO nd nd nd 9.18 13.74 13.72 THOM . \. \1 ll I2 C3615 BaO nd nd nd (J NATRR18930 A nd nd no SrO 0.05 0.05 0.06 O.07 0.07 0.06 1 TETRANATRRl 8930 .i TETRANATR86-53A tr Na,O 15.41 15.46 14 99 5.08 0.11 0.04 \ K,O 0.02 0.02 0.02 0.02 nd nd Sum 87.85 87.62 86.88 85.02 84.45 83.36 I 10 12 14 16 H.Of 9.48 9.48 9.48 12.37 13.78 13.78 Sum 97.33 Na+K 97.10 96.36 97.39 98.29 97.14 Cations based on formula unit with Si + Al : 40 si 23.941 23.862 24.293 24jt02 24.078 24.069 8 . Scol Al 16.059 16.138 15J07 15.898 15.922 15.931 40 40 40 40 40 +7 40 o() Fe"- IJ- Mg +o 5.292 Z"esS Z.WA Ba Sr 0.015 0.014 0.017 0.020 0.023 0.01I T 0.015 0.014 0.017 5.252 7.900 7.996 uD+ Na 1s.509 15.620 15.232 5.239 0.114 0.040 + K 0.013 0.012 0.014 0.011 9,3 z, 15.522 15.632 15.246 5.250 0.114 0.040 + Cationsum 55.537 55.646 55.263 50.502 48.014 48.036

Tlale 7, Crystallineseries in the systemNarO-CaO-AlrO'-SiOr- H.O-

1 Gonnardite series (measured values from x = 0.3J.2) l,Na16 3,CarAl j6*,5i"o-,060. nHrO (on the ioin Na16Alr6Sir4O@-Na4ca6Alrosiroo@) 2. Tetranatroliteseries (measureed values from x = 0.4-3.9) loNa,6,CaAl16*,Siro-,Ooo. 16HrO (on the join Nal6Al,6siroooo-Na,rcalAl2osirooso) 3. Thomsonite series (measured values from x = O-2) !4Na4*,Ca6 Alro ,Siro*poo 24HrO (on the join NaooaoAlrosiroooo-NascaoAlr65iroo6o)

. The svmbol tr : vacant Na and Ca sites

cept H, in the structure. Since that time a large number of crystal structure refinements of natrolite have been reported (Meier, 1960; Torrie et al., 1964; Peacor, 1973; Alberti andYezzalini, 1981; Hesse,1983; Pecharet al., 1983;Artioli et al., 1984;Kirfel et al., 1984;Baur et al., 1990),as well as of tetranatrolite(Mikheeva et al., 1986; Mazzi et al., 1986;Pechar, 1989), mesolite (Pechar, 1982; t1101 Artioli et al., 1986),scolecite (Fiilth and Hansen, 1979; Fig. 6. Slice parallel to c-[110] of the natrolite framework. Si Joswiget al., 1984; Kvick et al., 1985),thomsonite (Al- and Al tetrahedra share corners to form chains with a 6.6'A berti et al., 1981; Pluth et al., 1985; Stahl et al., 1990), repeatextending in the c direction; adjacent chains are linked in edingtonite (Galli, 1976), and tetraedingtonite (Mazzi er the [ 10] direction by sharingthe bridging Si and Al tetrahedra. al., 1984). The crystal structure of gonnardite is un- The adjacent chains are translated with respectto one another known. Suffice to say, the crystal structures of the natro- by 1-2c/8 (= 1.65 A). The intraframework Na and HrO are not lite minerals thus far examined, with the exception of shown. This figure and Figures7 , 8, 12, I 3, and 2 1 are basedon parametersgiven Artioli et al. (1984). gonnardite, are generally well understood. However, ex- the by cept for one of the tetranatrolite structure determinations, all the structure determinations made thus far have been of crystals with end-member or close to end-member compositions. What are not understood are the structural mechanisms by which these minerals accommodate the variable amount of Na, Ca, and HrO within the channels of the [(Al,Si)ooOro]framework to form the gonnardite, thomsonite, and tetranatrolite series.In the following, a review of the nature of the crystal structuresof the natrolite minerals is presentedwith a view to obtaining a better understandingof the interrelationship betweenthe structural elements and the chemical compositions.

The structure of the [(Al,Si)*Orol framework The crystal structures of the natrolite group minerals are based on a pseudo-tetragonalunit cell with a = l3.l and c = 6.6 A, the unit cell of tetranatrolite, the simplest of these structures. More complex crystallographic relations, causedby symmetry and dimensional adjustments required by compositional variations, are found in the other members of this mineral group (Table 2). The basic structure of the natrolite minerals consistsof (Si,Al)Ootetrahedra linked by cornersto form chains oriented parallel to c; each chain is also linked to adjacent chains in one of three ways to form the three-dimensional framework. Slices of the natrolite framework taken par- NATROLITE allel to the c-[ l0] and (001) planes are shown in Figures F\g. 7. Slice of the natrolite framework structure projected 6 and 7, respectively. The natrolite framework is char- on (001). Four tetrahedral chains are linked togetherto form the acreized by the spiral linkage of adjacent tetrahedral large intraframework channels that contain Na and HrO (not chains, each chain being translated with respect to an shown). ROSS ET AL.: CRYSTALLINE SERIESIN THE NATROLITE GROUP 695

NATROLITE THOMSONITE

r _t_r"t i\

t1101 Fig. 8. Perspectivedrawing ofthe natrolite structure showing the natrolite-type linkage among four tetrahedral chains (front chains are white, rear chains are shaded).Each chain is translat- Fig. 10. Perspoctivedrawing of the thomsonitestructure ed with respect to an adjacent chain by approximately 1.65 A showingthe thomsonite-typelinkage among four tetrahedral (+2cl8) such that the blocks offour adjacentchains form a spiral chains(chains in the front arewhite, those in the rearare shaded). edifice. The left pair of chainsare translatedparallel to c by approxi- mately1.65 A relativeto the right pair of chains.

adjacentchain by +2c(natrolite)/8 (= 1.65 A). The spiral linkage among four natrolite chains is shown in perspective in Figure 8. The framework of tetranatrolite, scolecite, and mesolite is also formed through the natrolite- type linkage of the chains and is topologically identical to that of natrolite. In the crystal structures thus far determined, Al and Si were found to be completely ordered in the scoleciteand mesolite structures and in some, but not all, of the natrolite structures.Al and Si are partially to completely disordered in tetranatrolite. A secondway of linking the tetrahedral chains is found in the thomsonite structure. Within slices perpendicular to the a axis. the tetrahedral chains are not translated with respectto one another (Fig. 9), but within slices petpen- THOMSONITE dicular to the b axis, adjacent chains are translated with one anotherby +s116.msonite)/8(= 1.65 A), Fig. 9. Slice parallel to (100) of the thomsonite framework. respectto The tetrahedral chains within this slice are linked togetherwith- the same chain relationship as seenin the c-[l l0] slice of out any relative c-axis translations. This figure and Figures 10, natrolite shown in Figure 6. Figure 10, a perspective 12,14,17,18, and 21 arebased on the parametersofStehl et drawing of the thomsonite structure, showsthe thomson- al. (1990). ite-type linkage of the tetrahedral chains. None of the 696 ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP

crystal structure studies of thomsonite reported any Si- of the thomsonite type have intermediate-sized cages(Fig. Al disorder in the tetrahedral sites. l0), and those with linkage of the edingtonite type have The third type oftetrahedral chain linkage, the eding- the largest cages(Fig. I l). The size of the cageswithin tonite-type linkage, is found in the edingtonite and tetra- eachframework is also affectedby the amount of rotation edingtonite structures (Fig. l l). In these structures there of the tetrahedra(Gottardi and Galli, 1985,p. 37). is no relative c-axis translation ofthe tetrahedral chains. Natrolite and tetranatrolite. Each channel cage in the Al and Si are completely ordered in edingtonite and com- natrolite structure contains two Na atoms, the Na atoms pletely disordered in tetraedingtonite. being coordinated by four O atoms from the framework and two HrO molecules to form NaOo(HrO), polyhedra. The arrangementof the cations and HrO molecules These polyhedra are in the shape of a distorted trigonal The Si-Al-O framework of the natrolite group minerals prism and link by sharing edges to form a continuous contains continuous channels oriented parallel to the c chain (Fig. l2). A perspective view of the complete na- axis. The channelsare composed oflinked cages,spaced trolite structure looking down the channels is shown in 6.6 A apart, that contain the Na, Ca, and Ba cations and Figure 13. The channels in the tetranatrolite structure the HrO molecules.The size and shape of these cagesis contain polyhedra nearly identical in shapeto those found related particularly to whether the linkage of the tetra- in natrolite (Fig. l2). In tetranatrolite, however, up to hedral chains is of the natrolite, thomsonite, or edington- 25o/oof lhe Na in the cagescan be replacedby Ca, and a ite type. Crystal structures with linkage of the natrolite single cagecan contain zero or one but not two Ca atoms. type have the smallest cages(Fig. 8), those with linkage Scolecite.The scolecitechannels contain Ca atoms coordinated by four framework O atoms and three HrO moleculesto form a distorted pentagonalbipyramid. Each cagewithin the channelscontains one Ca atom and one vacancy; thus the CaOo(H,O), bipyramids do not link together as in natrolite but are isolated from one another Gig. la). To expressthe concept of vacancieswithin the crystal structure, the formula for scolecitecan be written as lrCarAl, uSi2oO80' 24HzO. Mesolite. The mesolite structure contains two types of channels.One type contains linked NaOo(HrO), polyhedra that are nearly identical to those found in natrolite (Fig. l2); the second type contains isolated CaOo(HrO). polyhedra nearly identical to those found in scolecite(Fig. 14). One-third of the mesolite channelscontain Na poly-

---_-b _tr10t _-*b -_ trlOl

natrolite tetranatrolite nesolite thomsonite Fig. 12. Comparison of the Na-bearing coordination polyhedra of the natrolite minerals. In natrolite, tetranatrolite, and t1r0l mesolite, the NaO.(H.O), and (Na,Ca)O4(H,O),polyhedra are EDINGTONITE nearly identical distorted trigonal prisms. The (Na,Ca)O.(HrO), Fig. I l. Perspectivedrawing of the framework of the eding- polyhedra about the hybrid Na,Ca site in thomsonite are in the tonite crystal structure showing the edingtonite-type linkage form of regular square antiprisms. Number in brackets is the among four tetrahedral chains (chains in front are white, those cation coordination number. Na,Ca refersto a disorderedsite in in the rear are shaded).There is no c-axis translation between which Ca randomly replacesup to 50o/oof the Na. The tetrana- the tetrahedral chains. This figure and Figures 14 and 2l are trolite drawing is based on the parametersof Konnert and Ross basedon the parametersof Galli (1976). (sample Rl 8930, unpublished data). ROSSET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP 697

MESOLITE Fig. 15. A slice of the mesolite structure projected on (001). Al tetrahedra are shaded.The larger spheresare Ca atoms; the smaller spheresare Na atoms (HrO not shown). This figure and Figures 12, 14, arrd I 6 are basedon the parametersof Artioli et al. (1986).

Fig. 13. A perspectivedrawing ofthe natrolitecrystal struc- tureviewed looking down the intraframework channels that contain Na atoms (smallerspheres) and HrO molecules(larger spheres). hedra and two-thirds contain Ca polyhedra, as seen in Figure 15, which shows a slice of the structure projected on (001). The mesolite structure may be thought of as composedof polysomes,each polysome consistingof one (010) slice of natrolite and two (010) slicesof scolecite. ITICa In theory, a polysomatic series(Thompson, 1978) can be constructed of polysomescomposed of rn slices of scolecite with n slices of natrolite. The relative arrangement of the Na and Ca polyhedra within the channels is seen in Figure 16, which showsa slice of the mesolite structure "''"@..-b "'?s*EN'""@_-b --b -_b l@s@ tTlC a t61Na

Fig. 14. Comparison of the coordination polyhedra occur- MESOLITE ring in edingtonite, [BaO(HrO)o]; thomsonite, [CaO4(HrO)J; Fig. 16. A slice parallel to the c-[130] plane of the mesolite scolecite,[CaO(HzO),]; and mesolite, [CaO.(H,O).]. The scole- structure (excluding the Si-Al tetrahedra) showing the configu- cite drawing is based on the parameters of Fiilth and Hansen ration of the CaOo(HrO)r and NaOo(HrO)' polyhedra. The Ca (r979). polyhedra are separatedby vacant cation positions. 698 ROSS ET AL.:CRYSTALLINE SERIES IN THE NATROLITE GROUP

b Hzo @ @ Na,Ca @ @

Cae @ Can

@ @ l6lC a THOMSONITE projected (001). trl(Na,Ca)tsi(l\a,Ca) tsl(Na,Ca) Fig. 18. A sliceof thethomsonite structure on THOMSONITE TheAl tetrahedraare shaded. The small,intermediate, and large spheresare Na, Ca,and HrO, respectively.The Ca atoms occupy Fig. 17. A sliceparallel to the b-c planeof the thomsonite a split-atomposition, and only oneofthe two positionspictured (excluding showingthe configu- structure the Si-Al tetrahedra) (Ca^or Ca")is filled. ration of the (Na,Ca)O.(H.O)oand CaOo(H,O),polyhedra. The Ca polyhedraare separated by vacantcation positions. Edingtonite. In the edingtonite structure each channel cagecontains a Ba atom and a vacancy. The Ba atom is parallel to the c-[130] plane. The formula of the unit-cell coordinated by six O atoms and four HrO molecules to contents of mesolite is E,uNa,uCa,6Alo8sirroro0.64}i{2O, form the complex polyhedra shown in Figure 14. which expressesthe l/l ratio of Na to vacanciesand Na The Na-bearing polyhedra found in natrolite, tetrana- to Ca. trolite, mesolite, and thomsonite are compared in Figure Thomsonite. The thomsonite framework also contains 12. The Ba- and Ca-bearing polyhedra found in eding- two types of channels. One type contains one Ca atom tonite, thomsonite, scolecite,and mesolite are compared and one vacancy per cage,similar to that found in scole- in Figure 14. cite. The single Ca atom can occupy one of two positions, 0.52 A, apart, about a split-atom site. The isolated Trm srnucruRAl BAsrs FoR THE GoNNARDTTE, CaOo(HrO), polyhedra are in the form of very distorted TETRANATROLITE, PARANATROLITE, AND trigonal prisms. These Ca coordination polyhedra appear THOMSONITE CRYSTALLINE SERIES as hexagonal bipyramids in Figure 17, bu| becauseCa A summary of the essentialfeatures of the nine natro- occupies only one of two possible positions about the lite mineral structures is presentedin Figure 19; five of center of this bipyramid, the pair of O atoms on the right these minerals have structures with the natrolite-type side or on the left side of the bipyramid lie outside the framework, one with the thomsonite- and two with the Ca coordination sphere. edingtonite-type framework, and one possibly composed The second type of channel contains two atoms per of more than one type of framework. To the right of each cage. In end-member thomsonite, EoNaoCa.Alrosi2oo8o.mineral name given in Figure 19 is recorded the cation 24H2O, there is one Na and one Ca atom in each cage, content and cation coordination number within a single but these atoms are disordered within the channels. In channel cage. As presented previously, natrolite, scole- the more sodic varieties of thomsonite, the channelscon- cite, and mesolite show only small deviations from the tain approximately 70o/oNa and 30o/oCa (Fig. 5, Table ideal formulas given in Table l; thus there is no compel- 4). The (Na,Ca)Oo(H,O)ocoordination polyhedra about ling evidence that these phasesform a crystalline series the hybrid Na,Ca site are in the form of regular square with other members of the natrolite group. The compo- antiprisms (Fig. l7). A slice of the thomsonite crystal sitions ofedingtonite and tetraedingtonitealso show little structure projected on (001) is shown in Figure 18. deviation from the ideal composition given in Table l. ROSS ET AL.: CRYSTALLINE SERIESIN THE NATROLITE GROUP 699

The tetranatrolite and paranatrolite series NRTROTIIETYPE OF FBRMEIUORK pARANATROLIIS. t6l1 t8l1Na,Ca) The tetranatrolite series(series II, Table 7, Figs. 4, 5) NATROLITE: t6lN a involves the substitution of Ca for Na simultaneously I6tNa t61Na" ttt(Na,Ca) with the replacementof Si by Al. No vacant sites appear tEIlNA,CA) TETRANATRO"'"". in the channel cages (n : 0). The principle of Al-Al trtlna,Ca) avoidance in a framework of corner-sharing tetrahedra, tt'fi t6lg t7l6 tTlg plus SCOLECITE: MESOLITE: r review of the many reported chemical analyses,in- r6lNa tr" tr" dicates that no more than 20 Al atoms can appear in the formula unit of any of the natrolite minerals (Table l). TIIOMSONITETYPE OF FRNMEUOR( Thus, in the tetranatrolite series,if maximum Al : 20, THoMSONITE: ttl1ys,6s; t6lgs then the maximum Ca: 4. This maximum value of four t8l1Ns,Ca)tr atoms of Ca is approachedin sample 86-53A, where Ca : 3.833(Table 5, analysis23). The Al-Si and Na-Ca sub- EIIINEIONITEIYPE OFTRRMEU'ONK stitutions and the presenceoftetragonal spacegroup I42d EDINGTONITE' trolB a TETRAEDINGTONITE: llolBa require at least partial Al-Si disorder in one or both tet- tr tr rahedral sites and complete Na-Ca disorder within the FBNMEUORK channel cagesites. The three tetranatrolite structuresthus MIITEOIYPES OF far determined (Mazzi et al., I 986; Mikheeva et al., I 98 6; GONNARDITE: t61Na * t8l1Ng,gsl t6lc a Pechar, 1989) show complete Na-Ca disorder and partial t61Na * tEllpa,Ca) U to nearly complete Al-Si disorder in both the T1 and the (natrolite) (thomsonite) T2 sites. A recent structure refinement of tetranatrolite Fig. 19. Classificationofthe nine natroliteminerals accord- R18930 (J. A. Konnert and M. Ross,unpublished data) ing to thetype of tetrahedralframework and composition of the also showscomplete Na-Ca disorder and nearly complete channelcages within the framework.Each cage can contain two Al-Si disorder in the Tl and the T2 sites. Na atoms,two atomsthat includeboth Na andCa, one atom of plus The meso- The cloudy crystalsin samplesR18930 and Rl6517 Caplus a vacancy,or oneatom ofBa a vacancy. and possiblythe paranatrolitestructures con- are composedmostly of tetranatrolite and twinned natrolite, thomsonite, tain two typesof cages.The gonnarditestructure may be com- lite, respectively, and both phasesappear to be decom- posedof mixedslabs of natrolite,floNa'uAl,uSirnOro l6HrO, and position products paranatrolite. of Since natrolite devi- thomsonite,EoNaoCarAlroSir'O8o'24Hzo. Channel cation va- ates little from the ideal composition Na,uAlr6si24os0' cancyis indicatedby tr. l6HrO, dehydration of a Ca- and Al-rich paranatrolite (e.g., CaoNa,2Al2oSi2oO80'24HrO)to form natrolite requires a large changein the Al/Si ratio and thus a complete reorganization of the tetrahedral framework. Such It is suggestedthat the tetranatrolite series,depicted in a reaction should be very slrrggish;the preferred reaction Figure 5 and defined in Table 7, also definesthe parana- where minimal trolite seriesas troNa,u-"Ca,Al, 24HrO, where would be the formation of tetranatrolite, u**Siro"Oro' chemical and structural changes are required. A Ca- and x = 4. Paranatrolite has the natrolite-type framework, Al-poor paranatrolite, possibly the precursor to the which is inherited by tetranatrolite or natrolite on dehy- twinned natrolite in sample Rl6517, could much more dration. On the basis of the estimate of HrO content in readily form natrolite, since the transformation requires paranatrolite of 24 H'O per formula unit (Chao, 1980) little changein the composition or crystal structure. Chao and the chemical composition of tetranatrolite presented (1980) indicates that the symmetry of paranatrolite may here, it is postulated that the paranatrolite framework be monoclinic. If this is so, then the origin ofthe twinning contains two types of channels.Each cagewithin the first in the cloudy crystals of sample Rl65l7 may be ex- type of channel contains two Na atoms coordinated by plained if monoclinic paranatrolite can transform into four O atoms and two HrO. TheseNaOo(HrO), polyhedra the orthorhombic noncentric natrolite in two orienta- link to form chains similar to those found in natrolite tions, that is, by forming rieht-handed and left-handed (Fig. l2). Each cage within the second type of channel structuresrelated to one another by a 90'rotation about also contains two atoms (either Na-Na or Na-Ca), both a common c axis. coordinated by four O atoms and four HrO molecules. Our chemical studies show that the phase from Nor- These (Na,Ca)Oo(HrO)opolyhedra link to form chains way, reported as "gonnardile" by Mazzi et al. (1986), is similar to those found in thomsonite (Fig. 17). In the actually tetranatrolite;the composition is (Na,K),r..G, oo- second type of channel, Na and Ca are statistically dis- Al,ruosi2,o5o8o'24.8H2Oand is recordedin Figure 5 by tributed on the cation sites with a maximum of 50o/oCa the "x" mark. In the Fourier difference maps obtained occupancywithin the channel. during their crystal structure refinement, weak extra peaks appeared, which suggestedthat there were additional HrO The thomsonite series molecules in the channel cages.These peaks could per- The thomsonite series (seriesIII, Table 7, Figs. 4, 5) haps be due to diffraction from residual domains of un- also involves the concomitant replacementof Ca with Na dehydrated paranatrolite within the tetranatrolite crystal. and Al with Si. In contrast to the tetranatrolite series, 700 ROSS ET AL.: CRYSTALLINE SERIESIN THE NATROLITE GROUP where cagevacancies are zero, there are four vacant cage 4, Fig. 4) and proved that the analysesobtained with the sitesper formula unit, all of which are found in the chan- electron microprobe were not from a mixture of phases. nels that contain only Ca (in the tetranatrolite and thom- The single-crystalelectron diffraction patterns of thomsonite series the vacanciesare fixed at 0 and 4, respec- sonite were consistentwith spacegroup Pncn as reported tively). The composition limits of series III are defined in the literature (Table 2). The strongersubcell reflections by 16 < Al = 20; note that the chemical analysesof the in the single-crystalelectron diffraction patterns of gon- natrolite minerals rarely record less than 16 or greater nardite are consistentwith spacegroup Fdd2 of natrolite than 20 Al atoms per formula unit. In the theoretical (Table 2). However, extra rows of reflections appear in sodic end-member, loNarCaoAl,uSiroO,o. 24H2O, the hy- the c*-[10]* and a*-[031]* nets (Fig. 20), indicating a brid Na,Ca site contains only Na. An X-ray structure doubling ofthe repeat in the a, b, and c directions over determination of a thomsonite sample from Death Val- those of the subcell. The subcell of gonnardite may be Iey, California (Alberti et al., l98l), and neutron diffrac- similar to that of natrolite, but the supercellis complex. tion studies at 29-7 and 13 K of thomsonite from an un- Lattice-fringe imaging of the gonnardite crystals could not known locality (Pluth et al., 1985;StAhl et a1.,1990) show be accomplishedbecause the finely focusedelectron beam no statistically significant Al-Si disorder in the six T sites but complete disorder of Na and Ca in the hybrid Na,Ca site. In all three determinations, the Ca site was found to be split into two positions, 0.3 A apart, with approximately 100/oof the Ca replaced by Sr. Even though the crystal structure of thomsonite has been determined by the most modern techniques,there is still some question as to whether the structure is fully defined. Pluth et al. (1985) observed 58 reflections that violated the extinc- tion criteria for spacegroup Pncn. In our examination of three thomsonite samples,we found that the samplefrom Drain, Oregon, showed violations of the two n glides, indicating that the spacegroup for this sample is Pmc2r, P2cm, or Pmcm rather than Pncn (Table 2\.

The gonnardite series The gonnarditeseries (series I, Table 7, Figs.2, 3,4) is a* the most complex of the three seriesin that the number + ofcation vacanciesin the channel cagesvary linearly with Ca content, approximately along the join Na,uAl,uSiroOro- NaoCarAlroSiroOro.We presenthere a structural model to explain this series. Diffraction data for gonnardite. Repeated and unsuc- cessfulattempts were made to obtain good single-crystal X-ray photographsof gonnardite for symmetry and crystal structure analysis.The patterns were all poor because + [031]* the crystal fragments examined were invariably multi- crystalline. The distributions of the more intense reflections in the precessionphotographs of gonnardite from sample R10022 are very similar to that seen in the natrolite photographs. The X-ray powder patterns of the natrolite minerals are very similar, but the pattern of gonnardite shows distinct differencesfrom those ofnatrolite. tetranatrolite, and thomsonite. With the hope that electron diffraction methods might be more informative, we asked Kenneth Livi (Department of Earth and Planetary Sciences,Johns Hopkins University) to examine sample Fig. 20. Single-crystalelectron difraction patterns of gon- R10022. Dispersed grain mounts were prepared from a nardite sample R10022. Patterns are indexed on a subcell cor- responding grotp (Table (A) fine fraction of the ground sample and examined with a to space Fdd2 ofnatrolite 2). The c*-[ 10]* net showing rows (arrows) ofdiffirse reflectionshaving Philips 420ST electron microscopeequipped with instru- a nonorthogonalintensity distribution. Theserows lie parallel to mentation for analytical electron microscopy (AEM). Sin- I I 0]* and betweenthe rows of strong spots of the natrolite-like gle crystals of gonnardite and thomsonite from sample subcell. (B) The a*-[031]* net showing extra rows (arrows) of R10022 were easily identified by AEM, which gave com- reflectionsoriented parallel to a* and lying betweenthe rows of positions very similar to those reported here (Tables 3, strong reflections of the subcell. ROSS ET AL.: CRYSTALLINE SERIESIN THE NATROLITE GROUP 701 caused decomposition. Electron difraction and AEM, tensive chemical variation of gonnardite samplesreport- however, were undertaken without crystal decomposi- ed in Figures2,3, and 4 and the complex crystallographic tion. relations of sample R10022 can be understood. A series A structural model for the gonnardite series.The com- composed of mixing z slabs of end-member natrolite, positional variation and complex unit cell of gonnardite EoNa,uAl,uSi24O8o'l6H2O,:uu.tth n slabs of end-member can be explained if the gonnardite seriesis a polysomatic thomsonite, EoNaoCarAlrosir.O8''24HrO, fairly well de- series composed of mixed domains of natrolite and scribes the gonnardite compositions given in Figures 2, thomsonite. There are two ways in which two different 3, and 4, although there is considerable scatter in gon- natrolite group framework structures can be joined on a nardite H-45-8 (Fig. 2), perhaps due to the presenceof a coherentinterface: (l) at an interfacejoining(Tl0) ofna- second phase.Howevgr, the gonnardite phasesin the three trolite with (010) of thomsonite, with the c axes parallel samplesfrom Puy de Dome, France(157727,15273, and (Fig. 2lA) and (2) at an interfacejoining (ll0) ofeding- 138378),have compositionsthat deviate significantlyfrom tonite with (100) of thomsonite, with c axesparallel (Fig. trend line I (Fig. a). The compositions of gonnardite and 2lB). Two polysomatic seriescan thus be constructedby thomsonite in sample 157727 (Fie. lE) are plotted in interleaving slabs of natrolite and thomsonite or eding- Figwe 22. The gonnardite data points fall approximately tonite and thomsonite. If gonnardite is indeed composed on trend line IV. Trend IV defines a polysomatic series of interleaved slices of natrolite and thomsonite, the ex- composedof slicesof end-member natrolite and slicesof sodic thomsonite of composition EoNa,, Ca. , Al,, , - Sir,rOro'nHrO. Proof of the existenceof such mixed structuresmust await lattice-fringe imaging by TEM using cold stagesand very low energyelectron beams. Last- ly, there is the question of whether gonnardite has a true stability field or rather is metastable with respect to natrolite + thomsonite or natrolite * mesolite. The textures of coexisting gonnardite and thomsonite in the Magnet Cove samples(Figs. lA, lB) suggest,however, that these two minerals are in equilibrium.

Drscussrolq Although a great deal is now known about the composition and structure of the natrolite minerals, little is

21 EDINGTONITE THOMSONITE THOM1 57727 I GONN1 57727 ! 20 tt)r 'r 19 r\ \l It la.tl- *rffil-J -\ ffi!\+-+au- \- \ il t\\ ! 17 \ \\ \ -r\ 16 a Fig. 21. The two possible ways of joining two different tetrahedral frameworks on a coherent interface. Al-bearing tetrahedra are shaded.The numbers 0, 2, 4, and 6 give the height of 15 the central tetrahedra above the (001) plane in multiples of 0 4 10 12 14 16 c(natrolite)/8 (Alberti and Gottardi, 1975; Gottardi and Galli, 1985, p. 37). (A) Projection of the natrolite and thomsonite Na+K structures on (001). The two lattices are joined at a coherent interface (dashed line), which is parallel to (Tl0) and (010) of Fig.22. Compositional trends for coexistinggonnardite and natrolite and thomsonite, respectively. (B) Projection of the thomsonite in sample 157727 from Puy de Dome, France (Fig. edingtonite and thomsonite structures on (001). The coherent lE). Trends I and III and small closedcircles are as in Figure 5. interface (dashedline) is parallel to the (lT0) and (100) planes Trend IV lies on the join Na,uAl,uSiroOro-EoNarrCzIur- of edingtonite and thomsonite, respectively. A1r85si2r rOro, also denoted by small closed circles. 702 ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP known of their stability fields (Gottardi, 1989). Experi- (U.S. Geological Survey, Reston) for his help with various aspectsin the mental studies of zeolite equilibria are greatly hampered use of the electron microprobe, Harvey Belkin (U.S. Geological Survey, Reston)for his help in samplepreparation, Judith Konnert (U.S. Geolog- by slow reaction rates and difrculties in nucleation. Sen- ical Survey, Reston) for her crystal structure aoalysisof the Mont Saint- derov and Khitarov (1971) reported that natrolite-type Hilaire tetranatrolite, and Kenneth Livi (Johns Hopkins University) for and analcime-type zeolites formed under stable condi- analyzing one of our thomsonite-gonnarditespecimens with the trans- tions from amorphous materials in the temperaturerange mission electron microscopeand furnishing us with severalelectron dif- patterns of 100-200 We not fraction and chemical analyses.Iastly, we wish to acknowledge "C. are aware of any other reports of that our present understandingof the structure of the natrolite minerals equilibrium synthesis of any of the nine natrolite min- is, in no small way, due to the studiesof the late ProfessorGlauco Got- erals. Quantitative estimatesof the conditions of natural tardi and his colleaguesat the University of Modena-we dedicate this formation of these minerals are also scarce; however, paper to the memory of this distinguishedrnineralogist. Kristmannsd6ttir and T6masson (1978) reported that RrrnnrNcrs crrBn thomsonite, mesolite, and scolecite formed in the Ice- landic geothermal areasin the temperature range of 65- Alberti, A., and Gottardi, G. (1975) Possiblestructures in fibrous zeolites. 100 (1971) presents NeuesJahrbuch fiir Mineralogie Monatshefte,9,396-4ll "C. Carpenter a theoretical activity- Alberti, A., and Vezzalini, G. (1981) A partially disordered natrolite: activity diagram indicating stability fields at low P and Z Relationships between cell parameters and Si-Al distribution. Acta for natrolite, mesolite, and thomsonite and comparesthis Crystallographica,B37, 781-788. diagram with naturally occurring thomsonite- and meso- Alberti, A., Yezzalin| G., and'fazzoli, V. (1981) Thomsonite: A detailed lite-bearing assemblages refinementwith crosschecking by crystal energycalculations. Zeolites, observed in the altered basalts r,9r-97. at Cape Blomidon, Nova Scotia. Alberti, A., Pongiluppi, D., and Vezzalini, G. (1982) The crystal chem- The natrolite minerals occurring in the Magnet Cove istry ofnatrolite, mesolite and scolecite.Neues Jahrbuch fiir Mineralo- syenites are formed as a result of late-stagereaction of gie Abhandlungeu 143, 23 1-248. the primary nepheline,the latter having reequilibrated or, Artioli, G., Smith, J.V., and Kvick, A. (1984)Neutron diffractionstudy possibly, of natrolite, Na,AlrSirO,o zHrO,at 20 K. Acta Crystallographica,C40, crystallized at temperatures as low as 500 "C r658-r662. (Flohr and Ross, 1989).The metasomaticalteration may Artioli, G., Smith, J.V., and Pluth, J.J. (1986) X-ray structure refinement be the result of autometasomatism,or deuteric alteration, of mesolite. Acta Crystallographica,C42, 93'7-942. in which early magmatic phasesreact with their own re- Baur,W.H., Kassner,D., Kim, C.-H., and Sieber,N.H. (1990)Flexibility sidual fluids to form a suite of low-temperature minerals and distortion ofthe framework ofnatrolite: Crystal structuresofion- exchangednatrolites. EuropeanJournal of Mineralogy, 2, 7 61-7 69. such as natrolite, gonnardite, and thomsonite. In the case Birch, B. (1989) Chemistry of Victorian zeolites. In W.D. Birch, Ed., of the alteration of nephelineoccurring in the mafic ijolite Zeolites of Victoria, specialpublication 2, p. 9l-1O2. The Mineralog- xenoliths, the metasomatism may have occurred when ical Society of Victoria, Melbourne, Australia. the xenoliths were entrained in the volatile-rich magma Carpenter,A.B. (1971) Graphical analysisofzeolite mineral assemblages garnet pseudoleucite jacupiran- from the Bay ofFundy area, Nova Scotia. In American Chemical So- of the host syenite. The ciety Advancesin Chemistry Series,101, 328-333. gite may have experiencedlow temperature metasoma- Chao, G.Y. (1980) Paranatrolite, a new zeolite from Mount St-Hilaire, tism that was related to the formation of nearby mineral Qu6bec.Canadian Mineralogist, 18, 85-88. deposits or was metasomatizedby circulating late-stage Chen, T.T., and Chao, G.Y. (1980) Tetranatrolite from Mount St-Hilaire, -84. hydrothermal fluids during the waning stagesof igneous Qu6bec.Canadian Mineralogist, 18, 77 Deer, W.A., Howie, R.A., and Zussman,J (1963) Rock-forming silicates, activity at Magnet Cove. Autometasomatism cannot be vol. 4: Framework silicates.Longman, London. ruled out. The literature cited above would suggestthat Dowty, E. (1991) Atoms (Macintosh version 1.0).Shape Soflware, Kings- the Magnet Cove zeolites formed below 200 "C. port, Tennessee. The many reports on natural occurrencesof the natro- Fiilth, L., and Hansen,S. (1979) Structureofscolecite from Poona, India. lite group Acta Crystallographica,B35, 1877-1880 minerals and the need to understand the con- Flohr, M.J.K., and Ross, M. (1989) Alkaline igneous rocks of Magnet ditions under which these minerals crystallized indicate Cove, Arkansas: Metasomatized ijolite xenoliths from Diamond Jo that renewedefforts should be made in determining their quarry. American Mineralogist, 74, ll3-131. stability fields. Our present study suggestsa number of -(1990) Alkaline igneousrocks of Magnet Cove, Arkansas: Miner- geochemistry -98. problems that may be encounteredin equilibrium exper- alogy and of syenites.Lithos, 26, 67 Foster, M.D. (1965a) Compositional relations among thomsonites,gon- iments besides those of kinetics and nucleation; for ex- nardites, and natrolites. U.S. Geological Survey Professional Paper 504- ample, the presenceof at least three crystalline series,the E, El-E10. presenceof possibly metastablephases and the dehydra- - (1965b) Composition of zeolitesof the natrolite group. U.S. Geo- tion products of paranatrolite, the difficult of phaseiden- logical Survey ProfessionalPaper 504-D, Dl-D7. (1976) powder Galli, E. Crystal structure refinement of edingtonite.Acta Crystal- tification by conventionalX-ray methods,the role lographica,832, 1623-1627. of Al-Si and Na-Ca order-disorder in obtaining equilib- Gottardi, G. (1989) The genesisofzeolites EuropeanJournal ofMiner- rium, and choosing geologically pertinent experimental alogy,1, 479-487. variables. Gottardi, G., and Galli, E. (1985) Natural zeolites.Springer-Verlag, Ber- lin. AcrNowr,nocMENTS Gunter, M.E., Knowles, C.R., and Schalck,D.K (1991) Intergrown natrolite/mesolite "single" crystals from the Columbia River basalts. We thank Jeffrey Post (U.S. National Museum, Washington, DC) for GeologicalSociety of America Abstractswith Programs, 23, A219. helping us with sample acquisition and for his much appreciatedadvice Hesse, K.-F. (1983) Refinement of a partially disordered natrolite, on our X-ray powder diffraction work and crystal drawings,James McGee NarAlrSirO,o.2HrO. Zeitschrift fiir Kristallogaphie, | 63, 69-7 4. ROSS ET AL.: CRYSTALLINE SERIES IN THE NATROLITE GROUP 703

Hey, M.H. (1933) Studieson the zeolites.Part V. Mesolite. Mineralogical Pabst, A. (1971) Natrolite from the Green River Formation, Colorado, Magazine,23, 421-447. showing an intergrowth akin to twinning. American Mineralogist, 56, -(1936) Studies on the zeolites. Pa( IX. Scoleciteand metascole- 560-569. cite Mineralogical Magazine,24, 227-253 Pauling,L. (1930)The structureof somesodium and calcium aluminosili- Joswig,W., Bartl, H., and Fuess,H. (1984) Structurerefinement of scole- cates. Proceedingsof the National Academy of SciencesU S.A., 16, cite by neutron diffraction. Zeitschrift fiir Kristallographie, 166, 219- 453-459. 223. Peacor,D R. (1973) High-temperature,single crystal X-ray study of na- Kile. D.E.. and Modreski. P.J. (1988) Zeolites and related minerals from trolite. American Mineralogist, 58, 676-680. the Table Mountain lava flows. Mineralogical Record, 19, 153-184. Pechar,F (1982) Precisedefinition ofthe crystalline structure ofnatural Kirfel, A., Orthen, M., and Will, c. (1984) Natrolite: Refinement of the mesolite. Acta Montana, 59, 143-152. crystal structure of two samples from Marienberg (Usti nad Labem, -(1989) An X-ray determination of the crystal structure of natural CSSR).Zeolites, 4, l40-146. tetragonalnatrolite. Zeitschrift fiir Kristallographie, 189, 191- 194. Knstmannsd6ttir, H., and T6masson,J. (1978) Zeolite zonesin geother- Pechar,F., Schiifer,W., and Will, G. (1983) A neutron diffraction refine- mal areasin Iceland. In L.B. Sand and F.A. Mumpton, Eds, Natural ment of the crystal structue of natural natrolite, NarAl'Si,O'o'2HrO zeolites,occurrence, properties, and use, p.277-284 Pergamon,New Zeitschrift fiir Kristallographie, 164, 19-24 - York. Pluth, J.J., Smith, J.V., and Kvick, A. (1985) Neutron diffraction study Krogh Anderson, E , Dans, M., and Peterson,O.V (1969) A tetragonal ofthe zeolite thomsonite. Zeolites. 5, 74-80. natrolite.Meddelelser om Gronland, l8l, 1-19. Rychly, R., and Ulrych, J. (1980) Mesolite from Homi Jilove, Bohemia. Kvick, A, Stehl, K., and Smith, J.V. (1935) A neutron diffraction study Tschermaks mineralogische und petrographischeMitteilungen, 27, 201- of the bonding of zeolitic water in scoleciteat 20 K. Zeitschrift fiir 208. Mineralogie,17 l, 141-154. Senderov,E.E., and Khitarov, NI (1971) Synthesisof thermodynami- Mazzi,F, Galli, E., and Gottardi, G. (1984) Crystal structurerefinement cally stablezeolites in the NarO-AlrOr-SiO.-HrO system.In American of two tetragonaledingtonites. Neues Jahrbuch fiir Mineralogie Monat- Chemical Society Advancesin Chemistry Series,l0l, 149-154. shefte.8. 373-382- Stehl, K., Kvick, A., and Smith, J.V. (1990) Thomsonite, a neutron dif- Mazz| F., Larsen, A.O., cottardi, G., and Galli, E. (1986) Gonnardite fraction study at 13 K. Acta Crystallographica,C46, 1370-1373. has the tetrahedral framework ofnatrolite: Experimental proofwith a Taylor, W.H., Meek, C.A., and Jackson,W.W. (1933) The structuresof sample from Norway. Neues Jahrbuch fiir Mineralogie Monatshefte, the fibrous zeolites.Zeitschrift fiir Kristallographie, 84, 373-398. 5,219-228. Thompson, J.B., Jr. (1978) Biopyriboles and polysomatic series.Ameri- Meier, W M. (1960) The crystal structureof natrolite. Zeitschrift fiir Kris- can Mineralogist, 63, 239-249. tallographie, | 18, 430-444. Torrie, 8.H., Brown, I.D., and Petch, H.E. (1964) Neutron diffraction Mikheeva, M.G., Pushcharovskii,D.Yu., Khomyakov, A.P., and Yam- determination ofthe hydrogenpositions in natrolite. CanadianJournal nova, N.A. (1986)Crystal structureof tetranatrolite.Soviet Physicsand of Physics,42, 229-240. Crystallography,31, 254-257. Ulrych, J., and Rychly, R. (1983) Re-examination of the natrolite group Nawaz, R. (1988) Gonnardite and disordered natrolite-group minerals: zeolites from the Ceske Stredohori Mts., NW Bohemia, Czechoslovakia. Their distinction and relationswith mesolite,natrolite and thomsonite. Acta Universitatis Carolinae-Geologica, Rost Volume, (l-2), 33-52. Mineralogical 1{egazine,52, 2O7-219. Nawaz,R., Malone,J.F., and Din, V.K. (1985)Pseudomesolite is meso- MeNuscnrpr REcETvEDMancs 4, 1992 lite Mineralogical Magazine,49, 103-105 Mlt.ruscnrpr AccEprEDMencn 20, 1992