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The Crystal Structures of One-Layer Phlogopite and Annite

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The Crystal Structures of One-Layer Phlogopite and Annite American Mineralogist, Volume 58, pages 889-900, 1973 The CrystalStructures of One-LayerPhlogopite and Annite RosEnr M. HezEN, lNo CnnnrEs W. BunNnlvr Department ol Geological Sciences,HarDqrd Uniuersity, C ambridge, M ass achusen s 0 2 I 3 8 Abstract The crystal structures of natural fluorophlogopite and annite have been refined by least- squares analysis of three-dimensional X-ray diffraction data to weighted R-values of 4.1 percent and 4.4 percent respectively. Both of these lM trioctahedral micas possess monoclinic spacegroup symmetry C2/m. Averagebond lengths for phlogopite are (Al,Si)-O 1.649,Mg-(O,F,OH) 2.064,innerK-O 2.969 and outer K-O 3.312A; while those of annite are (Al,Si)-O 1.659,(Fe,Mg,Ti)-(O,OH) 2.108,inner K-O 3.144 and outer K-O 3.216A. Atomic fractional coordinates closely conform to the "ideal" mica structure as defined by Pabst (1955). The only significant deviations from this ideal model are tetrahedral '7.5" rotation (o) and octahedral flattening (p). For phlogopite a : and 'y' - 58.95', while for annite d: 1.5' and,l, - 58.40'. These values agree with distortionspredicted by Hazen and Wones (1972). Anisotropies in apparent atomic vibrations may be bbrrelated with positional disorder of aluminum and silicon in the single tetrahedral site. Magnitudes of these apparent vibrations are greater in annite, reflecting the more extensive cation substitutional disorder in that specimen. No ordering was detected in the octahedral sheet of annite. At least 6 percent octahedral vacancies are indicated from both least-squaresrefinement and chemical analysis. These vacancies are evenly distributed between Ml and, M2, and thus do not represent solid solution between trioctahedral micas and the dioctahedral variety in which only the M2 sites are occupied. Introduction and illustrated by several authors (e.g., Bragg et al a The first researcher to describe an X-ray diffraction 1965, p. 254 et seq). ln the ideal mica structure, fit between examination of mica was Mauguit (1927,1928), who sheetofregular octahedrais constrained to rings of determined the symmetry, unit cell dimensions, and two sheets of interconnected hexagonal oxygens cell content of several natural specimens. Shortly tetrahedra. Furthermore, basal Ol and 02 oxygen and thereafter the basic crystal structure of mica was are coplanar in (001), as are apical 03 the proposed almost simultaneouslyby Pauling (1930)and hydroxyl groups. The above conditions imply that site Jackson and West (1930, 1933). While several sub- interlayer cation is in the twelve-coordinated A sequent studies established the existence and geometry formed by a hexagonal prism of basal oxygens. geometrical con- of complex twinning and stacking arrangements in further consequence of ideal mica these layer silicates (e.9., Hendricks and Jefferson, straints is the orthohexagonal unit-cell relationship, y'\a. 1939; Smith and Yoder 1956; Ross, Takeda, and b: micas is most Wones, 1966), it was not until 1959 that a three- The structure of real trioctahedral from dimensional structure analysis of a trioctahedral mica conveniently discussed in terms of deviations the was carried out by Takeuchi and Sadanaga(1959). A the ideal model. Such deviations result from list of several recent studies on the oneJayer triocta- differing sizes of ideal octahedral and tetrahedral it hedral mica crystal structure is given in Table l sheets,which share the 03 oxygens. For example, Pabst (1955) presented theoretical atomic co- is well known that the ideal AlSi, tetrahedral sheet ordinates for an "ideal" C2f m, one-layer, trioctahedral of phlogopite is larger than the regular Mg3 octa- mica; and this model has been subsequently described hedral sheet. Thus, these two sheets must coincide 889 890 R. M. HAZEN, AND C, W, BURNHAM Tesrr. 1. Previous Studies of One-Layer Trioctahedral difficulty of obtaining undeformed cleavage frag- Mica Crvstal Structures ments of layer silicates, the specimen was carefully nenen! Authors (Date) Speclnen examined for single crystals of a size appropriate Condltlons/Results for X-ray diftraction studies. Eventually, a suitable Takeuchl & ^4r | ! | rvPrrJ f rr u c T.^fr^ht. ?n fllm Sadanaga Naturaf R=131 euhedral hexagonal plate 60p thick, and 400p in (r959,1966) maximum width, was selected. The structural for- Zvyagln & PhLogopl te 2D Electronography Franklin phlogopite, calculated from a wet lillshchenko Nat ura I R . I7t mula of (1962) chemical analysis (seeTable 2), is (Ko 77Na616Bae.,65) Stelnflnk Fe rr1- Ph logop 1 t e Yc^tF^ht^ ?n ftln Mg3 which closely (1952) Natural R - 13i n,,Al1n5Si2 e5Olo,iro(F1 311OH6 6), approximates the ideal phlogopite formula, KMgr Donnay et. -aI. Ferrl-Annlte lc^tn^^l ^ ?n f{ lh ( 19'64tt Synthetlc R-2lfi AlSiBOlo(F,OH)r. Optical properties of the crystal Franzlnl -et. -al. 51x Blotltes z atomlc fractlonal selectedate a: 1.530,F : y = 1.558and 2V = 0". (1953) NatUral coordlnates fron 001 data The indices of refraction are slightly higher than Takeda & Llthlum FIuor-M1ca lhl c^tF^hl ^ those of pure synthetic fluor-phlogopite (y = 1.550, Donnay Synthetl c 3D counter ( 1966) R = 4.81 Hatch et al, 7957), and considerably lower than the McCauley(1968) Fluor-Phlogoplte lhl <^fF^h{. indices of synthetic hydroxy-phlogopite (y = 1.587, and Synthetlc 3D counter McCauleyet. and R = 6.1.1 Hazen and Wones, 1972). a1. (r973r Ba- Ll -Fluor- Ph logop 1 te Synthetlc Sp ecimen D escri ption- A nnite taJ hLl ; af -.1 Iron Blotlte Tc^fF^n{. in fl th ( 1959)- Natural R.14i Specimensof an iron-rich, igneous biotite from the Takeda & F luor- Poly I1 thlonl t e Isotroplc, generously Burnhan Synthetlc 3D counter Pikes Peak Granite, Colorado, were ( 1959) R = 5.1' provided by Dr. F. Barker of the United States Geological Survey. After opticat and X-ray examina- tion of more than thirty separated annite grains, an either by expansion of the octahedral sheet or by undeformed, approximately rectangular plate, 250p.X contraction of the tetrahedral sheet in the (001) 400p X l00p thick, was found and selectedfor further plane. Radoslovich and Norrish (1962) suggestfour study. Wet chemical analysisof Pikes Peak annite was mechanisms to accomplish this: 1) bond length provided by Barker (Table 2). The resultant structural alteration within the ideal sheets,2) tetrahedral sheet formula, calculated on the assumption that (O * tilting or corrugation, 3) octahedral sheet flattening, oH+F+Cl):12,is and 4) tetrahedral sheet rotation. Octahedral flatten- ing and tetrahedral rotation are believed (Ko orCaoo.)(Fell, Fe3]rnTio ,rMgo ,rMno ou to be the "rNao most important distortions in trioctahedral micas Alo onno,,)(Al, ,osi,,,XOro 3sOHl'sFO.2rClo ou). (Donnay, Donnay, and Takeda, 1964a). A gamma-ray resonant absorption (Mossbauer) The present study is an attempt to determine the u'Fe spectrum of in Pikes Peak annite was measured extent of deviations from Pabst's ideal mica struc- by Professor Roger Burns of the Massachusetts ture for two natural specimensand to compare these Institute of Technology, and confirmed the ratio deviations with values predicted by Hazen and Fe'*:Fe'* : 10:l in octahedralcoordination. Due Wones (1972). In addition. careful examinations of to the possibility of local variations in biotite com- anisotropic atomic thermal vibration ellipsoids and position, the singlecrystal under investigationwas also octahedral occupanciesand cation distributions have analyzed by electron microprobe (Table 2). No zoning been made to aid our understandine of trioctahedral was observed, and the composition determined by mica crystal chemistry. microprobe analysis is in close agreement with wet chemical data. Optical properties of Pikes Peak annite Experimenfal areot: 1.624,P: "y : l.672and 2Z: 0'. These Spe cimen D ercri ption-P hlogopit e values are similar to those obtained by Wones (1963) for a biotite with Fel(Fe + MC) : 0.75. Specimens of colorless euhedral phlogopite, em- bedded in calcite from the marbles of Franklin. New X-ray Diffraction Jersey (Harvard Museum #99276), were kindly Unit cell parameters of phlogopite and annite supplied by Professor Clifford Frondel. Due to the were determined using a precision back-reflection ONE.LAYER PHLOGOPITE AND ANNITE 891 'Il^B.rB Weissenbergcamera and measurementsof both Cu 2. Chemical Analvses Ka1 and Ka2 reflections. These data were refined using a least-squares program LCLse (Burnham, oxlde Franklln PlkcE P.ak Annltc Phlogopl te1 l{et Analysls2 Mlcroprobe3 1962) that corrects for systematic errors due to film ut-l lr* ta uj---L ltea-tq u!-L Alea_La shrinkage, specimen absorption, and camera ec- 919r- \2.0 2.95o 33.96 2.81 33.92 2.84 centricity. Results of these measurementsare listed lAr20r r2,7 1.050 r3.to 1.19 rr.6z t.t6 r (rv) in Table 3. Careful examination of a- and b-axis 4 .000 4.00 lr.oo Olevel Weissenberg photographs revealed no twin- tAl2O,(VI) O.OOo 0,09 0.0r lFe20! 0.00 0.000 3.06 t ning or complex stacking arrangements. Precession Feo 9.25 0.010 32.0/l Z.iZ 35.\2 2'.'rg M8O 28.6 2.980 0.97 0.12 0.89 0.11 photographs confirmed the diffraction symbol MnO 0.08 0.000 0,65 0.05 0.611 0.05 as N10 0.02 0.000 0.00 0.00 2/mC-/-, resulting in C2, Cm and C2/m as possible zno 0.01 0.000 0.00 0 ,00 T1O2 0'2? 0.008 3.55 0,22 0.23 spacegroups 3.61 . C2 / m was selectedas the trioctahedral r (vr) 2.998 2,89 2,93 mica space group, based on the discussion of Pabst |xao 8. !5 0.759 8.117 0.88 8.9! 0.95 (1955) and the experience of previous investigators lNaro 1.19 0,160 0.119 0.07 0.10 0, 02 IrLlrO 0.01 0.000 0.00 0,00 (seeTable 1).
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