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 sym- metry 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,temperature factors -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.