Lead Article Earth's Deep Interior*
561Lead ArticleActa Crvst. (1991). B47, 561-580Crystal Chemistry of Six-Coordinated Silicon: a Key to Understanding theEarth's Deep Interior*BY LARRY W. FINGER AND ROBERT M. HAZENCarnegie Institution of Washington, Geophysical Laboratory and Center for High Pressure Research,5251 Broad Branch Road NW, Washington, DC 20015-1305, USA(Received 26 November 1990; accepted 8 April 1991)feature of all these low-pressure mineral structures isthe presence of silicon cations exclusively in fourA survey of high-pressure silicates reveals 12 distinctcoordination (ttV]si) by oxide anions. The polymerizhigh-density structural topologies with octahedral Si.ation of SiO4 groups dictates many mineral propertiesSeven of these structure types - stishovite, perovskite,and it provides the basis for most silicate classificationilmenite, hollandite, calcium ferrite, pyrochlore andschemes.K2NiF4 type - contain only six-coordinated silicon.Research on silicates synthesized at high pressuresOther high-pressure silicates, including those with theand temperatures plays a major role in our efforts togarnet, pyroxene, wadeite, anhydrous phase B andunderstand the earth's deep interior. Cosmochemicalphase B structures, contain both tetrahedral andassumptions regarding the earth's bulk composition,octahedral Si. Five systematic trends among thesecoupled with seismological investigations of radialdozen structures suggest the existence of other, as yetinhomogeneities, establish important constraints forunobserved, possible mantle Si phases. The criteriamodeling our planet. Even so, actual crystallineare: (1) structures like rutile, hollandite and calciummaterials, erupted from depths of more than 100 kmferrite formed from edge-sharing chains of siliconor produced in high-pressure laboratory apparatus,octahedra; (2) germanates synthesized at room pressprovide our best opportunities for deriving a detailedure with octahedral Ge; (3) isomorphs of roompicture of the inaccessible 99"8% of the earth's solidpressure oxides with 3 or 4 transition-metalvolume. The first experiments on common rockcations; (4) high-pressure magnesium silicates relatedforming silicates at pressures up to 10 GPa (100 kbarto room-pressure aluminates by the substitution 2AIor approximately 100000 atmospheres) revealedM g Si; and (5) the homologous structures in systemstrikingchanges in mineral structure and properties.M g - S i - O - H that includes phase B and anhydrousSergei Stishov's seminal investigation of SIO2, forphase B. Each of these criteria can be used to predictexample, demonstrated the transition from a relaother potential octahedral Si phases. Of special intertively open quartz framework of corner-sharing siliest are predicted structure types that fulfill morecate tetrahedra to the dense rutile-type structure ofthan one criterion: diaspore-type (MgSi)O2(OH)2,stishovite with edge-sharing chains of silicateaerugite-type Mgl0Si3Oi6 , sphene-type CaSi2Os,benitoite-type BaSi409, gibbsite-type MgSi(OH)6 and octahedra (Stishov & Popova, 1961). The corresponding increase in d e n s i t y - m o r e than 66%, from 2.65pseudobrookite-type Fe2SiOs.to 4 . 4 1 g c m -3, between 0 and 8 G P a (Ross, Shu,Hazen & Gasparik, 1990)- has profound implicaIntroductiontions for interpretation of seismic velocity data.Subsequenthigh-pressureexperimentshaveSilicon and oxygen are the most abundant elementsdemonstrated that all common crustal silicatesin the earth's outer layers. Silicates comprise theundergo phase transitions to new structures with sixcommonest minerals on the earth's surface and precoordinated silicon (tVnsi) at pressures betweensumably they dominate throughout the earth's mantle8 GPa (for pure SiO2) and about 30 GPa, which corre(to a depth of about 2900 km). Many hundreds ofsponds to the pressure at the top of the earth's lowersilicate structures have been determined andmantle. Many researchers now assert that thecatalogued (see e.g. Liebau, 1985), but only about 50dominant mineral structure type in the earth's lowerdifferent structures account for the vast majority ofmantle - indeed, the structure that may account forall crustal silicates (Smyth & Bish, 1988). A commonmore than half of the solid earth's v o l u m e - i sof theapproximatecomposition* Editorial note: This invited paper is one of a series of compre- perovskitehensive Lead Articles which the Editors invite from time to time (Mgo.88Fe0.12)SiO3, in which silicon occurs in a coron subjects considered to be timely for such treatment.ner-linked array of octahedra. Silicate perovskites,Abstract0108-7681/91/050561-22503.00O 1991 International Union of Crystallography
562CRYSTAL C H E M I S T R Y OF S I X - C O O R D I N A T E D SILICONmixed with the oxide magnesiowiistite (Mg,Fe)O, arethus believed to account for the relatively high seismicvelocities of this region, from 670 to 2900 kin, in whichvelocities increase smoothly with depth (Fig. 1).Mineral physicists identify silicon coordination number as the major crystal-chemical difference betweenthe earth's crust and lower mantle: silicon is virtuallyall four-coordinated above about 250km, but isentirely six-coordinated below 670 km.The upper mantle and transition zone, which lieabove the lower mantle, possess a much more complex seismic character with depth. This region,extending to a depth of 670 km, displays several discontinuities and changes in slope of the velocitydepth profile (Fig. 1). Such features might be causedby either compositional variations or phase transitions. However, given the suspected pattern of mantleconvection and the well documented variety of high-½: piVPi!o li'[ .1.o:dO00I0(}0:]000Depth (km)Fig. 1. Velocity-density profile for the earth's crust and mantle.MgHi() 3 5o"-":gO,.-iP ". ".PV"".Ll,.):: . . t - ;i'": . ,; . .i l '. :?. . -:::i . pressure phase transitions in silicates, the latter explanation seems the more plausible. Specific phasetransitions have been proposed for each of the majorseismic features and mineralogical models have beenproposed that account for most of the complexitiesbetween 200 and 670 km (Fig. 2).One of the most fascinating aspects of the earth'stransition zone is the appearance of a group of highpressure silicates with both ['V]si and tVtJsi. The stability of these minerals is apparently confined to a rathernarrow pressure range from approximately 10 to30GPa. Within these limits, however, are silicatestructures of remarkable complexity and great topological interest. Five mixed-coordination highpressure silicate structures are now known for relatively simple chemical systems that contain Si andone or two other cations, but recent high-pressureexperiments on more-complex systems suggest thatmany other mixed silicon coordination structures mayawait discovery.While earth scientists have studied six-coordinatedsilicon in high-pressure environments, other researchers have focused on an intriguing group ofroom-pressure synthetic silicates with IWlsi. Whencoupled with other electronegative cations such asphosphorus or carbon, silicon can become an octahedrally coordinated network former. Polyhedra inthese unusual materials tend to be linked entirely bycorner sharing (i.e. 0 atoms are two-coordinated),thus leading to framework structures quite distinct incharacter from the high-pressure silicates of theearth's deep interior, all of which have O atoms in atleast three coordination. Octahedral corner sharingalso occurs in a variety of organic molecular crystalswith [VUsi (see, for example, Flynn & Boer, 1969)and there are several dozen silicon-organic compounds for which [Wlsi occurs in aqueous solution(Liebau, 1985). Molecular crystals with six-coordinated silicon are not considered in this review.The objectives of this review are to describe allknown non-molecular silicate structures with sixcoordinated or mixed-coordinated silicon, to identifycrystal-chemical similarities among these structuresand to suggest other possible high-pressure silicatestructure types.Review of structures with six-coordinated silicon15t)X10:1100015001,.!. . . . .20001J2,500' 000Temperature (K)Fig. 2. The phase diagram for MgSiO 3 reveals three t v, JSi phases perovskite (PV), garnet (GT), and ilmenite ( I L ) - as well aspyroxene (PX), spinel (SP), stishovite (ST),/3-Mg2SiO4 (/3) andliquid (L) (from Fei et al., 1990).There are fewer than 20 known structures with SiO6polyhedra (Table 1). These silicates can be dividedconveniently into three groups. Above about 25 GPa,corresponding to the earth's lower mantle, all silicatesstudied to date are observed to transform to one ofseven dense structures in which all Si is six-coordinated. These structures- rutile, perovskite, ilmenite,hollandite, calcium ferrite, pyrochlore and K2NiF4are well known room-pressure topologies for transition-metal oxides. In the high-pressure silicate
LARRY W. F I N G E R A N D ROBERT M. HAZENTable 1. Compositions and calculated densities ofsilcates with SiO6 octahedraMineralStructureCompositionnametype(a) High-pressure phases with S i O 6 groups onlySiO2CaSiO3MgSiO MgSiO3ZnSiO3KAISi308BaAI2Si208CaAI2Si20 NaAISiO4Sc2Si207In2Si207CasSiO4Stishovite-RutileC u b i c perovskiteO r t h o anditeC a l c i u m ferritePyrochlorePyrochloreK 2 Ni F4(b) High-pressure phases with SiO6 SiO 4 groupsMgSiO3MajoriteGarnetMnSiO3GarnetNa( hydrous phase BMgt2SiaO ,(OH)2Phase B(c)Compoundswith SiO 6 synthesizedSiP207-1SiP2OT-IIISiP2OT- ,)ThaumasitePo calc*3-514-323.283.093.443.37at r o o m p r e s s u r eZrP207-Table 2. The SiO2 stishovite structureTetragonal,P42/mnmV 4 6 . 6 , 3.(g c m -3)4.294.254.103.815-253-915.33.93'914.286-343- 563-223-053-1 l2"662"371.87* Po calc density calculated from unit-cell parameters at roompressure and temperature.isomorphs silicon occupies the octahedral transitionmetal site, while other cations may adopt six or greatercoordination.At pressures between about 10 and 20 GPa somesilicates form with mixed four and six coordination.These high-pressure phases, all of which may occurin the earth's transition zone, include silica-richmodifications of the well known garnet, pyroxene andwadeite structures, as well as complex new magnesium-bearing phases designated 'phase B' and'anhydrous phase B'.The third group of rV lsi silicates, including avariety of silicon phosphates, is distinguished by relatively open framework structures with corner sharingbetween silicon octahedra and other polyhedra ofelectronegative cations, notably [tVlp. We describeeach of these structures in the following section.High-pressure silicates with all rV ]siThe stishovite structure. Stishovite, the form of SiO2synthesized above 10 GPa, is believed to be the stableform of free silica throughout most of the earth'svolume. In addition to its assumed role in mantlemineralogy, stishovite has elicited considerable interest as a product of the transient high-pressure hightemperature environments of meteorite impacts(Chao, Fahey, Littler & Milton, 1962). The discovery563( O14 4hl,Z 2,SiteSymmetrySi2(a)04(f)mmmmma : b 4-18,xc2.67 ,yz0000.306x0of stishovite grains in sediments near the CretaceousTertiary boundary layer (McHone, Nieman & Lewis,1989) has provided support for the hypothesis that alarge impact, rather than volcanism, led to a massextinction approximately 65 million years ago.Stishovite has the simple rutile (TiO2) structure(Table 2; Fig. 3), with edge-linked chains of SiO,octahedra that extend parallel to the c axis andoctahedra corner linked to four adjacent chains. Twosymmetrically distinct atoms - Si at (0, 0, 0) and O at(x, x, 0) with x approximately 0 . 3 - define the structure in space group P42/mnm.The first stishovite structure refinements wereobtained by powder diffraction on small syntheticsamples (Stishov & Belov, 1962; Preisinger, 1962;Baur & Khan, 1971). A much improved refinementwas presented by Sinclair & Ringwood (1978), whosynthesized single crystals up to several hundredmicrometres in diameter. Subsequent single-crystalstructure studies by Hill, Newton & Gibbs (1983)under room conditions and by Sugiyama, Endo &Koto (1987) and Ross et al. (1990) at high pressure,amplify the earlier work (Table 3).The silicate perovskite structure. Synthesis and structural description of silicate perovskites (CaTiO3) haveposed a significant challenge to earth scientists sinceRingwood (1962, 1966) originally suggested theexistence of perovskite forms of MgSiO3 and CaSiO3.High-pressure transformations from pyroxene andgarnet structures to perovskite in the analogous systems Ca(Ge,Si)O3 and Ca(Ti,Si)O3 (Marezio,Remeika & Jayaraman, 1966; Ringwood & Major,1967a, 1971; Reid & Ringwood, 1975) supported thishypothesis. Pure silicate perovskites were first produced at the Australian National University (Liu,1974, 1975a, b, 1976a, b,c; Liu & Ringwood, 1975) andresults were quickly duplicated in Japan and theUnited States (Sawamoto, 1977; Ito, 1977; lto &Matsui, 1977, 1978, 1979; Mao, Yagi & Bell, 1977).These workers demonstrated that above pressures ofabout 27 GPa many silicates transform to the perovskite structure, in which silicon octahedra form athree-dimensional corner-linked network, whilelarger R cations fill positions with oxygen coordination of eight or greater. By the late 1970s many earthscientists were persuaded that the earth's 670 km seismic discontinuity, which divides the transition zonefrom the lower mantle, coincides with a perovskitephase-transition boundary, and that perovskite ofapproximate composition (Mgo.gFeo,)SiO3 is a
564CRYSTAL C H E M I S T R Y OF S I X - C O O R D I N A T E D SILICONTable 3. Stishovite structure refinementsDistances are given in / and angles in o(1)(2)(3)(4)acXo4.1772 (7)*2-6651 (4)0-3062 (2)4.1773 (1)2-6655 (I)0.30608 (6)4-1797 (2)2.6669 (I)0.30613 (7)4.1801 (6)2.6678 (6)0.3067 (3)Si-O [4]Si-O [2]1.7568 (5)1.8089 (10)1.7572 ( 1)1.8087 (2)1-7582 (2)1.8095 (3)1-7564 (6)! -8130 (10)Mean Si-O1.774O-Si-OOctahedral volume (/ 3)tQuadratic elongationtAngle variancet1.77481.34 (5)7.359 (4)1-0081 (3)27.381-34 (I)7-361 ( I )1-0081 (1)27.31.77581.35 (2)7.373 ( I )1.0080 ( 1)27.31.77581-17 (5)7-369 (6)i-0084 (2)28.4References: ( I ) Sinclair & R i n g w o o d (1978), R 0 . 0 1 5 ; (2) Hill et al. (1983), R 0 . 0 1 2 ; (3) S u g i y a m a et al. (1987), R 0 . 0 1 5 ; (4)Ross, Shu et al. (1990), d a t a collected on crystal in h i g h - p r e s s u r e cell, R 0.014.* P a r e n t h e s i z e d figures r e p r e s e n t e.s.d.'s.t O c t a h e d r a l v o l u m e c a l c u l a t e d with V O L C A Lby R o b i n s o n , G i b b s & Ribbe (1971).( H a z e n & Finger, 1982). Q u a d r a t i c elongation and angle v a r i a n c e are as d e s c r i b e dTable 4. The cubic ( a) and orthorhombic ( b ) silicateperovskite structures( a ) CaSiO3, cubic, P 4 / m 3 2 / m3.567 (1) A, V 4 5 - 3 7 ( 8 ) / 3 ,SiCaO(O ),a b c SiteSymmetryxyzl(a)I(b)3(d)m3 rnm3m4/mmrn0 0 00(b) (Mg,Fe)SiO3, o r t h o r h o m b i c , P b n mb 4.93, c 6-90/ , V 162.4/ 3MgSiO!02Z I,(D2h),16 Z 04, a 4 - 7 8 ,SiteSymmetryxyz4(c)4(b)4(c)8(d)m1mI0.51 0-100'200-5600-470-200-55-* F r o m M a o et al. (1989).dominant lower mantle mineral (Anderson, 1976; Liu,1977a, 1979; Yagi, Mao & Bell, 1978).The simplest perovskite variant is the cubic form,represented by CaSiO3, which is stable above about15 GPa (Table 4a; Fig. 4a). This phase, first synthesized by Liu & Ringwood (1975), has a structurethat is completely specified by the cubic cell edge, a,because all atoms are in invariant special positions.The Si- and O-atom positions, for example, are(0, 0, 0) and (1/2, 0, 0), respectively, so the Si-O distance of the regular silicon octahedron is a/2.Similarly, the octahedral volume is a3/6. Calciumsilicate perovskite cannot be quenched metastably toroom pressure; samples invariably transform to glassupon release of pressure. Nevertheless, equationof-state measurements of CaSiO3 by Mao, Chen,Hemley, Jephcoat & Wu (1989) to 134 GPa definethe structure as a function of pressure and allowreasonable extrapolation to room-pressure values(Table 4a).The corner-linked silicate perovskite frameworkwill tilt to accommodate divalent cations smaller thanCa. Thus, the structure of (Mg,Fe)SiO3, widelythought to be the earth's most abundant mineral, isorthorhombic (Table 4b; Fig. 4b). Silicon occupiesnear-regular octahedral coordination, while magnesium is in a larger site with eight nearest-neighborO atoms. Orthorhombic cell parameters possess a2v x 2x/2x 2 relationship to the simple cubic axes.Initial studies of this structure were performed byYagi, Mao & Bell (1978, 1982) and lto & Matsui(1978) on powders. Recent synthesis by Ito & Weidner(1986) of single crystals has led to much more precisestructure refinements under room conditions(Horiuchi, Ito & Weidner, 1987) and at high pressure(Kudoh, lto & Takeda, 1987; Ross & Hazen, 1990),as recorded in Table 5.All silicate perovskite structure studies based onX-ray diffraction indicate complete ordering of Si andthe divalent cations in the octahedral sites and thelarger sites, respectively. Recently, however, Jackson,Knittle, Brown & Jeanloz (1987) examined a polycrystalline iron-bearing silicate perovskite of approximate composition (Mgo.9Feo.l)SiO3 with EXAFS.Observation of significant [Vl]Fe led them to proposethat some Si enters the eight-coordinated larger site.Single-crystal studies by Kudoh, Prewitt, Finger,Darovskikh & lto (1990) and powder diffraction databy Parise, Wang, Yeganeh-Haeri, Cox & Fei (1990)on iron-bearing samples do not support this interpretation - they find all Fe in the larger site, as would beexpected from crystal-chemical arguments. Kirkpatrick, Howell, Phillips, Cong, lto & Navrotsky(1991) came to the same conclusion based o n 29SiNMR spectroscopy of MgSiO3 perovskite.The ilmenite structure. The ilmenite (FeTiO3) andcorundum (a-m1203) structures have long beenrecognized as likely candidates for high-pressure silicates in which all cations assume octahedral coordi-
LARRY W. FINGER A N D ROBERT M. HAZEN565Table 5. MgSiO3 perovskite structure refinementsDistances are given in , and angles in (1)abcXMg(2)(3)Xo Yo xo2YO2zo24-7754 (3)4.9292 (4)6.8969 (5)0.524 (6)0.561 (5)0.096 (8)0.468 (12)0-201 (9)0"205 (8)0-558 556Si-O1 [2]Si-O2 [2]Si-O2 [2]1.79 (1)1.79 (1)1.79 (1)1.79 (1)1.76 (3)1-82 (3)YMsMean tahedral volume (A3) *Quadratic elongation87.988.689.47.65 (17)i-002 (15)6.3Angle 87 (4)4.9313 (4)6-9083 (8)0.5141 (1)0.5560(I)0.1028 (2)0.4660 (2)0-1961 (1)0"2014 (2)0.5531 (1)4-745 (2)4.907 (1)6.853 (2)0"5169 (14)0.5573 (10)0.1087 (24)0.4659 (22)0.1965 (15)0"2037 (15)0.5558 (7)4.772 (2)4.927 (I)6-8977 (I)1'5131 (7)0.5563 (4)0.1031 (12)0.4654 (9)0.1953 (7)0"2010 (6)0.5510 (4)1.8005 (3)1.7827 (7)1.7960 (7)1.797 (3)1.794 (7)1.769 (7)1.801 (I)1.795 (4)!-779 (3)1.7931.7871.79288.66 (5)88.49 (5)89.43 (3)88.6 (4)88.8 (4)89.4 (3)88.2 (2)89.0 (2)89.59 (5)7.63 (15)1.001 (15)2.37-702 (3)1.0005 (3)1.67-60 (3)1.001.47.657 (14)1.005 (16)1-7References: (1) lto & M a t s u i (1978), based on powder X R D , (2) Yagi et al. (1978), based on powder X R D , R 0-10, (3) Horiuchi e tal. (1987), single crystal in air, R - 0.035; (4) Kudoh e t ai. (1987), single crystal at 4.0 GPa, R 0.079; (5) R o s s & H a z e n (1990), singlecrystal in pressure cell at room pressure, R - - 0 . 0 4 2 .* See Table 3.Table 6. Refinement of the MgSiO3 ilmenite structure, from Horiuchi et al. (1982), R 0.049Rhombohedral, R 3 (C32d), Z 6, a 4.7284 (4), c 13-5591 (16) , ,, V 262-5/ 3. Distances are given in / and angles in oMgSiOSiteSymmetryxyz2(g)2(g)6(I)331000-3214 (5)000.0361 (4)0-35970 (12)0-15768 (10)0-24077 ( 1 ! )Si-O [3]Si-O [3]Mean Si-OO-Si-OO-Si-OO-Si-OO-Si-OOctahedral volume (, 3)*Quadratic elongationAngle variance1.830 (2)1-768 (2)Mg-O [3]Mg-O [3]1-79980.886.196.497-2Mean Mg-O(l)(I)( 1)(I)7.59 (1)1.01552.82-163 (2)1.990 (2)2.077O-Mg-OO-Mg-OO-Mg-OO- Mg-O70-590.194.5101.2Octahedra[ volume ( 3)Quadratic elongationI1.30 (1)1.04 (l)141-6Angle variance(1)(1)(I)( 1)* See Table 3.nation [J. B. Thompson in Birch (1952)]. (The twostructures differ only in the lack of ordered cationsin corundum.) Ringwood & Seabrook (1962) demonstrated such a transformation in the germanateanalog, MgGeO3, and other high-pressure germanateisomorphs were soon identified. The silicate endmember MgSiO3 was subsequently produced byKawai, Tachimori & Ito (1974) and this material wasidentified by Ito & Matsui (1974) as having theilmenite (R3) structure, in which silicon and magnesium must be at least partially ordered (Table 6,Fig. 5).Horiuchi, Hirano, Ito & Matsui (1982) synthesizedsingle crystals of MgSiO3 ilmenite and documenteddetails of the crystal structure. Silicon and magnesiumappear to be almost completely ordered in the twosymmetrically distinct cation positions (Table 6). Silicate ilmenites are unique in that each silicon octahedron shares a face with an adjacent magnesiumoctahedron- no other known silicate structure displays face sharing between a silicon polyhedron andanother tetrahedron or octahedron. Magnesiumsilicon ordering may be facilitated by this feature, foronly in a completely ordered silicate ilmenite can facesharing between two silicon octahedra be avoided.The stability of silicate ilmenites is rather restricted,in terms of both pressure and composition. Pressuresabove 20 GPa are required to synthesize the MgSiO3
566CRYSTAL C H E M I S T R Y OF S I X - C O O R D I N A T E D SILICONTable 7. Refinement of the KAISi308 hollanditestructure, from Yamada et al. (1984), R 0-137Tetragonal,I 4 / m ( C ] h ) , Z 2, a 9 . 3 2 4 4 (4), c 2 . 7 2 2 7 ( 3 ) / ,V 2 3 6 . 7 , 3. D i s t a n c e s a r e g i v e n in A a n d a n g l e s in o.SiteKI t M( AI 4SI)Ol02M-OIM - O I [2]M-O2M - O 2 [2]Mean M - OOI-M-OIOI-M-OISymmetryOrthorhombic,xyz' 2(b)4/m008(h)8(h)8(h)mmm0.348 (4)0-143(5)0-541 (6)0"170 (3)0-219(5)0.162(6)1-97 (4)1.71 ( i )1.80(4)1.82(4)1.8185 (3)106 (3)Table 8. Refinement of the NaAISiO4 calcium ferritestructure, from Yamada et al. (1983), R 0.039, withrevised and corrected atomic coordinates000OI-M-O2O1-M-O2OI-M-O2O2- M - O 2O2- M - O 279 (2)89(3)96(2)91 (3)97 (3)Octahedral volume (, 3),Quadratic elongationAngle variance7.62 (23)1-02 (4)60.5* S e e T a b l e 3.phase, but above about 25 GPa perovskite formsinstead (Fei, Saxena & Navrotsky, 1990; see Fig. 2).Addition of more than a few atom percent iron formagnesium stabilizes the perovskite form at theexpense of ilmenite; 10% iron completely eliminatesthe ilmenite field. Of the other common divalentcations, only zinc has been found to form a stablesilicate ilmenite-ZnSiO3 (Ito & Matsui, 1974; Liu,1977b).The hollandite structure. Feldspars, includingKAISi3Os, NaAISi308 and CaA12Si208, are the mostabundant minerals in the earth's crust. Accordingly,a number of researchers have examined high-pressurephase relations for these minerals (Kume, Matsumoto& Koizumi, 1966; Ringwood, Reid & Wadsley, 1967;Reid & Ringwood, 1969; Kinomura, Kume &Koizumi, 1975; Liu, 1978a, b). All of these investigators concluded that feldspars ultimately transformto the hollandite (BaMn8016) structure at pressuresabove about 10 GPa. Hollandite-type silicates havethus been proposed as a primary repository for alkalisin the earth's mantle.The ideal hollandite structure is tetragonal, I 4 / m ,with double chains of edge-sharing (Si,AI) octahedra(Fig. 6; Table 7). Large alkali or akaline-earth cationsoccupy positions along large channels that runparallel to c. The structure of KAISi308 hollanditewas refined from powder diffraction data by Yamada,Matsui & Ito (1984). They detected no deviationsfrom tetragonal symmetry and so assumed completedisorder of aluminium and silicon on the one symmetrically distinct octahedral site. Natural hollandites, however, are typically monoclinic (pseudotetragonal) owing to ordering of Mn 3 and Mn 4 orother octahedral cations, as well as distortion of thechannels. Single crystals of KAISi308 hollandite haverecently been synthesized at the Mineral PhysicsInstitute, State University of New York (Jaidong Ko,personal communication) and these samples mayPbnm ( D 6 ), Z 4, a 1 0 . 1 5 4 6 (8), b 8 . 6 6 4 2 (8),c 2.7385 (4)/ ,SiteV 240.93 (3)/ (c)mmmm0.236(3)0-577 (4)0.556 (3)0.303 (5)0-388 (5)0.479 t0.438 (6)4'4 4IO10203040.339(3)0.890 (3)0.398 (3)0.635 (5)0-982 (5)0.216 *,0.430 (4)I t 43a* Octahedral sites treated as disordered AI Si.t Published coordinates are approximately correct, but they yieldseveral unreasonably short octahedral bond distances. Refined 03coordinates, 0.201 (5) and 0-461 (5), have been replaced in thistable for consistency.: z coordinate of 04 was incorrectly given as 1/4 in the originalpaper.reveal if aluminium-silicon ordering lowers theapparent tetragonal symmetry.A number of other silicate hollandites have beensynthesized but not fully characterized by X-raydiffraction. Reid & Ringwood (1969) made hollandites with compositions approximating SrAI2Si2Osand BaAI2Si208 (though reported alkaline-earth contents are significantly less than 1.0), while Madon,Castex & Peyronneau (1989) described synthesis of(Cao.sMgo s)A12Si208 hollandite. Given the 1:1 ratioof aluminium to silicon in these samples, ordering ofAI and Si into symmetrically distinct octahedra ispossible, though structure-energy calculations by Post& Burnham (1986) suggest that octahedral cationsare disordered in most hollandites. This propositionis supported by Vicat, Fanchan, Strobel & Qui's(1986) ordering studies of synthetic hollandite, . 4 . . 3 .-. ,( K 1.33 n6.67Mnl.33ui6),which displays diffusediffraction effects characteristic of some short-rangeorder, but long-range disorder of Mn 4 and Mn 3 .The calcium ferrite structure. High-pressure studiesof NaAIGeO4 (Ringwood & Major, 1967a; Reid,Wadsley & Ringwood, 1967) and NaA1SiO4 (Liu,1977c, 1978a; Yamada, Matsui & Ito, 1983) revealedthat these compounds adopt the orthorhombic calcium ferrite (CaFe204) structure in which all Si andAI are in octahedral coordination. Yamada et al.(1983), who synthesized NaAISiO4 at pressures above24 GPa, used X-ray diffraction to identify their polycrystalline product and propose atomic coordinates.The basic topology of the high-pressure NaAISiO4structure is thus well established (Fig. 7; Table 8).Bond distances calculated from their refined coordinates, however, yield unreasonably short cationoxygen distances, so details of the structure remainin doubt.The calcium ferrite structure bears a close relationship to hollandite (Yamada et at, 1983). Both struc-
ACTA C R Y S T A L L O G R A P H I C A ,VOL. B47, 1991--FINGER & HAZENPLATE 5Fig. 3. The SiO stishovite structure, after Smyth & Bish (1988)./5' /.A- " h" " 2 !ic,";/;Fig. 5. The MgSiO3 ilmenite structure, based on the refinement ofHoriuchietal.(1982). w5:"/5':"X',)St', :-" \\: L-,',\'-""),',7," ',\',,,x/'' \\.- . W. \Fig. 6. The KA1Si308 hollandite structure, based on the refinementof Yamada et al. (1984).",X (b)Fig. 4. The cubic (a) and orthorhombic (b) silicate perovskitestructures, after Hazen (1988).Fig. 7. The NaAISiO4 calcium ferrite structure, modified from therefinement of Yamada et al. (1983).[ To face p. 566
ACTA CRYSTALLOGRAPHICA,VOL. B47, 1 9 9 1 - - F I N G E R & HAZENPLATE 6ILi!iiil j It!,. /,,/iFig. 8. The K2NiF4 structure, after Jorgensen (1987).Fig.11. Thestructure of K2[vqsi JVlsi3Og,& Prewitt (1983).afterSwanson4Fig. 9. The structure of MgSiO3 garnet, after Smyth & Bish (1988).Fig. 12. The structure of anhydrous phase B, from Finger et al.(1989). i '',\' " 1tiI \ " Fig. 10. The structure of Na(Mgo.sSio.5)Si206 pyroxene, fromAngel et al. (1988).Fig. 13. The structure of Mgt2Si4Olg(OH) 2 (phase B), from Fingeret al. (1989).
LARRY W. F I N G E R A N D ROBERT M. HAZENTable 9. Refinement of silicate pyrochlore structures,from Reid et al. (1977), R 0.031 for Sc2Si207 andR 0-015 for In2Si207Cubic, F d 3 m(O ,), Z 8 .Sc2Si207" a 9 . 2 8 7 ( 3 ) , ,V 800"98 (1) / 3. In2Si OT: a 9 - 4 1 3 ( 3 ) , , V 8 3 4 . 0 3 ( I ) , 3. Distances are given in A and angles in o.Sc, InSiOl02SiteSymmetryxyz16( )16(d)8(a)48(f)3m3m ,3m0 ,Sc: 0"4313 (21)In: 0.4272 (15)0 ,80mmSi-O2 [6]Sc2Si2071-761 (7)O2-Si-O2 [6]O2-Si-O2 [6]92.5 (7)87.5 (7)Octahedral volume ( 3),Quadratic elongationAngle variance7.26 (3)1.002 (9)6.7is567Table 10. The structure of K2NiF4-type Ca2SiO4, afterLiu (1978b)Tetragonal, 14/mmm (D417), Z 2 , a 3 . 5 6 4 ( 2 ) , c 11.66 (1), ,V 148.1 (!) , ,3. Distances are given in A and angles in .CaSiOl02Si-Ol [4]Si-O2 82 (I)-1-78x0y0-0.36000 0000-0-15O1-Si-Ol [4]OI-Si-O2 [8]z*9090* Fractional c o o r d i n a t e s not reported.In2Si2071.800 (5)94.0 (6)86.0 (6)7.71 (2)1-005 (7)17.8* See Table 3.tures consist of double octahedral chains which arejoined to form 'tunnels' parallel to c that accommodate the alkali or alkaline-earth cations. In hollanditefour double chains form square tunnels, whereas incalcium ferrite four chains define triangular tunnels.The pyrochlore[(Na,Ca)2(Nb,Ta)zO6(OH,F)]structure. Thortveitite, S c 2 S i 2 0 7 , contains Si207groups and Sc in distorted octahedral coordination.The structure is unusual in that the Si-O-Si linkageis constrained to be collinear because the O atom lieson a center of inversion. Reid, Li & Ringwood (1977)studied high-pressure transformations of 8 c 2 S i 2 0 7and its isomorph, In2Si207, from the thortveititestructure to the pyrochlore structure at 12 GPa and1273 K. They
known non-molecular silicate structures with six- coordinated or mixed-coordinated silicon, to identify crystal-chemical similarities among these structures and to suggest other possible high-pressure silicate structure types. Review of structures with six-coordinated silicon
Amendments to the Louisiana Constitution of 1974 Article I Article II Article III Article IV Article V Article VI Article VII Article VIII Article IX Article X Article XI Article XII Article XIII Article XIV Article I: Declaration of Rights Election Ballot # Author Bill/Act # Amendment Sec. Votes for % For Votes Against %
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EARTH / MOON WHITE EARTH / MIAMI WHITE EARTH / MULTICOLOR EARTH / LIGHT GREY colors EARTH / OUTDOOR 2CM EARTH / LIGHT GREY p.22 EARTH / DARK GREY p.26 EARTH / DARK GREY. 2 Drawing inspiration from the Dolomites in Northeastern Italy, Earth
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Using (A) Earth, Earth’s Moon, Mars Comparison worksheet, ask students to make a prediction using a drawing of the Earth, Earth’s Moon, and Mars, showing what they think the sizes are in relationship to each other. B. Look at the image of Earth and Earth
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