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Chemistry of the Rockforming Silicates Multiplechain, Sheet, And

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Chemistry of the Rockforming Silicates Multiplechain, Sheet, And REVIEWS OF GEOPHYSICS, VOL. 26, NO. 3, PAGES 407-444, AUGUST 1988 Chemistry of the Rock-Forming Silicates' Multiple-Chain, Sheet, and Framework Structures J. J. PAPIKE Institutefor the Studyof Mineral Deposits,South Dakota Schoolof Mines and Technology,Rapid City The crystal chemistry of 16 groups of multiple-chain, sheet, and framework silicates is reviewed. Crystal structuredrawings are presentedto illustrate crystal chemicalfeatures necessary to interpret chemicaldata for eachmineral group. The 16 silicategroups considered in this revieware the amphibole; nonclassical,ordered pyriboles; mica; pyrophyllite-talc;chlorite; greenalite;minnesotaite; stilpnomel- ane; prehnite;silica polymorphs; feldspar; nepheline-kalsilite; leucite-analcite; sodalite group; cancrinite group; and scapolite.Electron microprobe analyses should be augmentedby independentdeterminations of Fe2+/Fe3+ and H20 for manyof the silicategroups discussed and by determinationsof CO32-, SO42-, S2-, and Li in someof the others.However, microprobe data augmentedas suggestedwill still be ambiguousfor someof the silicategroups considered here because the structuresare not completely determinedor are variable,with disparatedomains and/or structuralmodulations, e.g., pyriboles, greena- lite, minnesotaite,and stilpnomelane.Nevertheless, the most rigorousway to interpretsilicate mineral chemicaldata is basedon the crystal structuresinvolved. CONTENTS Deer et al. [-1963a, 1962, 1963b] for mutiple-chain, sheet, and Introduction ............................................. 407 frameworksilicates, respectively. The Reviewsin Mineralo•Iy Amphibole .............................................. 407 series gives additional valuable information [Ribbe, 1983; Pyriboles (nonclassical, ordered) ........................... 412 Veblen, 1981; Veblen and Ribbe, 1982; Bailey, 1984]. Other Mica ................................................... 413 amphibole reviews are by Cameronand Papike [1979] and Pyrophyllite-talc ......................................... 416 Chlorite ................................................. 418 Hawthorne[-1983]. A generalreview of silicatecrystal chemis- Greenalite ............................................... 420 try for 12 silicate groups is provided by Papike and Cameron Minnesotaite ............................................ 422 [1976]. Stilpnomelane ........................................... 422 The elementsconsidered for each mineral group are tabu- Prehnite ................................................ 425 lated in Table 1. Table 2 lists recommended normalization Quartz-tridymite-cristobalite .............................. 427 Feldspar ................................................ 429 schemesfor microprobedata and criteria for good analyses. Nepheline-kalsilite ....................................... 432 The following text documents the rationale for these rec- Leucite-analcite .......................................... 436 ommendations. Sodalite group ........................................... 437 It is my hope, as with part 1 of this synthesis,that this Cancrinite group ......................................... 439 Scapolite ................................................ 440 review will help in the rigorous interpretation of chemical Concluding statement .................................... 442 analyses of silicate minerals. AMPHIBOLE INTRODUCTION The amphibole group has the general formula The basic premise of this review of multiple-chain, sheet, Ao_•B2CsT8022(OH, F, C1, 0)2 , where A contains Na, K at and framework silicatestructures is the sameas that for part 1 the A site,B containsNa, Li, Ca, Mn 2+, Fe2 +, Mg at the M4 of this synthesis[Papike, 1987]: that true understandingof the site,C containsMn 2+, Fe2 +, Mg, Fe3 +, Cr 3+, A1,Ti 4+ at the chemistryof mineralscan be achievedonly by referenceto the octahedrally coordinated M1, M2, and M3 sites, and T con- crystal structuresinvolved. To help investigatorsin the inter- tains Si, A1 at the tetrahedral sites.The published work on this pretation of silicate chemical data for those minerals con- important group of rock-forming silicates is voluminous, and sideredin this review,I have assembledstructure drawings for only a relatively brief review is presented here. For more ex- 16 groups of multiple-chain, sheet, and framework structures. tensivecoverage the reader is referred to Deer et al. [1963a], Some of the drawingsare new, and somehave been adapted Papike and Cameron [1976], Cameron and Papike [1979], from previouslypublished drawings. However, as in part 1 of Veblen [-1981], Veblen and Ribbe [-1982], and Hawthorne this synthesis,all diagrams presentedhere have been selected [1983]. to accurately portray sufficient detail to the non- Important aspects of the C2/m structure are illustrated in crystallographerso that the mineral chemistrycan be interpre- Figure la. The M1, M2, and M3 octahedra, which accommo- ted as rigorously as possible. date the C cations, share edges to form octahedral bands Becauseof length limitations, referencinghas been stream- parallel to ½. M1 and M3 are usually coordinated by four lined. References necessary to convey the essentials of the oxygens and two (OH, F) groups, whereas M2 is crYordinated structural chemistry of each mineral group are emphasized. by six oxygens.When present,A1 and Fe3 + usuallyprefer the Readers who require more detailed information are referred to M2 site, especially when Na occupiesthe adjacent M4 site. The M4 site, which accommodates the B cations, can be con- Copyright 1988 by the American Geophysical Union. sidered six-coordinated (a distorted octahedron) when oc- Paper number 8R0240. cupiedby Mn 2+, Fe2+, and Mg and eight-coordinatedwhen 8755-1209/88/008 R-0240505.00 occupied by Na or Ca. The double tetrahedral chains cross- 407 408 PAPIKE' CHEMISTRY OF SILICATE MINERALS TABLE 1. Inventory of Important Elements to Determine for Each Mineral Si4+ A13+Fe 3+ Ti4+ Mg2+ Fe2+ Mn2+ Ca2+ Ba2+ Li + Na+ K + Rb+ Cs+ H20 F- C1- CO32- SO42- S2- Amphibole group X X X* X X X X X X* X X X* X X Pyriboles X X X X X X X X* Mica X X X* X X X X X X* X X X X X* X Pyrophyllite-talc X X X X X X X X X X X* Chlorite X X X* X X X X X* Greenalite X X X X X X* Minnesotaire X X X* X X X X X X X* Stilpnomelane X X X* X X X X X X X* Prehnite X X X* X X X X X X X* Silica polymorphs X X X X X X X X X X Feldspar X X X* X X X X X X X Nepheline-kalsilite X X X X X X X X Leucite-analcite X X X X X X X X X X* Sodalite group X X X X X X X X* X Cancrinite group X X X X X X X* X X* X* Scapolite X X X X X X X* X* X *Determinations recommendedto supplementmicroprobe data. TABLE 2. Recommended Normalization Schemesfor Microprobe Data and Criteria for Good Analyses Normalization Criteria for Good Analyses Fixed Compare Number 100% With of Anhydrous Total Table Oxygens Fixed Number of Cations Sum Silicons Selected Cation Sum Analyses Amphibole 23 no Si, •VAl = 8 Mn, Fe, Mg, Ti, Cr, A1 -> 5 octahedral cations in excess of 5 (exoct) should sum to -<2 exoct, Ca < 2 exoct, Ca, Na, Li = 2 site occupancy (Na, K) -< 1 Pyriboles Clinojimthompsonite 34 no 12 v• A1, Mg, Fe2+, Mn, Ca = 10 Jimthompsonite 34 no 12 v=A1, Mg, Fe2+, Mn, Ca = 10 Chesterite 57 no 2O • A1, Mg, Fe2+, Mn, Ca - 17 Mica 11 no Y• Si, AI-> 4 Si-<4 octahedral cations -< 3 X cations -< 1 Pyrophyllite 11 no VIAl = 2 Talc 11 no •Mg, Fe, AI- 3 Chlorite 14 no Octahedral cations = 5.5-6.0 Si- 2.34-3.45 Greenalite 7 no Octahedral cations = 2.73-2.85 Si = 2.06-2.13 Minnesotaite 11 no X Stilpnomelane Y•Si, A1,Fe 3+, Mg, X Fe2+, Mn = 15 Prehnite 11 no Ca + Na = 2 • Si, A1, Fe3+, Ti, Mg, Fe2+, Mn = 5 Quartz 2 yes Tridymite 2 yes iVAl + •VFe3+= 2(Mg, Fe2+, Mn 2+, Ca) + Na + K Feldspar 8 yes • Na, K, Ca, Ba, Fe2+, Mg= 1 •AI, Si, Fe3+,Ti= 4 Nepheline-kalsilite 16 yes • Si, A1, Fe3+ = 8 Leucite 6 yes •K, Na, Ca= 1 vSi A1, Fe3+ Ti= 3 Analcite 6 no VNa, K, Ca = 1 • A1, Si = 3 Sodalitegroup • Si, A1, Fe3+ = 12 no Cancrinite group Y• Si, A1 = 12 no Scapolite • Si, A1 - 12 no It is assumedthat Fe2+/Fe3+ is determinedindependently, and therefore cations are not adjustedto estimateFe2+/Fe 3+ PAPIKE.' CHEMISTRY OF SILICATE MINERALS 409 AMPHIBOLE Csin • A O3 J b Fig. la. Thecrystal structure ofa C2/mamphibole projected down a [fromPapike and Cameron, 1976]. X, o8 // o8 ',,, I b C 2/m I b Pnmn P2•/m PROTOAMPHIBOLE AMPHIBOLE I-BEAMS Fig. lb. Amphibolestructural topology [from Papike and Cameron, 1976]. 410 PAPIKE.' CHEMISTRY OF SILICATE MINERALS AMPHIBOLE M3 MI M2 M4 Fig. lc. The crystal structure of C2/m amphibole projected down [001]. Stippled areas outlined I beams depicted in Figure lb [from Cameron and Papike, 1979]. link the octahedral strips and are composed of two indepen- tetrahedra. The large and highly distorted A polyhedron dent tetrahedral sites, T1 and T2. The T1 site, which shares occurs between the tetrahedral chains. This site can be vacant, three oxygens with other tetrahedra, is more regular and partially filled, or fully occupied by Na and/or K in the C2/m smaller than T2. The T2 site shares two oxygens with adjacent structures. The 03 site is usually occupied by an (OH)- group or, less commonly, by F- and C1-. Oxyamphiboles predomi- FE-MG-MN AMPHIBOLES nantlyhave 0 2- at the 03 position. The major topologic aspects of the structures are summa- rized in Figure lb. Each "I beam" represents a tetrahedral- ORTHORHOMBIC 1.0 octahedral-tetrahedral (T-O-T) unit. The rotational aspect of MAGNESIO-ANTHOPHYLLITE MAGNESIO-GEDRITE the tetrahedral chain relative to the octahedral strip is S if the tetrahedra and
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