4. Mineralogical background II: Silicates (among others Nesosilicates, Cyclosilicates, Inosilicates)

Transkript

1 4. Mineralogical background II: Silicates (among others Nesosilicates, Cyclosilicates, Inosilicates) The manuscript is under construction. Please suggestions an Thank you, Rolf Kilian

2 SiO 4 -Tetrahedron: basic module silicates Silicium has here the coordination number four, and it is surrounded by four oxygen-ligand Tetrahedron Charges and proportions are illustrated. 2

3 Classification of silicates In the silicates are SiO 4 -Tetrahedrons not linked or are linked unidimensional, two-dimensional and three dimensional on common oxygen atoms: Nesosilicate (isolated SiO 4 -Tetrahedron) Sorosilicate (two cross-linked Tetrahedron) Cyclosilicate (ring-shaped crosslinking) Inosilicate (unidimensional crosslinking) Phyllosilicate (two-dimensional crosslinking) Tectosilicate (three-dimensional crosslinking) 3

4 Nesosilicates (Nesosilicate, Orthosilicate) isolated [SiO 4 ] 4 -Tetrahedron Si : O -ratio = 1 : 4 Connection through intermediate cations Basic module SiO 4 -Tetrahedron O 2- For example: olivine, garnet, zircon, andalusite, disthene, sillimanite, staurolithe, topaz, titanite O 2- Si 4+ O 2- O 2- Si 4+ q Si 4+ O 2-4

5 disthene Ideomorphic garnet staurolithecruciform twin titanite (sphene) 5

6 Olivine (Mg,Fe 2+ ) 2 [SiO 4 ] orthorhombic Mantelperidotit, vorwiegend aus Olivin Series of mixed crystals between forsterite (Mg 2 [SiO 4 ]) and fayalite (Fe 2 [SiO 4 ]) 6

7 Olivine Hardness 6 1 / 2 7 Density 3,2 g/cm 3 (forsterite); 4,3 g/cm 3 (fayalite) subject to Fe content Poor cleavage (shelly breakage) 7

8 Olivine occurs in In mantle peridotites: prevalent in the upper mantle crustal rocks mafic cumulates Marbles magmatic rocks, but only in SiO 2 -poor, especially in basanitic und basaltic fusion 8

9 Serpentinization Olivine can be to changed in magnetite and serpentine. 3 Fe 2 SiO 4 + O 2 2 Fe 3 O SiO 2 3 Mg 2 SiO 4 +SiO 2 +2H 2 O 2 Mg 3 Si 2 O 5 (OH) 4 9

10 Polyhedron model of the crystal structure of olivine The isolated [SiO 4 ]-tetrahedrons are linked by the corners with the Mg oder Fe 2+ octahedrons (yellow) [SiO4]-tetrahedron Mg/Fe 2+ -octahedron 10

11 Temperature Melt Olivine+ melt Composition of the mixed crystals Residual melt Fosterite Weight % fosterite Fayalite 11

12 Depth Oceanic crust oceanic Lithosphere upper Mantle Mid-ocean ridge Low - velocity Zone (Asthenosphere) Olivine Continental crust continental Lithosphere silicium oxygen Transition range: olivine to spinel Olivine 600 Spinel structure Spinel Density springs lower Mantle Perovskite structure and magnesium oxide S-Wave velocity Perovskite 12

13 Garnet The group of the garnet forms a complete mixed crystal series with the generic formula A 2 +3B 3 +2[SiO 4 ] 3 with A 2+ = Ca, Mg, Fe 2+, Mn, Na B 3+ = Al, Fe 3+, Cr 3+, Ti 4+ u.a. Mg 3 Al 2 [SiO 4 ] 3 Pyrope (in mantle peridotites) Fe 3 Al 2 [SiO 4 ] 3 Almandine (in mica schist) Mn 3 Al 2 [SiO 4 ] 3 Spessartine Ca 3 Al 2 [SiO 4 ] 3 Grossularite (in marble) Ca 3 Fe 2 [SiO 4 ] 3 Andradite Ca 3 Cr 2 [SiO 4 ] 3 Uvarovite 13

14 Garnet crystallize cubic hardness 7 density 3,3-4,4 g/cm 3 Idiomorphe Almandin-Kristalle aus dem Ötztal, Tirol, Österreich 14

15 Formed mainly metamorphic; for example in schist, gneisses, amphibolites, eclogites, granulites, peridotites To form necessary different high pressures In pelitic system: already in the upper to middle cruste in mica schist often as idioblast 15

16 Garnet structure characterized by insular SiO oder AlO 4-4 -tetrahedrons (blue) as well as octahedric coordinated cations. 16

17 Garnet Pyroxene Eclogite xenolith from the Roberst Victor Mine, South Africa. 17

18 Aluminum silicates Andalusite Disthene Al 2 SiO 5 with the three modifications: andalusite, disthene, sillimanite Sillimanite Differences in cristallographic, physical properties and formation conditions. predominant metamorphic in pelitic (clayey) systems 18

19 Pressure Experimentally determined phase diagram of the Al 2 SiO 5 -Polymorphous Disthene divariant field invariant point Sillimanite divariant field PT-path a sinking and ehxumation Andalusite divariant field Temperature C) ( 19

20 Andalusite Al 2 SiO 5 or Al [6] Al [5] [O/SiO 4 ] hardness 7 1 / 2 predominant metamorphic in pelitic (clayey) systems 20

21 Andalusite-Chiastolite from Lancaster Massachusetts often thick-columnar crystals with rectangular section sometimes with dark cross through embedded inclusions (often graphite variety chiastolite) 21

22 Disthene Al 2 SiO 5 or Al [6] Al [6] [O/SiO 4 ] triclinic pronounced hardness anisotropy Disthene from the Ural 22

23 Acicular sillimatite crystals from Williamstown, Australia 23

24 Staurolite hardness 7 density 3.8 g/cm 3 monocline, pseudorhombic (Fe 2+,Mg,Zn) 2 (Al,Fe 3+,Ti) 9 O 6 [(Si,Al)O 4 ] 4 (O,OH) 2 predominant formed metamorphic in Al-rich systems Cruciform twin are frequent 24

25 Zircon Zr[SiO 4 ] ca Gew.% SiO 2 67 wt.% ZrO 2 a high percentage of Hf (HfO 2 1 wt.%) many zircons contain U and Th and can therefore be used as chronometer ( 235 U 207 Pb; 238 U 206 Pb; 232 Th 208 Pb). especially heavy rare earth (SEE 3+ ) can be incorporated at the place of Zr

26 Zircon occurrence in intermediated to acid magmatites in all clastic sediments in heavy metal placers as detritic zircongrains, which remain by metamorphose and partial dissolution and further form grow seaming at old grains Zoned zircon at SEM 26

27 Zircon crystals from Frederiksvärn, Norway The economic significance: Raw material for the extraction of Zr and Hf; gemstone industry. 27

28 Titanite CaTi [SiO 4 ] (O,OH,F) hardness 5 yellow, brown, red brown; greenish in differentiated alkaline-rich Ca-rich magmatites in metamorphites: green schist, amphibolites, gneisses, eclogites 28

29 Topaz Al 2 [SiO 4 ](F,OH) 2 hardness 8 Gemstone varieties Topaz from Spitzkopje, South-west Africa in pegmatites or as secondary form in Al-rich acid granites 29

30 Sorosilicates [Si 2 O 7 ] To form by low metamorphic conditions: epidote group, vesuvianite, pumpellyite, lawsonite In well silica-poor high temperature metamorphic and magmatic systems melilite group 30

31 Vesuvianite tetragonal Idiomorphic vesuvianite crystal from Burgers, Canada Ca 19 (Al,Fe 3+,Ti) 10 (Mg,Fe 2+,Mn) 3 [Si 2 O 7 ] 4 [SiO 4 ] 10 (O,OH,F) 10 Hardness 6-7 with many colors depending on chemism Occurrence: as concretions in sediments of the Eifel maars By high temperatures in SiO 2 and calc-rich material components 31

32 Epidote / Zoisite Ca 2 (Al,Fe 3+ )Al 2 [O/OH/SiO 4 /Si 2 O 7 ] Yellow-green to dark green, pistachio green ("Pistazite") Stengelige Zoisitkristalle aus Stammbach, Fichtelgebirge metamorphic, mainly occurring in green schist, often in diaclases 32

33 Lawsonite orthorhombic CaAl 2 [Si 2 O 7 ](OH) 2. H 2 O can be formed in the subducted oceanic crust and by low geothermic gradient and is stable up to very high pressure (10 GPa = 300 km depth). The mineral can incorporate up to 10 wt. % water in crystal lattice and can resist the pressures according about 200 km depth Water transport in the Earth s mantle! 33

34 Pumpellyite W 2 XY 2 [Si 2 (O,OH) 7 ][SiO 4 ](OH,O) 3 often: (Ca,Na,K) 2 (Al,Fe 3+,Fe 2+,Mg)Al 2 [Si 2 (O,OH) 7 ][SiO 4 ](OH,O) 3 Example for a chemical very variable crystal lattice Reaction rim between diabase and quartz grain together with plagioclase, smectite and Fe-Hydroxide Foto: Dr. Alfred Schuster 34

35 Cyclosilicates annular linked [SiO 4 ] 4- -tetrahedrons Si : O -ratio = 1 : 3 [Si 4 O 12 ] 8- - and [Si 6 O 18 ] 12- -rings Common characteristics are a very high hardness (7-8) and diverse cations, especially placed in the ring structures. This results in very different colors and gemstone varieties. For example beryl, tourmaline, cordierite 35

36 Cyclosilicates cordierite, rounded beryl 36

37 Beryl Be 3 Al 2 [Si 6 O 18 ] hexagonal Gemstone varieties: green (= emerald) blue (= aquamarine) green, blue, yellow, reddish; transparent to opaque Hardness 7 1 / 2-8 Occurrence in pegmatites 37

38 hexagonal, columnar crystals 38

39 Beryl Projection at 39

40 Beryl- channels Approx. Layer of Si-atoms in Si 5 O 18 rings. Layer of Be and Al atoms. place place Approx. 40

41 Turmalin Härte 7 1 / 2 Dichte 2.9 bis 3.2 g/cm 3 triklin oft chemisch zonariert unterschiedliche Kationen beteiligt (Na,K,Ca)(Mg,Fe,Mn,Li,Al) 3 (Al,Mg,Fe 3+ ) 6 [Si 6 O 18 ](BO 3 ) 3 (O,OH) 3 (OH,F) 41

42 Turmalin säulige Kristalle, häufig in radialstrahligen Aggregaten ("Turmalinsonnen") keine Spaltbarkeit; jedoch Querbrüche der Säulen häufig schwarz (Fe-reich), aber auch grün, blau, rot, braun, rosa, farblos (siehe oben!) pegmatitisch, magmatisch, metamorph vorkommend 42

43 Cordierit (Mg,Fe 2+ ) 2 [Al 4 Si 5 O 18 ] Luchssaphir bzw Wassersaphir aus Ceylon Entsteht bei hohen Temperaturen (T 520 C) in Al-reichen Metamorphiten und Magmatiten 43

44 Luchssaphir bzw Wassersaphir aus Ceylon 44

45 Kettensilikate (Inosilikate) zu Ketten verknüpfte [SiO 4 ] 4 -Tetraeder auf 2 Sauerstoffatome eines Tetraeders gehören gleichzeitig zwei Tetraedern an Si : O -Verhältnis = 1 : 3 negativ geladene Ketten werden durch Kationen zusammengehalten. Beispiele: Pyroxene, Wollastonit. 45

46 Pyroxene: Allgemeine Formel: A 2 B 2 [(Si,Al) 2 O 6 ] mit A = Na, Ca, Mg, Fe 2+, Mn B = Mg, Fe 2+, Mn, Fe 3+, Al, Ti, Cr 46

47 Kristallstruktur von Pyroxenen Orthopyroxene rhombisch C Klinopyroxene monoklin 47

48 48

49 49

50 50

51 Idiomorpher Augit Enstatit Wollastonit mit Grossular (Schweden) Bronzit mit Plagioklas 51

52 Chemische Zonierungen in Pyroxenen von Mantelxenolithen reflektieren die Abkühlungs- und Aufheizungsgeschichte des Erdmantels 52

53 Bändersilikate jeweils zwei Tetraederketten sind zu einem Band oder einer Doppelkette verknüpft Si : O -Verhältnis = 1 : Beispiele: Amphibole 53

54 Amphibole - Hornblenden Ca 2 (Mg,Fe) 5 [Si 8 O 22 ](OH) 2 Basaltische Hornblende Aktinolith Aktinolith (Tschecheslowakei) Krokydolith Tigerauge NaCa 2 (Mg,Fe 2+ ) 4 (Ti, Fe 3+ ) [Al 2 Si 6 O 22 ](OH) 2 Tremolit Na 2 Fe 2+ 3 Fe 3+ 2 [Si 8 O 22 ](OH) 2 54 Ca 2 (Mg,Fe) 5 [Si 8 O 22 ](OH) 2

55 Amphibol-Gruppe Allgemeine Formel: A 0-1 X 2 Y 5 [(Si,Al) 8 O 22 (OH,F,Cl) 2 ] mit A = K, Na X = Na, Ca, Mg, Fe 2+, Mn Y = Mg,Fe 2+, Mn, Fe 3+, Al, Ti, Cr 55

56 Amphibol-Struktur OH M1 M2 M3 M4 b ~

57 Orthoamphibole = orthorhombisch Klinoamphibole = monoklin prismatische Kristalle mit meist sechseckigem Querschnitt; oft auch nadelig oder faserig 2 gute Spaltbarkeiten im Winkel von ~ 55 bzw. ~

58 Amphibole: Allgemein Härte 5 1 / 2 Dichte 2.9 bis 3.2 g/cm 3 magmatisch (u.a. Hornblende, Riebeckit) metamorph (u.a. Hornblende, Tremolit, Aktinolith, Glaukophan, Riebeckit) 58

59 Schichtsilikate (Phyllosilikate) zu Schichten vernetzte [SiO 4 ] 4 -Tetraeder vernetzte Sechserringe drei Sauerstoffatome gehören gleichzeitig zwei Tetraedern an Si : O -Verhältnis = 1 : 2.5. Beispiele: Biotit, Muskovit, Chlorit, Serpentin, Talk, Kaolin. 59

60 Schichtsilikat-Strukturen In Schichtsilikaten Wechseln sich Tetraeder- und Oktaeder-Schichten ab. Rechts: Tetraeder-Basiseinheit 60