Howieite, a New Type of Chain Silicate 87

American Mineralogist, Volume 59, pages 86-97, 1974

Howieite,a New Type of ChainSilicate

H. R. WnNx Departmentol Geologyand Geophysics,Uniuersity ol California at Berkeley

Abshact

Howieite, one of the new iron silicate minerals from the Franciscan formation, representsa new type of chain silicate, hybrid betweena single and a double chain. In effect, a four-octahedron-wide ribbon of the octahedral layer of mica is sandwichedbetween two hybrid [Si uO'z]-chains. As for the tetrahedral sheetsin mica, the apical oxygens of these chains face (and are shaped by) this ribbon. The resultant structural units run parallel to z, their flat sides being parallel to (120) in howieite, which is thus analogous to (001) in the sheet silicates. Where the flat sides of two such structural units are adjacent, the basal oxygensof their [SieOrz]-chainsform the interstice that accommodates the large Na-atom. Those basal oxygensmore distant from the Na-atom, and thus at the edgesof the structural units, are shared by the octahedral ribbons of neighboring structural units. The structure was refined with 35 independent anisotropic atoms to an R-factor of 4.5 percent in space-groupPl. But partial order of divalent and trivalent ions on the 12 octahedral sitesindicate that the true symmetry of howieite may be Pl. The howieite structure could well be indicative of high pressuressince it representsa rhuch denser packing than in sheet silicates; thus its occurrencein the high-pressure, medium-temperature assemblagesof the Franciscan formation is plausible from crystal chemical considerations.

Introduction suggestedby Agrell, Bown, and McKie (1965) is: Howieite is one of the three new silicate minerals (Nar.s3Ca6.er),ou(Mgo auMn, nrFe'*u.nr)n.rn which have recently been found in the .riebeckite- stilpnomelaneschists of the Franciscanformation in . (Fet*, ssAlo62)2. 1e( Sirl s6Tio.o4)12. 36031. 3(oH)12. 6e. (Agrell Northern California et al, 1965). The crystal The following triclinic unit cell contains one for- usedin the structuredetermination was collectedat mula unit: the type locality at Laytonville quarry (Mendocino County,California); it is a cleavagefragment from a a : 10.176)A, b : 9.72 A', c : 9.56A, plumose aggregateof black bladed crystals (Figure 'Y: 1a) in carbonate-bearingstilpnomelane-riebeckite- a: 91.30. B:70.7" 109". spessartite-howieite-deeritelschist. Howieite is bi- In the course of the structure determinations.both axial negative(,a = 1.70I, I = 1.720,y : l.'734, formula and lattice constantshad only to be slightly 2V : 65') with strong dispersion( o ) r) and modified. Howieite was an interesting choice for a pleochroism(a golden,y green). It is distinguished structuredetermination because its lattice constants from the similar coexistingstilpnomelane by inclined are different from any known silicates,and there is extinction and higher birefringence. Howieite has thus possibility of finding a new silicate structure two moderatelygood cleavages,(010) and (100). type. Another stimuluswas local reminiscence,since The density is 3.378 g/cc. The chemicalformula howieite was first discoveredby Dr. Agrell of Cam- bridge University in the Berkeley mineralogycollec- tion. The structuredetermination proved indeed that 'Deerite, another one of the three new minerals,has been howieite possessesa new structure, intermediatebe- described as belonging to space group P2,/a with presence tween sheetand chain silicates.It can be described of submicroscopical twinning (Agrell et aI, 7965). Our as a collapsedsheet silicate possibly indicative of the precession and Laue photographs show diffraction spots geologicalconditions of high pressureduring meta- whose equivalencein shape and intensity indicate that the mineral may rather possessthe orthorhombic space group morphic recrystallizationin the Franciscanformatio'n. Pnma (62) (a = 18.885A, b - 3.182 A (needleaxis), Somebasic featuresof the howieitestructure have c - 10.337A). alreadybeen reported (Wenk, 1973). HOWIEITE, A NEW TYPE OF CHAIN SILICATE 87

b Fig. 1a. Plumose aggregateof howieite crystals in stilpnomelane-riebeckiteschist. b. Prismatic bladed crvstal, chosenfor the structure determination.

Experimental Procedures corrected for Lorentz and polaization effects. Material qnd Data Collection Standard deviations o of individual reflections were determined from counting statistics of integrated Most howieite crystals were either bent, showed count and background. An additional error of p : 4 asterism, or consisted of several slightly disoriented percent of the net intensity was added to the standard domains. After examination by Laue photographs deviation to avoid overweighting of intense reflections of many crystals on a precession camera, a small (Duesler and Raymond, l97l; Corfield, Doedens, nearly perfect crystal (0.02 X 0.1 X 0.15 mm) was and Ibers, 1967). Of 5322 reflections only 4047 found (Fig. lb). This crystal was then oriented on a reflections with intensity )3o were used in the computer-controlled Picker diffractometer. Lattice refinement. constants and orientation angles were refined by The data crystal had maximum point-to-point least squares methods from positions of 12 indg- distancesof 0.02 X 0.10 X 0.15 mm and was mounted pendent reflections(a : l0.l7o(4) A, b : 9.774(4)L, approximately along the z axis (Fig. 1b). The linear : : ^y c 9.589(4)A, o 91.22(5)", P : 70.76(5)", : absorption coefficient is 53 cm-'. The value of e-" 108.09(5)". Intensity data were then collected on ranged from a minimum of 0.6 to a maximum of the four-circle diffractometer using MoKa radiation 0.9 at 20 : 0' (the variation is somewhat higher than with highly oriented graphite as monochromator in this for high angle reflections). The actual range of the O-2O scan mode up to 2O angles of 60'. A scan integrated transmission factors is smaller than this rate of l"/min was used with a scan width of 1.4' f and no absorption correction was applied. As dis- A2O, where A2O is the distance between calculated cussed later, errors caused by absorption mainly maxima for Ka1 and Ka, peaks. Background was affected the anisotropic temperature factors, which counted for 4 seconds stationary on either side of the show a strong elongation parallel to y. peak. The stability of the crystal and the diffractometer was checked by measuring the 026,420, and 150 reflections after every two hundred reflections. These Solution of the Structure reflections showed constant intensity within I to 2 The structure was determined using a combination percent throughout data collection. The data were then of sign determination (program FasuN; Main, Woolfson, and Germaine, unpublished), Patterson, '9Estimatederrors in the leastsignificant digits are given Fourier (program Fonnar; Zalkin, unpublished), in parentheses. and least-squares(program Nucrs 6; Busing, Martin, H. R. WENK and Levy, 1962; modified by Ibers and Raymond) ,..- Old Cell techniques.Scattering-factor tables of Cromer and r-/----tr----] Mann (1968) interpolated for 0.1 electron formal \t charge were used for all atoms. A correction for \ .re(lC) \ Fe(2C)\ anomalous dispersion effects was applied to Fe a\ usingboth Af'and Af" (Cromer,1965). The scattering Fe(2)\- - { Fe(2) factors of Na and Fe (M) were modified to take care \\ . of minor substitutionsusing the chemicalanalysis by .fe(i) Fe(t )\\ Agrell et al (1965), ([Nao nrCao or], [Fe'*o.u3Mflo.r5 Mgo. onFe'*o.rrAlo ou). -\ First a Patterson map was calculated. Its main featureis a strongmaximumat u : 0, u : 0, w : 1/3 which can be interpreted as a chain arrangement of atoms parallel to the z-axis with a repeat of l/3 +X (3.2 A) that just about correspondsto an O-O dis- NewCell tance.A secondstep was to computeWilson statistics FIc. 2. Diagram indicating Fe(=M) positions and choice and usethem to calculatenormalized structure factors of unit cell in the beginning of the structure determination lEl for 400 strong reflections. Results of Wilson suggestedby the sign determination program (dashed) and statistics indicate that a centrosymmetricstructure in the refined structure (solid). is more probable(average lEl : 664,lEl > 1 : l8/o, lEl > 2 : 6.I%). Using program F.q,slaN,signs of 310 E's were determined and for one of several Refinement solutionsthe E map in space-groupPl showedstrong At this stagea trial to define the structure in the -0.19, peaksat l/2,0,0; l/2,0, 0.33; 0.18,0.06; centrosymmetricspace-group Pi (averagingpseudo' -0.19, 0.18, 0.39, and weakerones at 0.12,0.30, symmetrically related atoms) failed. The structure -0.11, -0.30, 0.12; 0.12, 0.30, 0.45; 0.20; 0.20, refined readily to R : 10 percent irt Pl with 69 -0.15, 0.26(Fig. 2, old setting);these all agreewith independent atoms. Now an attempt to use the the Pattersonmap. Refining scalefactors and atomic centric space-groupwas successful,and final refine- positions by least-squares,the R-value dropped to ments were done with four models using 3721 inde- .18percent. Using 1043reflections (lFI' > 5o,20 I pendentreflections (lF,l > 3o). 35') with unit weight and combining a one-cycle a) The first model assumed35 independentatoms full-matrix least-squaresrefinement with a difference with isotropic temperature factors, refining 146 Fourier, more atoms were added after each cycle parameters(atomic positions, temperaturefactors -8, and the R-value reached 29 percent after 5 cycles. a scalefactor, an extinction coefficientfZachariasen, On the Fourier maps, chains of atoms parallel to z 1968],and occupancyfactors for M and Na atoms). with a l/3 repeat were apparent from the start. It The refinement converged after a few cycles (un- was not possibleto refine it any further. Therefore weightedR : 6.1370,weighted R : 1.797). Final symmetry was lowered to Pl starting out new with parametersfrom this refinementare listed in Tables2 t heavy atoms which showed up consistently in and 6 and occupanciesin Figure 10. Thesedata were Fourier maps, one of them fixed at l/2, 0,0. This used in the calculation of interatomic distancesand refinement was much better, and at an R-value of bond angles in Tables 4-5 (with Program Onrrn). 18 percent the structure was essentiallysolved with b) An analogous refinement was done but with 12 M, 12 Si, 44 O, and 1 Na located.It appeared anisotropic temperaturefactors for all atoms, thus at this point that the sign determination program varying 321 parameters.This refinement also con- placed the center of symmetry at a pseudo-center verged well (unweighted R 4.5570, weighted Fe (2), and the origin wasnow fixed in the Na position R : 6.187d. Atomic positions were identical to (Figure 2). We mention this question of origin in the isotropic refinementwithin lessthan the estimated some detail to demonstratethe grade of resolution error. Coefficients0t; of the thermal ellipsoids are of a sign determinationprogram. The pseudo-centerlisted in Table 3. As can be seen,there is quite sub- which has been determinedat Fe (2) is surprisingly stantial anisotropy, particularly for the large Na asvmmetric! atom (compare also Figure 7). A final difference HOWIEITE, A NEW TYPE OF CHAIN SILICATE 89

Fourier summation showed no peaks larger than /z +l.O e/A", and - 1.5 e/4", or roughly l0 percent of an oxygenatom. The largestpeaks appearednear heavy atoms and are therefore interpreted as series termination ripples rather than any serious error in the final model (Figure 3). A list of F.'s and F"'s for this refinementis shown in Table l. c) The noncentric isotropic refinement (60 atoms with 288 parametersvaried) convergedslowly below R : 10 percent and large correlation coefficients (up to 0.975) betweenparameters of pseudocentro- symmetricallyrelated atoms were encountered.Esti- o-X mated errors of the parameterswere high, and after many cycles R (unweighted) reached 5.80 percent. Obviously the quality of the data set was insufficient to resolve the ambiguity as to whether howieite belongsto the Pl or Pl space-group,and also least- squares methods do not lend themselveswell to refiningstructures with a very strongpseudosymmetry. For other reasons,discussed below, we maintain that or2A o the true symmetryof howieite is Pl. d) In order to test the acentricity further in a -v2 model with fixed atomic positions and thermal parameterstaken from the isotropic Pl refinement, Frc. 3. r - 0.5 z sectionof a differenceFourier basedon the occupanciesof the M sites were refined in Pl. the last anisotropic refinement cycle. The section contains The refinement converged and showed significant the largest peak. Positions of close atoms are indicated. Contours are in 1/10 e A-'. Solid lines indicate positive, differencesbetween centrosymmetrically related sites, darrhedlines indicate negative topography. thus suggestingpartial order of Fe'*, Fet*, Mg, and A1 on the octahedral sites. Results are shown in Table6 and Figure8. same direction. These six-memberedrings are con- Another attempt at refining occupancyfactors of nectedwith one corner and extendas infinite garland- M and octahedral O positions in Pl also refined shaped chains parallel to the z-axis (Fig. 5). In the resulting in identical occupanciesas in the former six-memberedring, two tetrahedrahave three corners (Table refinement 6) and differences in atomic linked in the chain [Si(2)], four tetrahedrahave only positions of centrosymmetricallyrelated atoms of two [Si(l), Si(3)],thus forming a [SiuO"] unit which up to 0.5 A (5 standarddeviations). is a hybrid betweena single and a double chain for For all these refinements (a-d), the extinction which there is no representativeso far. Si-O distances coefficientwas very small(0 + 2 X l0-'). in the six independenttetrahedra are listed in Table 4. There is no significantdifference in the averageSi-O Description of the Structure distanceof the varioussites (1.621-1.631 A). The structure of howieite is displayedin Figures 4 The tetrahedral chain is attached with corners to and 5. An xy projection is easiestto visualizedue to an octahedralband consistingof four rows of octa- the formation of "chains" of all atoms parallel to hedra also extending parallel to the z-axis with a the z-axis. Aiming at some natural order in the lf3 z repeat. M-O distances and O-M-O angles labeling of the 35 independentatoms, we used the (Table 5) are fairly uniform and show relativelylittle repetition along the z-axis; for instance, O(2, 3) distortion. In this [M,rO'o] octahedralband: twelve meaning the second oxygen atom on the third z- oxygens-O(2,l), O(2, 2), O(3, 2), O(3,3), O(4, l), level. O(2, 3C) is its pseudocentrosymmetriccounter- O(4, 3) plus their centrosymmetricequivalents-are part. bonded to three M and one Si atoms; six oxygens- SiOr-tetrahedraare linked to form rings of six, O(2, 3), O(3, 1), O(4, 2) plus equivalents-areonly with the free corners of all of them pointing in the bonded to three M atoms; four oxygens-O(5, 1), TnsI-r 1. Observedand Calculated Structure Factors of Howieite*

* For all reflectionshkl usedin the refinement. F obtained from the last cycle of the anisotropic refinementin spacegroup P|. "'s TAsr,r 1. continued 92 H. R, WENK

Tnnr.n 1, continued

O(5, 3fare bonded to two M and one Si atom; distorted (Table 5), and averageM-O distancesvary four oxygens-O(I, 1), O(1, 3)-are bonded to one M between2.029 and2.l94A. Notice that the directionof and one Si atom; two oxygens-O(5, 2) plus its extension of the octahedral bands with respect to equivalent-are bonded to two M atoms only; and the orientation of the octahedron in howieite is two oxygens-O(1, 2) plus equivalents-are bonded different from amphibolesor pyroxenes. to one M atom only. The latter are characterized by The space-groupsymmetry of howieite is either large temperature factors. By analogy to other silicates, Pi or Pl. The structurecould be refinedsatisfactorily the octahedral corners which are not bonded to four in Pi to an R factor of 4.6 percent. Yet there are cations may be OH. The octahedra are more or less strong indications that Pl is only a pseudosymmetry HOWIEITE, A NEW TYPE OF CHAIN SILICATE 93 and that the true symmetry is Pl. The rather large Tesln 2. Atomic Coordinates and Isotropic Temperature Refined in SpaceGroup Pl* anisotropic vibration especially in the y-directio4 Factors for Howieite, (Table 3) may possibly be due to absorption effects. M(1,r) -0. 45599(9) 0 -29345(9) 0.23699(9) O.71(2) But divalent and trivalent ions with rather different M(1,2) -0.46434(B) 0.2609s(8) -0.42833(9) 0.48(2) : : M(1,3) -0.46138(9) 0,21 582(9) -0.09654(9) 0.68(2) M-O distances(Fe"*-O 2.17A; Fe''-O 2.045A: M(2,1) -0.15485(8) 0.42838(8) 0.30607(8) 0-56(2) : : M(2,2) -0.1s716(8) 0.42405(B) -0.36488(9) 0.60(2) Mg't-O 2.12 A; Al3*-o 1.93A according to M(2,3) -0. 14897(8) o. 43047(8) -0.03362(8) 0,46(2) Shannon and Prewitt, 1969) and very different Na 0 0 0 2.12<12' si(1,r) -0.21 392(r5) 0.08663Gs) 0.01664(16) 0.46(2') electron configurations are distributed over twelve si(1,2) -0.27312(r5) o 07690(1s) 0.32831(16) 0.48(2) si (2,1) 0. 0006s(r5) o.2207r(15) 0.47972(16) O.45(2) octahedral sites in the unit cell, and at least partiai si(2,2) -0. 00205(1s) 0.21802(1s) -0.26243(16) O.46(2' si(3,1) 0.27037(r5) 0.33955(15) -0.16660(16) 0.45(2) order is expected. There are in fact significant dif- si(3,2) a.27346(t5) o -34722(75) 0.14394(16) 0.53(2) (1, (4) a 4524(4) -0.0256(4) 1.05(6) ferences in refined occupancy factors (Table 6). o 1) 0. 4140 o(r,2) 0,3621(5) 0.07ss(5) 0.3153(s) 1.88(6) -0.4100(4) The smallest M(1, 2) site has also the smallest oc- 0(1,3) 0. 3960(4) o.o723(4) 0.83(5) o (2,1) -o.3779(4) o -2327(4' 0.0248(4) 0.72(6) factor and has presumably a high occupancy o(2,2) -0.328s(4) o.2r76(4) 0.3591(4' 0. 73(6) cupancy -0.3133(4) o (2,3) -0. 3408(4) 0.2244(4' 0.88(6) of Mg and Al. As has been discussedearlier, absorp- o(3,1) -0.0361(4) 0.3832(4) 0.0989(4) 0.75(6) o(3,2) -o.0277 (4) o.31 42(4) o.4293(4> 0.74(6) tion for the geometry of data collection is most 0 (3,3) -0.0300 (4) o.3114(4) -0.248r(4) 0.64(6) serious for high angle reflections, which thus do not o(4,r) o.265r(4) -0.4892(4) 0. r604(4) o.12(6) o (4,2) o.2851(4) -0.4s87(4) 0.4857(4) 0.95(6) contribute significantly to occupancy factors. There- o(4,3) o.2616(4) -0.4960(4) -0.1790(4) 0.91(6) o (s,1) -0.4313(4) -0.3249 (4) +o.2104(4\ 0.8s(6) fore we maintain that the encountered differences 0 (5,2) -o.4484(4) -o,3462(4) -0.4530(5) r.36(7) o(s,3) -0.4364(4) -0.3414(4) -0.107r(4) 0.86(5) are real and not artifacts. Notice also that occupancy o(6,1) -0. r288(4) 0.0996(4) 0.378r(4) 0.92(6) o(6,2) -o.1342(4) 0.0952(4) -0.1351(4) 0.81(5) factors for isotropic and anisotropic refinements are 0 (7,1) 0.1601(4) 0.2391(4) 0.2938(4) 0.86 (6) -0.2338(4) o(7,2) 0. 1506(4 ) 0.2300(4) 0 86(5) virtually identical and, although the deviation from 0 (8) -0.2084(4) 0. 0703(4 ) 0.1478(4) o.74 (6t -0.4239(4) o(9) 0.0047(4 ) 0.r631(4) 0.70(6) unity is only up to five times the estimated error, it o(10) o.2094(4) o.2t82(4' 0.0107(4) 0.78(6) seems to be significant. If there is partial order, then * Estimatederror of theleast significant digits in this and the order in the octahedral bands would destroy the followingtables is givenin parentheses. centrosymmetry.For instance the M(2,2) octahedron is related by a center to the directly adjacent octa- cients. But a refinement of just the M occupancies hedron M(2, 2C). An attempt to refine the structure in Pl produced significant differences in the pseudo- in Pl failed because of the high correlation coeffi- centrosymmetric atoms. A schematic display of the

o (4,1)|6

o ( t,t)s7 (t,2\2 ( t,3)ss

or2A Frc. 4. xy-projection of the structure of howieite, slightly schematic(atoms in chains along z-axis are drawn exactly on top of each other). z-coordinate,rounded to 2 decimals, given as subscriot. 94 H. R. WENK

o( t,2)06

o( 5, | )67

o(5,3C)

- X

o( 5,3)66

o(r,2c) ofi,3t7 o ( 5,2)65 u2 oL-t--;E Frc. 5. -xz-projectionof the structure of howieite, illustrating the octahedral band (unshaded) with the attachedtetrahedral chain (shaded). y-coordinaterounded to 2 decimalsgiven as sub- script. The shaded tetrahedra, whose apices point down, represent the [Si"O"] repeat unit of the chain.

Tanlp 3. Anisotropic Temperature Factors. gtr. of all Atoms Tasls 4. Interatomic Distances and Angles of Tetrahedra in in a Centric Refinement of the Howieite Structure" Howieite

corr€apondlng angle O-51-O P,105 sr(r,1)-o (1,lc) r.594(4' o(r.,1c)-o(2,1) 2.628<6' 109.812) H(1,1) 155(8) 302(8) (9) (6) -72(6' -s(6) -0(2,1) I91 87 7.6L7(4) o(L,rc)-o(6,2) 2.659(5) 1U.0(2) x(1,2) 101(8) 175(8) 129(8) -26(6) -r1(5) -o(5,2) 48(s) 1.637(4) o(1,rc)-o(8) 2.664(6' rr0,3(2) M(r,3) 160(8) 307(8) 155(8) 106(6) -38(6) -18(6) -o(8) 7.646<4) o(2,1)-0(6,2) 2.700(6) (2) M(2,r) 12r(8) 23t(8) 1:J8(9) -22(6) 7rr.4 50(5) 3(6) averaae L.624 0(2,r)-o(8) 2.688(5) M(2,2) 127(8) 252(8) 1s4(9) 68(6) -32(6) -26(6\ r1r.7(2) 0(6,2)-0(8) 2.5se(6) (2) tr(2,3) 87(8) r93(8) t32(9) 38(6) -35(6) -6 (5) ro2.4 averege 2.650 ro9.4 Na 1L9(52) 1429(66) 756(s5) 8se(at) 39(3e) 306(4 3) si(1,1) rr7(r2) r83(r2) 128(13) 55(9) -23(10) e(s) s1(1,2)-o(1,3C) 1.58s(4) o(r,3c)-o(2,2) 2.70r(6) 1r4.0(2) si(1,2) rl1(r2) 219(13) 126(13) 45(r0) -44(7O) -40(10) -o(2,2) 1.636(4) o(r,3c)-o(6,1.) 2.605(5) 107.9(2) si(2,r) r34(r2\ 185(12) r27(13) 83(10) -35(10) -15(9) -0(5,1) 1.635(4) 0(1,3c)-o(8) 2.674(6) rrr.8(2) si (2,2) 136(72) 792(12) 1r0(13) 72(9) -46(10) -1e(9) -o(8) 1.545(4) 0(2,2)-o(5,1) 2.694(6> 1r0.e(2) si(3,1) 95(12) 222(13) r4r (r3) 7s(10) -3e(10) -25 (10) average 7.625 o(2,2)-o(8) 2.66)-(5) 108.4(2 siG,2) 135(12) -22 -19 ) 25r(13) t22(r3) 122(70) (r0) (r0) o (6,1)-o (8) 2,s72(6) 103.3(2) averaSe 2.651 109.4

€'loq si ( 2,r)-o ( 3,2) r.613(4) 0(3,2)-0(6,1) 2.640(5' 109.9(2) _o 0(I,r) 18(3) 34(4) s0(4) 4(3) -13(3) - r /1\ (6,1) 1.613(4) 0(3,2)-o(7.1) 2.650(6) r09.7(2) 0(I,2) 6s(5) ArG) 67(5) r2(4\ -22(L' -9!l:1) L.627(4) o(3,2)-o(e) 2.687(s) 111.9(2) siii -u\', o(r,l) 22(3) 17(3) 2s(4) 0(3) 2(3' o(3) I.631(4) o(5,1)-o(7.r) 2,647<5) 109.6(2) o(2,r) 23(3' r8(3) 26(4) t(3) -7(3) 7(3) averase I.621 0(6,r)-o(9) 2.633(5> r08.5(2) o(2,2) 1e(3) r8(3) 26(4) 8(r) -2(3) 3 (3) o(7.r)_o(9) 2-622(6' t07.2(2) o(2,3) r8(3) 3O(3) 27(1' 10(3) -6(3) -1 (3) average 2.647 109.5 o(3,1) r3(3) 27(3) 26(4) 7(3) -s(3) 2(3) sr(2,2)-o(3,3) r.610(4) o(3,3)-0(6,2) 2.661(s) r10.3(2) 0(3,2) 19(3) 2s(3) 2s(4) 11(3) -9(3) -40) -o(5,2) 1.532(4) 0(3,3)-o(7r2) 2.672(s) 11r.0(2) o (3,3) 19(3) 2L(3) t8(3) -4(3) 10(3) +o(3) -o(7 '2' 1.631(4) o(3,3)-o(9) 2.654(5) 110.8(2) o(4,1) 23(3) 29(3) -7(3) -r(3) r8(4) 17(3) -0 (9) 1.516(4) O(6,2)-o(7,2' 2,602<6) 105.8(2) a(4,2) 22(3) 38(4) -9(3) 26(4) 14c) -2(3) average r.622 o(6,2)-o(9) 2.64s(6) 109.A(2) 0(4,3) 24(3) 28(3) 26(4' (3) _3 _s 14 (3) (3) o(7,2)-o(9) o(5,r) 13(3) 3s(4) 28(4) -6(3) -5(3) 2.6:s(5) r09.8(2) 10G) average 2.648 109.5 o(s,2) 58(4) 57(L) 31(4) 36(4) -2s(4) -10(3) o(s,3) 14(l) 34(4) 26(4) r1(3) -r(3) 4 (3) si (3,1)-o(4,3) r.624(4) o(4,3)-o(5,lc) 2,720(6> r14.6(2) -o(5,lc) 0(6,1) 25(4) 30(3) 40(4) 1o(l) -24(3) -s (3) 1,609(4) 0(4,3)-o(7,2) 2.670(6) 109.9(2) -o(7,2) 0(6,2) 19(3) 32(3) 21(4) 8 (3) 0 (3) 2(3) r.637(4) 0(4,3)-o(10) 2,67L(6) 109.r(2) o(7,1) 2s(3) 3s(3) 20(L) r5(3) -5(3) -r (3) -o (10) 1.6s5(4) o(5,rc)-0(7,2) 2.678(6) 11r.2(2) o(7,2) 24(3) 33(3) 33(4) 13(3) -16G) -14(3) average 1.631 o(5,1c)-0(10) 2,626(6) 107.r(2) 0(8) r8(3) 38(3) 2o(3) t4G) -9 (3) -6(3) o(7,2)-o(r0) 2.602(6> r04.4(2) 0(9) 29(3' 31(3) ro(3) t6(3) -5(3) -2 (3) avetage 2.66\ 709,4 0 (I0) 24(3> li (3) r5(3) 150) -3(3) -2 (3) sr(3,2)-o(4,1) 1.630(4) o(4, r) -o(5,3c) 2.702(s) r13.7(2) -o(5,3c) r.s95(4) o(4,1)-o (7 ,1) 2.686(6) 110.5 (2) * The form -o(7,1) 1.639(4) o(4,r) -o(r0) 2.665(5) 108.5(2) of the thermal ellipsoid is -o(10) r.654(4) o(s,3c)-o (7,1) 2.6s0(5) 1r0.0(2) average 1.630 o(5,3c)-o (10) 2.548(5) 109.1(2) exP - o(7.1)-o(10) 2.605(5) 104.6(2) [fu1h2I FrnkzI FaaP* z7nhk | 2?nhl 1- 29zakl. 2.559 r09.4 HOWIEITE, A NEW TYPE OF CHAIN SILICATE 95

TasI-s 5. Interatomic Distances and Angles of Octahedra in Tasr,r 6. Occupancy*ff:fr.i,:.. M and Na Positionsin Howieite

cenEric \efltrencnl Acentr ic Ref incmcnc, isol roPic corrc6pondlng angle 0-M-o rr-occupan' ies isotroDic anlscroPlc only l2 :l-occu?irtcres l2 refined and 30 o-Positions 3.409(i) e8.2(2) M(r,1)-o(r,2) 2.256(4') o(1,2)-o(2,r) re f ined -o(2,1) 2.256(5\ o (r,2)-O(2,2) 3.rs8(6) 87.8(2) -o<2,2) 2.r00(4) o(1,2)-0(5,2C) 2,933(6) 84.3(2) 7 t6i: 1 6JA -o (4,3C) 2.183(4) o(1,2)-o(s,3c) 1.044(6) 89.1(2) rtz I -197, 6 raz -o (s,2c) 2,111(s) o(2,L)-o(212) 3.182(5) 88.5(1) * .99r(lr) .994(l1) -o (5,3c) o(2,1)-0(4,3C)* 2,929<6' 82.6(2) (4) 2.0s6(4) li[]'l], .seoor .991 l 000(1I) 99C(rr) averaSe 2,r94 o(2,1)-0(5,3c),. 2.902(6> 84.5( 1) o (2, 2)-o (4,3c) * 3 03r(6) 8s.o(1) 884( r0) . 894( l0) (2) 11[],1], o(2,2\-o(5,2C) 2.830(6) 79.7 .sao

M(1,3)-o(1,r) 2.126(4) o(1,1)-o(2,1) 2.628(6) 82.4 (7) -o(2,1) 2.283(4' o(1,1)-o(2,3) 3.095(6) 91.8(2) occupanciesover the twelve M-sites is shown in -o ( 2,3) 2.18r(4) o(1,1)-o(5,1c) 3.321(6) 10r .6 (2) -o (4, lc) 2.289<4> o(1,1)-o(5,rc) 3 044(5) 92.6 (2) Figure 8. As can be seen,high and low occupancies (r) -o (5,lc) 2.164(4) o(2,1)-0(2,3) * 3.324(s) 96.2 -o (5, 3C) 2.086(4) o(2,r)-o(4,lc)* 2.90s(6) 78.9(1) do not seemto be simply alternating.These symmetry 2.188 o(2,r)-o(5,3c)* 2.902(6' 83.1(1) o(2,1)-o(4,1c)* 2.954(6) 82 7 (2) considerationscannot be satisfactorilyresolved with o(2,1)-o(5,rc) 2.686(6t 76.4 (2) o(4,rc)-o(5,lc) 3.311(6) 96.0(1) X-ray data. Unambiguous proof for noncentric o(4,lC)-o(5,3c) 3'169(6) 92.1(2) o(5,rc)-o(s,3c) 3.349(s) 104.0(2) structure would have to come from more sensitive 3.058 89,9 averaile methodssuch as magneticresonance studies. M(2,r)-o(2,2) 2-167(4) o(2,2)-o(3,1) 3.137(6) 9s.2(2) 2(1) -o(3,1) 2.081(4) o(2,2>-o(f,2) ^ 3.223(s) 95 The structural and chemicalformula derived from -o(3,2) 2.r98(4) O(2,2)-o(4,2c)+ 2.148(6) 80.6(2) -o(3,3C) 2.1s0(4) o(2,2)-o(4,3C) 3.031(5) 88.8(2) -a (4,2C) 2.080(4) o(3,1)-o(3,2) * 2.742(8> 96.8(2) -o (4,3C) 2.15s(4) o(3,r)-o(3,3C)+ 2.172(6) 81.8(2) avelage 2.t-40 o(3,1)-o(4,3c)* 2.196(6) 82.4(2) o(3,2)-O(3,3c)* 2.847(6' 8r.8 (1) o(3,2)-o(4,2c) 2.865(5) 84 0 (2) o(3,3c)-o(4,2c) 3.297(6) ro2.4 (2) o(3,3c)-o(4,3C) 3.161(5) 94.2(2) o(4,2c)-o(4,3c) 3.181(5) 9't r(2) average 2-983 90.0

Ir(2,2)-o(2,3) 2.142(4) o( 2,3)-o ( 3,2) 3.209(6) 97.4(2) -o(3,2) 2. r29 (4) o(2,3)-o(3,3) * 1.278(s) 99.7(2) 88 1(2) -o (3,3) 2. 146G) o (2,3)-o(1 ,1c) * 2.954(6) -o(3,2c) 2.160(4) o(2,3>-0(4tzc' 2,6/'6(6, 76.9(2) 92.4(2) -o(4,1c) 2.108(4) o(3,2)-o(3,3) ,r 3.086(5) -o -o (4, 2c) 2. 114(4) o G,2) (3,2c) * 2.768(8) 80.4(2) average 2.133 o(3,2)-o(4,2c)* 2.855(6) 8s 0(2) o(3,3)-c(3,2c)* 2 841(6) 82 8(r) o ( 3,3)-o (4,rc) 2.861(5) 84.5 (1) o (3,2c) -o (4 , rc ) 3.r28 (6) 94 3(2) o(3,2c)-o(4,2c' 3.288(6) 100.5(2) o (4,1C)-o(4,2c) 3.196(6) 98.4(2) 3.011 90.0

M(2,3) -o (2,1) 2.064(4) o(2,1)-o(3,1) 3.071(s) 94'5(2) -o(3,1) 2.115(4) o(2,1)-o(3,3) + 3.r26(6) 94.6(2' -o(3,3) 2.190(4) o(2,r)-o(4,1c); 2.905(6) 87.1(2) -o(3,Ic) 2.069(4) o(2,r)-o(4,3c) 2,929(6> 86.5(2) -o (4,lc) 2.149(tt) o(3,r)-o(3,3) + 1.307(5) 8r.2(?' -0(4,3c) 2.2O3(4) o(3,.r)-o(r,rc); 2.742(a) 81.8(2) 2,r32 o(3,1)-o(4.3c); 2.7e6(6) 80.7(2) o(3,3)-o(3,lc)- 2.172(6' 8r.2(2) o(1,3)-o(4,lc) * 2.86r(6) 82.5(r) o(1,lc)-o(4,lc) 3.1s3(5) 96 7(2) o(3,1C)-o(4,3C) 3.218(6) 91.1(2) o(4,rc)-o(4,3c) 3 245(5) 96'4(2) awerage 3.010 84 4

11(1,1)-M(r,2) 3.202(2' 11(2,1)-M(2,2) 3.149(2) M(1,1)-M (1,3) 3.2r7(2) M(2,1)-11(2,1) 3.213(2) u(1,1)-M(2,1) 3.196(2) M(2,1)-!l(2,2) 3'259(2) M(r,1)-M(2,1) 3.2r1lt H(2,1)-11(2,3c) 3.238(2) M(1,2)-M(r,3) 3.191(2) N(2,2)-Il(2,3) 3.204(2) x(r,2)-M(2,r) 3 25s(3) M(2,2)-il(2,]c) 3.2s9(2) u (1,2) -M(2,2) 3.250(2) M(2,2)-M(2,2c\ 3.271(2) M(r,3)-H(2 ,2 ) 3.20s(3) 11(2,3)-M(2,lc) 3.213(3) -M(2, M(2,3)-II(2,3c) 3'162(2) u( r, 3) 3) 3.290(2) FIc. 6. a. Schematic diagram comparing the howieite average distance structure (a) with a sheet silicate structure. b. z-projection indieates shued ed7es in howieite, y-projection in sheet silicate (mica setting). 96 H. R. WENK

Frc. 7. Stereoscopicpair of drawings displaying the coordination of Na in the howieite structure. View is along x. Ellipsoids give 95 percent probability contour of thermal vibration. the structuredetermination is Na(Fe'*, Mg, Fet*, with a [SiuO,r]-chain.This chain is intermediate Al)', ilSi60,,lr, lO, OHl,.l. To achieve a charge between a double and a single chain. Therefore balance,at least l0 of the 44 oxygens-presumably howieite resemblesan amphibole or a pyroxene. the free tetrahedralcorners of the [SiuO,r]-chainas But it is easierto relate it to sheetsilicates. This is well as some octahedral anions-must be OH-. illustrated in Figure 6 which comparesa z-projection The structual building unit of howieite is the of howieite with a y-projection of a sheet silicate octahedral band which is decorated on both sides (mica setting).The stackingsequence in both of them consistsof an octahedrallayer, a tetrahedral layer, and between two tetrahedral layers a highly co- ordinated large cation site. In howieite this site is 1' mainly occupiedby Na. The site which has l2-fold coordination in the ideal structure is distorted such that eachNa has only 8 exygensas closestneighbors (Figure 7). The "sheetplane" (001) in sheet silicates (Figure 6b) is (120) in howieite (Figure 6a). In con- trast to sheet silicates, the layers do not extend indefinitely but are interrupted after four rows of octrahedra and a second set of "sheet units" is insertedwith a dispfacement-l/6 x and l/2 y resulting in an intricate linkage resembling a brick' frame. Octahedralunits partially occupy the loosely packedlarge cation layers and thereforethe howieite structure is denser than an ordinary sheet silicate with more M and lessSi and Na in the corresponding formula unit [glauconite: [NanM,rSi,u(O,OH)n., V : 947 L", p : 2.4 to 2.45 g/gc,; howieite: NaM,rSi,r(O,OH)n', V : 851At, p : 3.38g/ccl. Howieitemay be the high-pressurebreakdown product of a sheet silicate. Its formation can be plausibly expected to be favored by shearing under high confining pressure, conditions which most likely prevailed in the metamorphic recrystallization of the Franciscanformation in California. Frc. 8. Variation of occupancy factors and averageM-O distance in the 12 M positions (compare also Table 6) Howieite resemblesand has been mistaken for (schematic "tz-projection). Shading indicates octahedra of stilpnomelanewhich occurswith it. It is most readily high, medium, and low occupancy. distinguished by its inclined extinction; otherwise HOWIEITE, A NEW TYPE OF CHAIN SILICATE 97 optical properties are similar. The rather unstable Franciscan of the Laytonville district, Mendocino County, structure of stilpnomelane (Eggleton, 1972'1 with California. Amer. Mineral. SO,27 8. BusrNc, W. R., K. O. MlnnN, rtto H. A. LBw (1962) A warping sheetsof octahedral cations can indeed be FonlneN crystallographic function and error program' visualized as some intermediate state between a tl. S. Nat. Tech. Inform..Sero. Onxr-ru-308. stable sheet silicate and howieite. A rewarding ConrrBr.o,P. W. R., R. I. DoeoeNs,lNo J. A. IsEns (1967) project for investigation will be the stability field Studies of metal-nitrogen multiple bonds. I. The crystal (diethyl- of howieite and possibly breakdown reactions, and molecular structure of nitridodichlorotris phenylphosphene rhenium (V), ReNCL[P (C,Hu CuHu]o related ) ), especiallyin comparison to the structurally Innrg. Chem. 6, 197-204. coexisting minerals, stilpnomelane,amphibole, and Cnouen, D. T. (1965) Anomalous dispersion corrections deerite which form this interesting high-pressure, computed from self-consistent field relativistic Dirac' low-temperatureassemblage in the Franciscan for- Slater wave functions. Acto Crystallogr. 18, 17'23, mation. -, eNo I. MANN (1968) X-ray scattering factors computed from numerical Hartree-Fock wave functions. Acta Acknowledgments Cry stallogr. L 2A, 321-324. DUEsLER,E. N., exo K. N. RevuoNo (1971) Conforma- The mineral was collected on a field trip with Dr. tional effects of intermolecular interactions, The structure S. Agrell. The data set was collected on the Picker diffrac- of tris-(ethylene diamine) cobalt (III) monohydrogen tometer at the Lawrence Berkeley Laboratory through the phosphate nonohydrate. I norg. C he m. 10, 1486-1 492. courtesy of Dr. A. Zalkin. Mr. D. Greig assisted in the Eccr,moN, R. A. (1972) The crystal structure of stilpnome' initial data processing. I acknowledge the help of Dr. lane. Part II. The full cell. Mineral. Mag.38,693-:lll. S. Goldberg and Ms E. N. Duesler in questions of computer SneNNox, R. D., eNo C, T. PnewITT (1969) Effective ionic progriuns, and the stimulating discussionswith Dr. S. Ghose radii in oxides and fluorides. Acta Crystallogr. R25,925- and Professor D. Templeton. The calculations were done 946. partially on the CDC 6400 computer on the Berkeley Welr' H. R. (1973) The crystal structure of howieite. campus, partially on the CDC 7600 of the Lawrence N aturwissenschatten, 6O, 25 4-255. Berkeley Laboratory. ZecnenhseN, W. H. (1968) TeMe: Experimental tests of the general formula for the integrated intensity of real Refetences crystals. Acta Crystallogr. L 24, 212, AcRBLL,S. O., M. G. BowN, eNo D. McKrB (1965) Deerite, Manuscript receioed,Iune 21, 1973; accepted howieite, and zussmanite, three new minerals from the for publication, October 8, 1973'