American Mineralogist, Volume 78, pages204-209, 1993

Holotype buddingtonite:An ammoniumfeldspar without zeolitic HrO

J.H.L. VoNCKEN,* H.L.M. vAN RSERMUNDT** A.M.J. vAN DER EnnonN, J.B.H. Jl.NsnN Facultyof EarthSciences, Department of ChemicalGeology, Budapestlaan 4, 3508TA Utrecht,The Netherlands R. C. Eno uSGeorosicar',)lJii;'?'ix",iHi'f; i.'""':';:i%:1l|;'ffi].]:sT""'Mser''

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Holotype buddingtonite from the Sulphur Bank Quicksilver deposit, Lake County, Cal- ifornia, was studied by XRD, IR, STEM, SEM, TGA, DTA and wet chemical analyses. Synthetic anhydrous ammonium was studied also for comparison. The holotype buddingtonite specimen(NMNH no. I1697400) contains admixtures of FeSr,anatase and, most importantly, montmorillonite. Montmorillonite is capableof reversibledehydration, and its admixture explains the zeolitic behavior previously ascribed to buddingtonite. It is concluded that buddingtonite is an anhydrous ammonium feldspar: i.e., free of zeolitic HrO, with no zeolitic properties.

INtnooucrroN the HrO is not present in an ordered way (Kimball and Megaw, 1974). Moreover, structure and properties of Buddingtonite was first describedby Erd et al. (1964) synthetic ammonium feldspar are consistentwith the an- as an ammonium feldspar with zeolitic HrO, with the hydrous formula NHoAlSi3Os(Voncken et al., 1988; following formula: NHoAlSi3Os't/zH2O.The mineral was Voncken, 1990). In addition, the lattice parametersand studied by chemical analysis,XRD, thermal analysis,and X-ray diffraction pattern of synthetic ammonium feld- IR. Buddingtonite was found initially as a hydrothermal spar are essentiallyidentical to those of natural ammo- replacementof plagioclasoin certain parts of the Sulphur nium feldspar, suggestingthat differencesin properties Bank Quicksilver deposit,Lake County, California (White between natural and synthetic ammonium feldspar are and Robertson,1962), and later in oil shalesand hydro- due to impurities in the natural material. Pyrite, marca- thermal deposits (Gulbrandsen, 1974; Kimbara and site. anatase. and montmorillonite were found in the Nishimura, 1982;Loughnan et al., 1983).Buddingtonite sample initially characterizedby Erd et al. (1964). has gained importance recently becauseof spatial rela- This study presentsnew data on the natural holotype tionships with Au deposits (Kydd and kvinson, 1986). buddingtonite specimen studied originally in order to shed All authors have acceptedthe hydrous formula that was light on the nature of the zeolite-like behavior of the min- proposedfor buddingtonite.Pavlishin and Bagmut (1988) eral. Pure synthetic ammonium feldspar was used for studied the substitution of K by NHf, in feldsparsof gran- comparison. ites and pegmatitesbut did not mention the presenceof any structural HrO in their ammonium-bearing . Solomonand Rossman(1988), however, did find minor S,l,lupr,B DESCRIPTToN amounts of structurally bound HrO (0.02 wto/oHrO) in Two samples, taken from the holotype specimen of their ammonium-bearing feldspars, although there was buddingtonite(NMNH no. I1697400),were available for no evidence for HrO* substitution in the alkali-cation comparison with the synthetic material. The first sample srtes. was a 70-mg remnant of DW-1, initially describedby Erd Experimental studies of ammonium feldspars(Barker, et al. (1964). This is the purest fraction ofholotype bud- 1964; Hallam and Eugster, 1976) were inconclusive with dingtonite. The secondsample, designatedhere as 2821, respectto the zeolitic characterofbuddingtonite. A study has not been previously described.It consists of a 100- of the of holotype buddingtonite con- mg remnant of material, split from the original holotype cluded that buddingtonite has a feldspar structure in which specimen, that was crushed, treated with cold 1:l HCI and l:l HNO3, and sedimentedin H'O for over an hour * Present address: Faculty of Mining and Petroleum Engi- to determine the associatedclay minerals. Buddingtonite neering,Department of Raw Materials Technology, Delft Uni- was the predominant mineral in this fraction. Fyrite and versity of Technology,Mijnbouwstraat 120,2628 RX Delft, The marcasite were eliminated and very minor anatasewas Netherlands. present.The smectite content was significantly geat- ** Present address: Laboratoire de P6trologie M6tamor- still phique,C.N.R.S. U.R.A., 736, Universit6Paris 7, 4 placeJus- er than in DW-1. sieu,75252Paris Cedex05, France. Synthetic ammonium feldsparwas preparedfrom NH. 0003-004x/93l0 l 02-0204$02.00 204 VONCKEN ET AL.: HOLOTYPE BUDDINGTONITE 205

Fig. l. SEMphotograph of a syntheticammonium feldspar Fig.2.rR spectra,]1l:H.*:":1,]* oooo-oo0 cm, ror crystalgrown at 600"C and2 kbar.Flaky material on both sides naturalbuddingtonite and syntheticbuddingtonite. of the crystalis carbonsubstrate material. solution and an Al-Si gel at 600 "C and 2 kbar using the In the IR-spectra of the natural ammonium feldspars method of Voncken et al. (1988).A rapid-quenchpres- (Fig. 2), thereare two distinct bandsat 3750-3600cm ', sure vesselwas used, allowing cooling to room tempera- which are characteristicof montmorillonite. Thesebands ture within 2 s. If qugnching is insufficiently rapid, sec- are most clearly visible in the spectrum of buddingtonite ondary tobelite develops (Voncken et al., 1988). The sample 2821. To highlight absorption features of the synthetic sample consistedof pure buddingtonite. montmorillonite impurity, a KBr pellet with synthetic buddingtonite was put in the referencebeam when nat- ANALYTICAL TECHNIQUES ural buddingtonite was analyzed.Many of the resulting X-ray powder diffraction photographs were obtained bands can be ascribed to montmorillonite, which may with an Enraf Nonius FR552 powder diffraction camera, have NHo*; in the interlayer (Table l). usingCuKa radiation. For SEM analysis,the sampleswere SEM analysesof DW-l reveal typical buddingtonite Au-coated and studied using a Cambridge Stereoscan, crystalsof - 5 pm in length, as well as blocky grains (Figs. operatedat 10 KeV. For STEM-analysis,the sampleswere 3a, 3b). Sometimes thick plates were visible. Figure 3b ground for 5 min in alcohol in an agate mortar, and a illustrates small thin flakes, visible as overgrowths on drop ofthe suspensionwas transferredto a Cu grid bear- feldspar faces.The grain size of the thin flakes is = I pm. ing a C film on one side. STEM analyseswere performed The amount of the flaky material in 2821 is estimatedto with a JEOL 200C TEM, equipped with an energy-dis- be four times as largeas in DW-I. persive X-ray analyzer(Link SystemsLtd.), and operated A STEM study of the samples DW-l and 2821 con- at 120 or 200 KeV. The sample preparation for IR-spec- firms the presenceof platesand flakesin both. The small troscopy was as follows: 2.5 mgof sampiewas mixed with thin flakesare extremelyunstable under the electronbeam. 250 mg of KBr. Pelletswere pressedin vacuum and sub- No analysescould be obtained. The larger and thicker sequentlydried for one night at 120.C and analyzedus- plates,already observedby SEM, were identified as bud- ing a Perkin Elmer 580 IR spectrometer.Thermal anal- dingtonite by means of electron diffraction patterns. Iron yses(TGA and DTA) were obtained in air with a Dupont sulfide and what was probably TiO, were identified a few thermal analyzer at a heating rate of l0 "C/min. Wet times using qualitative energydispersive X-ray analyses. chemical analysesof DW-l were carried out for NHi, If the amounts of NH; in wt0/0,determined by wet which was determined by colorimetry after acid decom- chemical methods, are recalculatedto (NHo)rO, values of position and Kjeldahl distillation. 8.5 and 8.3 wto/oare found for DW-l. This agreesvery well with the results of Erd et al. (1964, Table 3), who Rrsur,rs reported 8.34 wto/o(NH.),O. They reported that the ho- A typical euhedral synthetic ammonium feldspar crys- lotype buddingtonite contains small amounts of K, Na, tal is shown in Figure l. The XRD pattern of the holotype Ca, and Ba. sample DW-l only showed buddingtonite lines, as did Erd et al. (1964) recordeda total weight loss of l2olo those of previous studies (Erd et al., 1964: Kimball and for sample DW-I, whereas our TGA data for sample Megaw, 1974; Voncken et a1., 1988).In the pattern of DW-l shows a total weight loss of 10.8 wto/o(Figs. 4a, sample 2821, vaguebroad lines were visible with d values 4b, Table 2). Thermogravimetric analyses(Figs. 4a, 4b) of 4.45,3.04,2.59,and 1.50A, which areinterpreted as show a loss of bulk NH, and HrO from the material, belonging to montmorillonite. starting at about 500 "C. There is a difference between 206 VONCKEN ET AL.: HOLOTYPE BUDDINGTONITE

TABLE1, lR vibrationsof DW-1.

Inten- Refer- cm 1 sity Interpretation ence 3710 sh 3703 OH-stretchmontmorillonite '|1 I 36s5 OH-stretchmontmorillonite 3630 s 1 3640 OH-stretchmontmorillonite [ 3630 OH-stretch,tobelite 3 3400 SN 3410 H,O (montmorillonite) 3290 3290-3300 NHo* 3070 s 3070 NHi 4-1 0 2850 m 2850 NHo* 1640 m 1630 H,O (montmorillonite) 1430 s 1430- 1 435 NHi 4-10 1130 VS 1120 montmorillonite 1050 vsl ) 1035 montmorillonite(broad band) 1000 vsl 920 Sh 915 montmorillonite 2,11 695 880 montmorillonite 2 785 m 795 montmorillonite 2 735 sn 731 buddingtoniteor 2,4 705 m 711,715 buddingtonite 4, 5 692 montmorillonite 2 630 sn ozE montmorillonite 2 580 St 575 montmorillonite 2 540 520 montmorillonite 2 470 SI 464 montmorillonite 2 430 st 425 montmorillonite 2 Note;Abbreviations: s : strong,m : medium,w: weak,sh : shoulder Referencenumbers: 1 : Mortlandet al. (1963);2: Van der Marel and Beutelspacher(1976); 3 : Vonckenet al (1987);4 : Erd et al. (1964);5 : Vonckenet al. (1988);6 : Shigorovaet al. (1981);7 : Vedder(1965); 8 : Solomonand Rossman(1988); 9 : Chourabiand Fripiat(1981); 10 : Krohnand Altaner(1987); 11 : Serratosa(.1962). ' A sampleo{ syntheticbuddingtonite was in the referencebeam during the analvsis.

the temperaturesrecorded at the start of the loss of NH3 and H,O from buddingtonite given by Erd et al. (1964) (608 "C) and the presentresults (500 "C). However, Erd et Fig. 3. (a) SEM photographof a buddingtonite-bearingag- al. (1964)applied a differentheating rale (12'Clmin) in gregatein holotypeseparate DW-I. (b) SEM photographof a thermal analysis. Several weight-loss stagescan be rec- buddingtonitecrystal in sample2821. Overgrowth by an appar- ognizedin the thermogravimetric analysisof DW-l (Fig. ent phyllosilicateis visible. 4a). The rise of the curve at temperatures greater than 850 'C is due to the rise of the base line, as was proved with a corundum blank (not shown).The total weight loss DW-l (Fig. 5b) may be attributed to the transition of for sample 2821 (Fig. 4b) is 140/0.It largely occurs in two anataseto rutile (Yamaguchi and Mukaida, 1989). stages.This sampleshows a largeweight loss below 150 Our IR data indicate the presenceof ammonium-bear- 'C (5.00/o). ing montmorillonite in DW-l and 2821. The thin flakes, The DTA patterns (Figs. 5a, 5b, 5c) display a large imagedwith SEM and STEM, and which decomposevery curve between 25 and 700 "C. This large curve is due to quickly, are interpreted as being ammonium-bearing a differencein thermal conductivity between the silicate montmorillonite. and the referencematerial (corundum) and does not rep- Thermal analysis also confirms the presenceof mont- resent thermal decomposition. A differential thermal morillonite in DW- I and 2821.In the DTA recording of analysisof pure synthetic NH4A1Si3O'is shown in Figure sample2821, peaks are found at 60, 110,and 150'C. In 5a. In Figures 5b and 5c, DTA patterns of DW-l and the DTA recording of DW-l (Fig. 5b), which according 2821 are shown. The DTA data are summarized in Ta- to SEM investigation contains smaller amounts of mont- 'C. ble 3. morillonite, only one peak is found at 100 Loss of interlayer HrO in smectitic minerals may take place in Drscussron several steps up to 200 "C (cf. Koster van Groos and The presenceof iron sulfide and anataseis consistent Guggenheim,1987a; Gotoh et al., 1988)'Release of NH, with DTA results. Exothermic peaks in sample DW-l from NHf-bearing montmorillonite is an endothermic between 350 and 490 "C can be attributed to decompo- process,which takesplace gradually between200 and 600 sition of pyrite or marcasite(Smykatz-Kloss, 1974). The .C (Chourabi and Fripiat, 198 l). Besidesloss ofinterlayer small exothermic peak at 980 "C in the DTA pattern of HrO and NH., dehydroxylation should also occur. De- VONCKEN ET AL.: HOLOTYPE BUDDINGTONITE 207

E I E- oo o 96 bq -o4 E Pse I o8 i o 92 Synlhstic buddingtonit€ 12

2@ 3@ 4@ 5@ 6@ 7@

400 500 6m 700 Tmptrature ('C)

Tmporature fC)

E oo

o z trI o t

n 500 6m 700 8@ 900 tooo 1tm

Temperature(oC) Fo P Fig. 4. (a) TGA curveof sampleDW-I. (b) TGA curveof sample2821.

E { hydroxylation, which is exothermic, may occur between 500 and 700 "C (Chourabiand Fripiat, l98l; Koster van Groos and Guggenheim,1987b). o lm 2@ 3@ 4@ 5@ 6m 7@ @ m 1m 11@ Differential thermal analysesof the natural budding- Tcmpdature (oC) tonite samples (Figs. 5b, 5c) do not clearly indicate if Fig. 5. (a) DTA curveof syntheticammonium feldspar. (b) deammoniationand dehydroxylationoccur. However, the DTA curveof sampleDW-l. Peaksmarked with an arroware weightJosscurve (Fig. 5b) showsa changein slope some- listedin Table3. (c)DTA curveof separate2821. Peaks marked what above 550 "C. This may be explained by the com- with an arroware listed in Table3. bined weight loss from buddingtonite and montmorillon- ite in the temperatureregion 500-550 .C, with respectto the loss from buddingtonite alone at temperaturesabove compositions.Intensities of the IR bands at 3710 and 550 "C. At about 800'C, the remainingA1-O-Si structure 3630 cm ' can be used to calculate the relative amounts of buddingtonite breaks down, as indicated by the DTA of montmorillonite in thesesamples. Values of I/Iofor curve of synthetic ammonium feldspar (Fig. 5a). A sim- the 37l0and 3630 cm I bandsfrom the DW-1 spectrum ilar but smaller peak is recorded for DW- l. The sample are 8.3 and 16.7respectively, and for 2821 they are 31.3 2821, containing less buddingtonite than DW-I, shows and 63.8, respectively.This suggeststhat 2821 contains no peak here. approximately four times as much montmorillonite as Although montmorillonite is shown to be present in DW-I, which agreesreasonably well with the estimates both holotype buddingtonite samples,the preciseamount from SEM photographs. of the clay mineral in DW-l and 2821 is problematic. It is desirable to evaluate the contribution of the im- Erd et al. (1964)accepted the total amount of impurities purities to the total weight loss ofholotype buddingtonite. in DW-l to be less than 50/0,of which less than 30/ois On thermal decomposition, a feldspar with the ideal for- FeSr. Calculation of the amount of montmorillonite on mula NHoAlSi.O, would lose 10.I wto/oaccording to the the basis of the chemical analysis is ambiguous, as the reaction 2NH4AISi3O8* 2NHr + HrO + AlrO3 + SiOr. analysis of DW-l includes buddingtonite and montmo- A weight loss of 9.60/owas found for syntheticNH4AISi3O8 rillonite, and the latter mineral may have a wide rangeof by Vonckenet al. (1988).The weightJossdata for DW-l 208 VONCKEN ET AL.: HOLOTYPE BUDDINGTONITE

TABLE2. A summary of weight losses detected with TGA ability to dehydratereversibly (e.g.,Serratosa,1962), and this perfectly explainsthe zeolitic behavior previously de- Synthetic NH4AtSisOs DW-1 2821 scribed and ascribedto buddingtonite. It is concluded that buddingtonite contains no zeolitic wt. loss wt. loss wt. loss r fc) (./") rfc) (%l rfc) ('/"1 HrO and should be consideredto be anhydrous ammo- nium feldspar. 0-150 1.0 0-150 5.0 150-360 0.6 150-850 9.0 360-410 0.5 AcxNowr-nocMENTS 410-550 3.0 550-800 4.2 We wish to thank P. van Krieken ard T. Zalm for performing the 800-900 9.6 800-850 1.5 thermal analysis,P.G.G. Slaatsfor the X-ray diffraction photographs,and Total J Pietersof the Electron Microscopy Sectionof the Faculty of Biology of qA 10.8 the University of Utrecht for assistancewith the STEM work. The work benefitedmuch from careful reviews by D. White, K' Bargar,J Rytuba, G. Rossman,and S. Altaner. can be conected for the contribution of impurities by RrrrnnNcrs crtno subtracting 1.00/ofor absorbedHrO and the loss of inter- (1964) Ammonium in alkali feldspars.American Mineralo- layer HrO from montmorillonite and 0.50/ofor the loss of Barker, D.S. gist,49, 85l-85E. volatiles from FeSr-decomposition.Therefore, 9 .3o/ote- Chourabi, B., and Fripiat, J.J. (1981) Determination oftetrahedral sub- mains for buddingtonite and montmorillonite decom- stitutions and interlayer surfaceheterogeneity from vibrational spectra position (dehydroxylation and ammonium loss) in DW- of ammonium in smectites.Clays and Clay Minerals, 29'260-268' D.E. (1964)Buddingtonite, l. As the amount of montmorillonite in DW-l is not Erd, R.C.,White, D.E., Fahey,J.J., and ke, feldsparwith zeolitic water.American Mineralogist, 49' weight an ammonium known, the contribution of montmorillonite to the 831-857. loss cannot be determined. The discrepancybetween the Gotoh. Y, Okada,K., and Otsuka,N. (1988)Synthesis of ammonium chemical data for DW-l and the thermogravimetric anal- montmorillonite. Clay Science,7, ll5'127 . in the ysescan probably be attributed to this unknown contri- Gulbrandsen, R.A. (1974) Buddingtonite, ammonium feldspar Formation, SoutheasternIdaho. Journal ofResearchofthe Correction of the weightJossdata of 2821 for the Phosphoria bution. U.S. GeologicalSurvey, 2, 693-697' loss of interlayer HrO from montmorillonite (a subtrac- Hallam, M., and Eugster,H.P. (1976) Ammonium silicate stability rela- tion of 50/o)leads to a value of 9olofor ammonium loss tions. Contributions to Mineralogy and Petrology, 57,227-244. and dehydroxylation. Again, there is an unknown contri- Kimball, M.R., and Megaw, H.D. (1974) Interim report on the crystal In W'S. MacKenzie and J. Zussman,Eds' bution of dehydroxylation of montmorillonite. The cor- structure of buddingtonite' The Feldspars,Proceedings ofthe NATO ASI on Feldspars,p 8l-86' rected weight lossesfor both natural buddingtonite sam- ManchesterUniversity Press,Manchester, U.K ples, however, suggestthat buddingtonite is anhydrous, Kimbara. K.. and Nishimura, T (1982) Buddingtonite from the Toshichi like synlhetic ammonium feldspar. This agreeswith the Spa,Iwate Prefecture,Japan. Kobutsugahu Zasshi (Journal ofthe Min- English results of Voncken et al. (1988), who show similarity in eralogical Society of Japan), 15,207-216 (In Japanese,with and properties between synthetic and natural abstract). structure Koster van Groos, A.F., and Guggenheim,S. (1987a) Dehydration ofa buddingtonite. Ca- and a Mg-exchangedmontmorillonite (SWy-l) at elevated pres- The presenceof montmorillonite in the holotype bud- sures.American Mineralogist, 72, 292-298. dingtonite sampleis essentialfor the interpretation of the -(1987b) High-pressuredifferential thermal analysis(HP-DTA) of montmorillonite and K-exchanged zeolitic behavior reported for buddingtonite. As the H'O the dehydroxylation of Na-rich American Mineralogist, 7Z' 1l7 0-ll7 5' labo- montmorillonite. content of montmorillonite varies with humidity, Krohn. M.D., and Altaner, S.P (1987) Near infrared detection of am- ratory conditions will have an influence on the weight monium minerals. Geophysics,52, 924-930. lossesdetected. The differencebetween the total weight- Kydd, R.A., and Levinson, A.A. (1986) Ammonium halos in lithogeo- loss value found by Erd et al. (1964) for DW-l and the chemical exploration for at the Horse Canyon carbonatehosted Use and limitations. Applied Geochemistry,I' present (l2olovs. 10.87Qis therefore not considered deposit,Nevada, USA: one 407-417. to be significant. Montmorillonite is well known for its I-oughnan,F.C., Roberts, F.I., and Lindner, A'W. (1983) Buddingtonite (NHo-feldspar)in the Condor OilshaleDeposit, Queensland,' Mineralogical Magazine,47, 327-334' Trer-e3. A summaryof reactionsdetected with DTA Mortland, M.M., Fripiat, J.J.,Chaussidon, J., and Ultterhoeven, J. (1963) Interaction betweenammonia and the expandinglattices of montmo- Synthetic rillonite and vermiculite. Journal of PhysicalChemistry, 67' 248-258' NH4AlSi3os DW-1 2821 Pavlishin, V.I., and Bagmut, N.N' (1988)NHi in feldsparsof chambered Ifc) rype rfc) Type rfc) Type pegmatites and enclosing ganites. Mineralogical Journal (of the U.S.S.R.), 10, 3, 69-7 | (in Russian,with English abstract). 60 endo Serratosa,J.M. (1962) Dehydration and rehydration studiesofclay min- 110 endo erals by infrared spectra' In E. Ingerson' Ed., International Seriesof 150 endo on Earth Sciences,Monograph Il' p. 412-418' Pergamon, 350-490 exo Monographs 550 endo or exo endo or exo New York. B.M., and 800 exo +850 exo Shigorova,T.A., Kotov, N.V., Kotel'nikova, Ye.N., Shmakin, 875 exo 980 exo Frank-Kamenetzkiy, V.A. (1981) Synthesis, diffractometry and IR- spectroscopyof micas in the seriesfrom muscovite to the ammonium Note: endo : endothermic,exo : exothermic. analogue.Geochemistry International, I 8, 76-82. VONCKEN ET AL.: HOLOTYPE BUDDINGTONITE 209

Smykatz-Kloss,W. (1974) Differential thermal analysis:Applications and Voncken, J.H.L., Konings, R.J.M., Jansen,J.B.H., and Woensdregt,C.F. results in mineralogy, 185 p. Springer-Verlag,Berlin. (1988) Hydrothermally grown buddingtonite, an anhydrous ammoni- Solomon, G.C., and Rossman,G.R. (1988) NHi in pegmaritic feldspars um feldspar(NH4AlSijOs). Physics and Chemistryof Minerals, 15,323- from the southernBlack Hills, South Dakota. American Mineralogist, 328. 73,818-821. White, D.E, and Robertson,C.E. (1962) Sulphur Bank, California, a ma- Van der Marel, H.W., and Beutelspacher,H. (1976) Atlas of infrared jor horspring Quicksilver Deposit. In Petrologicstudies: A volume to spectroscopyof clay minerals and their admixtures, 396 p. Elsevier, honor A.F. Buddington. GeologicalSociety ofAmerica, specialpaper, Amsterdam. 397-428. Vedder, W. (1965) Ammonium in muscovite. Geochimica et Cosmo- Yamaguchi, O., and Mukaida, Y. (1989) Formation and transformation chimica Acta. 29. 221-228. of TiO, (anatase)solid solution in the system TiO,-Al,Or. Joumal of Voncken, J.H.L. (l990) Silicateswith incorporation of NH4*, Rb* or Cs*, the American Ceramic Society,72, 2,330-333. 9l p. Ph.D. thesis,University of Utrecht, GeologicaUltraiectina No 65, Utrecht, The Netherlands. Voncken,J.H.L., Wevers,J.M.A.R., Van der Eerden,A.M J., Bos, A., and Jansen,J.B.H. (1987) Hydrothermal synthesisof tobelite, NHoAlrSi3AlOro(OH)r,from various starting materials and implica- Me,NuscnrgrREcETvED Aucusr 20. 1991 tions for its occurrencein nature. Geologieen Mijnbouw, 66, 259-269. Mer.ruscnrprAccEprED Aucusr 20. 1992