Geology 2142

1. Geology 2142 Systematic Mineralogy Vs Rock-based Classification

2. Classification Needed to allow us to talk about minerals in a coherent way Many different possibilities Chemical Morphological Lithology based Structural

3. Chemical Classification Based on the major anionic group SiO42- - silicates Cl-, F-, Br-, I- - halides O2- - oxides (CO3)2- - carbonates (NO3)2- - nitrates (BO3)3- or (BO4)5- - borates

4. Chemical Classification (cont) (SO4)2- - sulphates (CrO4)2- - chromates (WO4)2- - tungstates (MoO4)2- molybdates (AsO4)3- - arsenates Single elements � native elements S � sulphides S, As, Sb - sulphosalts

5. Chemical Classifications Useful as a first step in classification What is the next step? Morphological classification? Structural classification?

6. Morphological Classifications Developed around 1770 by Carolus Linnaeus Suggested that minerals be classified in the same way as plants and animals Based on external morphology Linneas was the first crystallographer Morphological classifications are ambiguous Different minerals can have identical or virtually identical external morphology

7. Rock Based Classification Group minerals according to the rocks that contain them Quartz and alkali feldspar Granitic minerals to an igneous petrologist Sedimentologists associate them with Arkose Metamorphic petrologist has no specific association These two minerals are common in many different metamorphic rocks

8. Structural Classifications Mineralogists not that interested in the rocks in which minerals occur More interested in understanding how and why certain elements bond together to form minerals Mineralogy involves recognition of patterns in the structure of minerals Best classification is based on structure

9. Chemical and Structural Division of minerals into CLASSES based on their anionic group Within each class recognize subclasses For the silicate minerals the sub-classification is based on the SiO4 tetrahedron In this course we focus on the silicates Form more than 90% of the earths crust Most of the remaining 10% is carbonate

10. The structure of silicate minerals Most rock forming minerals are silicates Silicon cation has a 4+ charge fits well in tetrahedral coordination with oxygen (2- charge) Silicon tetrahedra forms the basic building block of all the silicate minerals

11. Silicate Structural Types Structural type #shared O2- Si:O configuration Orthosilicate 0 1:4 isolated tetra Disilicate 1 2:7 double tetra Ring silicate 2 1:3 rings of tetra Single chain 2 1:3 chains Double chain 2/3 4:11 chains Sheet silicate 3 2:5 sheets Frameworks 4 1:2 framework

12. Structure and Physical Properties The arrangement of atoms has a large effect on the properties of a mineral Bonding type Hardness Packing Density Atomic arrangement and cation bond strength Cleavage

13. The Framework Silicates Dominant minerals in the earth�s crust 6 subgroups Silica group Plagioclase group Alkali feldspar group Feldspathoids Zeolite group Scaploite

14. Structure of the Framework Silicates Based on a TO4 tetrahedral framework T= Si4+ or Al3+ All four oxygens in a tetrahedra are shared with an adjacent tetrahedra Si4+ and Al3+ have mutual repulsion Leads to an open framework Framework can hold large cations Na+, K+, Ca2+ Need to substitute Al3+ for Si4+ in T-sites to fit them in

15. Structure and Physical Properties Open framework leads to low density quartz = 2.65 g/cm3 compare with Fo (3.27 g/cm3) much higher density even though Mg has lower mass than Si Fo based on CCP of oxygens Quartz has a framework structure The open framework collapses at high pressure phase transitions - common in SiO2

16. Properties of the SiO2 Polymorphs Name Crystal System Density (g/cm3) a-quartz Trigonal 2.65 b-quartz Hexagonal 2.53 a-tridymite Orthorhombic 2.26 b-tridymite Hexagonal 2.20 a-cristobalite Tetragonal 2.32 b-cristobalite Cubic 2.2 Coesite Tetragonal 3.01 Stishovite Monoclinic 4.35

17. Phase Transitions in SiO2

18. Sheet silicates Also known as phyllosilicates Abundant in the crust found in igneous and metamorphic rocks also in sediments: very fine-grained All contain OH- anionic groups said to be hydrous

19. Types of sheet silicate Muscovite Zinnwaldite Talc Paragonite Lepidolite Chlorite Glauconite Clintonite Serpentine Margarite Stilpnomelane Pyrophyllite Biotite Phlogopite most common in red classification requires knowledge of structure

20. The Sheet Silicate Structure Made of two types of sheet O: Octahedral sheets T: Tetrahedral sheets

21. The sheet silicate structure O and T sheets are joined to form layers layers stacked and bonded to form 1 unit cell volume between layers can be vacant of filled by an INTERLAYER cation or an OH- group

22. The sheet silicate structure The perfect basal cleavage of the sheet silicates is a result of weak bonding between adjacent layers

23. Octahedral sheets 2 planes of OH- groups (net charge 6-) octahedral (6-fold) sites between them filled by cations either Fe2+ / Mg2+ OR Al3+ / Fe3+ This is VERY IMPORTANT!

24. Octahedral sheets with 2+ cations If cation has 2+ charge then three out of three octahedral sites are filled called a TRIOCTAHEDRAL sheet Ideal formula Mg3(OH)6 = Brucite brucite consists of trioctahedral sheets held together by van der Waals bonds

25. Octahedral sheets with 3+ cations Cations with 3+ charge cannot fill 3 out of 3 octahedral sites one must be left vacant to maintain charge balance these are DIOCTAHEDRAL sheets ideal formula; Al2(OH)6 = gibbsite

26. Dioctahedral vs Trioctahedral Di and Tri refer to SITE OCCUPANCY not to the valence of the cations remember that trivalent cations make - DIOCTAHEDRAL sheets divalent cations male - TRIOCTAHDRAL sheets

27. Joining T- and O-sheets T-sheets always joined to an o-sheet Apical oxygen from the t-sheet replaces and OH- in the o-sheet Can combine to form TO sheets (1:1 layer silicates) TOT sheets (2:1 layer silicates)

28. TO Layers Made of three planes of anions 1) Basal oxygens of the T-sheet 2) OH- groups of the upper layer of the O-sheet 3) In the middle OH- from O and apical oxygens from T

29. TO Layers Combination of a dioctahedral sheet and a T-sheet Si2O52- + Al(OH)6 = Al2Si2O5(OH4) + 2(OH)- Kaolinite Combination of a trioctahderal sheet and a T-sheet gives Mg3Si2O5(OH)4 Serpentine

30. TOT Layers Formed by joining a T-sheet to both sides of an octahedral sheet Consist of 4 rather than 3 planes of anions Outer two planes are basal oxygens of the T-sheets Middle two planes are OH- from O-sheet and apical oxygens from T-sheet

31. TOT Layers Joining 2 T-sheets to a dioctahedral sheet Al2Si4O10(OH)2 =pyrophyllite Joining to 2 sheets to a trioctahedral sheet Mg3Si4O10(OH)2 = Talc

32. The 1:1 (TO) Layer Silicates Repeating TO sheets are electrically neutral layers bonded together by electrostatic (van der Waals and hydrogen) bonds weak bonds lead to a soft mineral hardness of ~ 2 repeat distance from layer to layer ~ 7 A c unit cell is either 7 or 14 A depending on if it is one or two layers thick Kaolinite (di) and Serpentine (tri)

33. TOT layer silicates Repeating TOT layers simple form both the Di- and trioctahedral sheets are neutral BUT Al3+ and Fe3+ can substitute for Si4+ means that the TOT layers can have a net negative charge Three different varieties depend on presence of a net negative charge also on the interlayer cation that balances charge

34. Simple TOT layer silicates TOT layers are neutral all T-site filled by Si4+ bonding of TOT - TOT layers depends on van der Waals and hydrogen bonds no cations between the layers TOT layer silicates are soft trioctahedral = talc = Mg3Si4O10(OH)2 dioctahderal = pyrophyllite = Al2Si4O10(OH)2 repeat distance is 9 to 9.5 Angtroms (unit cell may be 2x repeat distance)

35. Tot+c Structure Micas and brittle micas TOT layer with some T-sites filled by Al3+/Fe3+ micas have Al:Si ratio of 1:3 Dioctahedral layers = Al2(AlSi3O10)(OH)21- trioctahedral layers = Mg3(AlSi3O10)(OH)21- net negative charge is balanced by large 1+ cations eg K+ TOT+C (dioctahedral) KAl2(AlSi3O10)(OH)2 = muscovite KMg3(AlSi3O10)(OH)2 = phlogopite

36. Tot+c Structure Bonding both electrostatic bonds and stronger ionic bonds from interlayer cations micas are harder than TOT layer silicates H= 2-3 repeat distance larger than TOT because of interlayer cation Margarite, clintonite and Xanthophyllite

37. TOT-O sheet silicates Also called 2:1 + 1 layer silicates most common is chlorite (dioctahedral) structure derived from TOT structure of Talc with addition of an octahedral sheet between the TOT layers the extra O sheet is a brucite type sheet

38. Mica polytypes Polytypes are a special type of polymorph differ only in the stacking sequence of individual sheets

39. Mica polytypes Adjacent TOT layers keyed to each other by the interlayer cations These fit in the middle of the hexagonal rings of the t-sheets With systematic offset the repeat distance is a multiple of the 9.5 - 10 angstrom layer thickness

40. Mica Polytypes 6 possible polytypes Repeat distances from ~10 angstroms (1 TOT layer) to 60 angstroms (six TOT layers) Most important are the 1M and 2M1 polytypes 1M successive layers show same direction of offset Unit cell is 1 TOT layer thick 2M1 layers offset on vectors at 120� Unit cell is 2 TOT layers thick Both are monoclinic