American Mineralogist, Volume 69, pages 186-189, 1984 Willhendersonite,a new zeolite isostructural with chabazite Donalo R. Peacon Department of Geological Sciences University of Michigan, Ann Arbor, Michigan 48109 PBrp J. DUNN Department of Mineral Sciences SmithsonianInstitution, Washington,D.C. 20560 WrLlreu B. Stutr,toNs Department of Earth Sciences University of New Orleans, New Orleans, Louisiana 70148 ErreHenr TrllunxNs aNo Rr,lNneno X. Flscnen Institut fiir Geowiss e ns chaften, J ohannes Gutenberg-U niversittit Postfach 3980, D-6500 Mainz, Federal Republic of Germany Abstract Willhendersonite(KCaAl3Si3O12 . 5HzO) is a new zeolite which is isostructuralwith chabazite, (Ca,NaJ[AlzSi+Orz]. 6H2O. It occurs in the San Venanzo Quarry, Terni, Umbria, Italy in cavities of Quaternary lavas as "trellislike" twinned aggregates,and as tabular crystals in a limestone xenolith from the Ettringer Bellerberg near Mayen, Eifel, Germany. It is triclinic, space group PT. Single crystals from Mayen have the unit cell parametersa:9.23(2),b:9.21(2),c:9.52(2)A,a=92.7(l),F:92.4(l),7:90.1(l),y = 808A3,Z = 2. Willhendersoniteis colorless and transparent,and it has a vitreous luster. The Mohs' hardnessis 3. The measuredand observeddensities are2.l8 and2.20 glcm3, respectively.Willhendersonite is optically biaxial with a = 1.505(3),F = 1.511(3),y = 1.517(3)and2V: 87(3)".The strongestlines in the diffractionpattern are: (d, III")9.16, 100;5.18, 30; 4.09, 40; 3.71, 30; 2.907,60; 2.804,50. Introduction the same mineral. This paper is a description of results investigations. In late 1980,Dr. William A. Henderson,Jr. of Stam- independentlyobtained in the separate pleased new specieswillhender- ford, Connecticutsubmitted a specimento P.J.D. that he We are to name this Jr. The new had obtained several years earlier from Mr. Gianni Por- sonite in honor of William A. Henderson, prior to publication, by cellini of Rimini, Italy. Dr. Hendersonhad examinedthe mineral and namewere approved, New Minerals and Mineral specimen and had noted that it contained a mineral the I. M. A. Commissionon Italian material are pre' unknown to him. When this mineral could not be identi- Names. Type specimensof the ' under catalog num- fied following a study of its optical properties, Dr. Hen- servedin the SmithsonianInstitution (holotype)and NIvTNH148656 (cotype). derson made it available for more extensive studies. bersNuNn 148655 Preliminary data then indicated that it was indeed a zeolite mineral not correlatable with other known spe- X-ray crystallograPhY cies,and subsequentinvestigation confirmed this hypoth- Single-crystalprecession photographs of crystals from esis. Entirely independently,material was discovered near Mayen show that willhendersonite is triclinic, space near Mayen, Eifel, Germany by Dr. G. Hentschel of group Pl or Pl, with a = 9.23(2), b = 9.21(2),c = Wiesbaden and its structure and properties were de- 9.52(2)A, a = 92.7(l), I = 92.4(l),and 7 = 90.l( l)' with Z scribed by Tillmanns and Fischer (1981, 1982).In part = 2.Pl was confirmedas the correct spacegroup on the with the aid of Dr. Glauco Gottardi (pers. comm.), the basis of a refinement of the structure (Tillmanns and specimensfrom Italy and Germany were found to be of Fischer, 1981, 1982).The refinementshowed that will- 0003-0M)vE4l0I 02-0 r 86$02.00 186 PEACOR ET AL.: WILLHENDERSONITE lE7 hendersoniteis isostructural with chabazite, with com- plete Si-Al ordering as indicated by T-O distances; that is, the tektosilicate frameworks of both minerals are topologically similar, and consist in part, of double six- rings of tetrahedra, although the coordinations of Ca are very diferent. Crystals from Terni were also studied using single- crystal techniques. Initial results appeared to indicate that the space group is C2lm, C2, or Crn with cell parametersa = 12.95(4),b = 12.96(3),c : 9.45Q)4, and I = 93.6(2)".Subsequent detailed examination of photo- graphs of carefully selected, relatively perfect crystals showed that high-theta reflections on upper-level Weis- senbergphotographs are doubled. This confirmed that the crystals are triclinic, but twinned so as to give a diffrac- tion pattern nearly identical to that of a monoclinic phase. Lattice parameterscould not be directly measuredfor the triclinic unit cell because of complete overlap of most Fig. l. SEM photomicrograph of willhendersonite twinned- reflections. Cell parameterswere obtained by transform- crystal aggregatefrom Terni. Scale bar is 0.1 mm. NMNH ing those of the monoclinic cell of the twins, yielding the #148655.The crystals are flattened on {001}, the narrow faces; parametersa - b = 9.16,c : 9.45A. d - B : 92.5,and y each approximately perpendicularto {001},are {100} and {010}. = 90.0". These parameters are very similar to those determined for the Mayen crystals although the cell : translations are slightly smaller. The angles d = are which is apparently rhombohedral, with a 9.404 and a B :92". precisely the averageof a and B for the Mayen crystals. The cell parameters are similar to those for chabazite Powder X-ray diffraction data are listed in Table l. They were obtained using a polycrystalline sample of Terni crystal fragments mounted in a Gandolfi camera Table l. X-ray powder diffractiondata for willhendersonite. (l14.6 mm diameter),with siliconas an internalstandard. Thed-values are calculated using triclinic unit cellparameters as Because reflections related by the pseudomonoclinic parameters: transformedfrom refined,-pseudomonoclinic symmetrycould not be resolved,and because the unit cell a = b : 9.16,c : 9.454,a = = 92.5,7= 90.0.. B is almost perfectly metrically monoclinic, the diffraction patternwas indexedusing the pseudomonoclinicC-cen- d (obs.) d (Calc.) trtll llloz d (obs.) rlro tered unit cell and a least-squaresrefinement was carried 9.16 9.15 100 100 2.429 10 out leading to the cell parameters for the pseudomono- 5.18 5.l8 111 30 2.264 l5 given param- 4.71 4.72 002 5 2.209 I clinic unit cell as above. The triclinic lattice 4.57 4.58 200 5 ?.163 l5 eters for the Terni material as listed above were obtained 4.27 4.27 r0z 2 2.078 10 by transformation of those parameters. 4.09 4.09 210 40 2.042 10 4.10 LM 2.004 5 3.93 3.93 rrz 20 t.979 Morphology and twinning 3.5? 3.84 12T 20 1.875 5 3.81 rI2 1.845 Willhendersonitefrom Terni occurs only in very char- 3.7L 3.73 zTt 30 1.819 acteristic twinned specimenswhich have a very striking 3.70 rv 1.794 l5 3.05 3.06 2T2 l0 1.738 appearance.As illustrated in Figure I, the twinned aggre- 3.05 300 1.692 20 gates(which are less than 1.0mm in diameter)consist of 3.01 3.02 013 10 t.570 3,00 t22 crystals in three different orientations, each with faces 3.00 221 1.552 2 1.515 2 approximately perpendicular to those of the others, and 2.907 2.900 u3 60 1.489 2 arranged in most specimensto have a "trellis-like" 2.896 3T0 1.459 2 2.804 2.809 r3T 50 t.426 I appearance.The faces ofindividual crystals are indexable 2,791 131 2.746 2,747 r3r 2 1.408 5 using the triclinic cell as {100}, {010} and {001}. The 2.747 222 1.387 2 (Crystals 1.370 I crystals are tabular, flattened on {001}. from 2.5t4 2.510 222 I 1.314 2 Mayen display the sameforms.) The twinning is appar- 2.538 2.541 230 20 t.279 2 2.539 203 ently by rotation about [11], a pseudothree-fold axis in 2.508 2.511 o32 20 t.260 I 1.220 I willhendersonitewhich is supposedlya three-fold axis or 1.206 2 psuedothree-fold axis in other membersof the chabazite 1,191 1 r.t42 I group. Additional complex twinning of two types was ob- served within each individual of the twins related bv lE8 PEACOR ET AL.: WILLHENDERSONITE rotation around [11]. The single-crystaldiffraction pat- directly. Becausethe amount of H2O in zeolitesmay be terns clearly showedtwinning by reflectionacross (001). variableand not consistentwith spacegroup equipoint In additionthcy demonstratedtwinniqg either by reflec- requirements,this calculationof H2O content is simply tion across(110) or rotation around [l10] (referredto the the best possibleunder the circumstances,and may be triclinic cell). This plane and axis are the pseudosym- subject to change when sampleslarge enough for water metry elementsof the pseudomonocliniccell which was analysis are found. referred to above, as compatible with a twinning relation Material from Mayen was analyzed using energy dis- consistentwith Mallard's law. Becausethe twinned lat- persive techniques(lskv, 0.007 lr.A). Standardswere tice displaysonly subtledifferences from that of a mono- Al2O3for A1, SiO2for Si, wollastonitefor Ca and a glass clinic lattic_e,the twin law could not be distinguished with the composition 35Vo KAlSi2O 6 and 65VoMgCaSi2O6 between(110) or [10]. However, this twinning was also for K. ZAF corrections were applied with the program observedoptically as polysynthetically twinned lamellae MAcrc by Colby (1980).The results of the analysisare with approximatecomposition plane (ll0). also listed in Table 2. Water content was calculatedby difference leading to the formula K6 73Ca1.e3Si3 Chemistry Al3O12' 4.5H2O.Some loss of potassiumwas observed Willhendersonite from Terni was chemically analyzed in subsequentanalyses of the samespecimen. The crystal utilizing an ARL-sEMeelectron microprobe using an oper- structure analysis (Tillmanns and Fischer, 1981, 1982) ating voltage of 15 kV and a beam current of 0.15 g,A. confirmsthe orderingof Al and Si, and K and Ca which is Standards used were microcline (K), and bytownite implied by this formula.