Structure of olivine and mica

includes T-O-T structure

1. OLIVINE AND MICA GROUP SUBMITTED BY: ANANDHU.P.C I MSC GEOLOGY

2. OLIVINE GROUP Olivines are one of important rock forming mineral. This group consist of number of closely related mineral crystallize in ORTHORHOMBIC system. Structure of all the minerals of the group have independent tetrahedra linked by Sio 4 divalent atoms in six fold co-ordination. The seris have two end members Forsterite (Mg 2 SiO 4 ) and Fayalite(Fe 2 SiO 4 ).

3. Members of Olivine family The general formula of olivine series of minerals is R 2 SiO 4 , where R=Mg or Fe. The members of this group belongs toa countinues solid solution series between Forsterite and Fayalite . The olivine is considered as a mixture of the two end members , i.e (Fo 63 Fa 37 ) .

4. Diagrammatical representation of Olivine series

5. STRUCTURE OF OLIVINE SERIES Figure 1:  The atomic scale structure of olivine looking along the  a axis. Oxygen is shown in red, silicon in pink, and magnesium/iron in blue. A projection of the unit cell is shown by the black rectangle

6. The structure of olivine was determined by Bragg and Brown. The structure consist of individual Si-O tetrahedra linked by Mg or Fe atoms each of which has six nearest oxygen. The oxygen lie in sheets parallel to the (100) plane are arranged in approximate hexagonal close packing. The magnesium atom do not occupy one set of equivalent lattice positions:half are located at centers of symmetry and half on reflection plane .

7. T-O-T STRUCTURE Structure may be visualized as a somewhat distorted hexagonally close packed array of oxygen which one-eight of the tetrahedral sites occupied by divalent cations. There are two different sites , the M1 sites that forms the edge sharing chains parallel to c get streched out along that axis as though to keep the cations within the octahedra further apart. The M2 sites that are affixed to the sides of these chains are somewhat less distorted. The net result of this distortion is to decrease the symmetry from the ideal hexagonal to orthorhimbic and the crystal axis tend to elongate to the c axis.

8. T-O-T STRUCTURE OF OLIVINE

9. CRYSTALLOGRAPHY: The olivine series readily crystallize in ORTHORHOMBIC system. CLEAVAGE: No distinct cleavage. FRACTURE: Concoidal HARDNESS: 6.5 – 7 SPECIFIC GRAVITY: Increases from Forsterite to Fayalite due to the iron content . FORSTERITE - 3.3 FAYALITE- 4.4 LUSTER: Vitreous COLOUR: Olive green VARIETIES: Other olivine group minerals are Monticellite,Glaucochroite and Kirschsteinite. GEM Variety – Peridote Polymorphs- Ringwoodite

10. OCCURENCE: It is characterises the ultra-basic igneous rocks as Dunite,Peridotites,Serpentine and basic rocks like Norite,Gabbro,Dolerite,Basalt etc... The common associates are Chromite,Spinel,Pyrope etc... Olivine and Quartz never occur together . Forsterite is formed by dedolomitisation or contact Metamorphism of Magnesium rich sedimentary rock as dolomitic limestone rich in silica.

11. OPTICAL PROPERTIES OF MAJOR MINERALS

12. FORSTERITE (UNDER PLANE POLARISED LIGHT) Chemical formula : Mg 2 SiO 4 System: Orthorhombic Color: Colorless in thin section. Form: Forsterite usually occurs In euhedral to subhedral crystals. Habit: Granular masses or rounded, embedded grains. Relief: Fairly high,n>balsam.The Indices increasing iron content. Cleavage: Indistinct on {010}, {100}

13. CLEAVAGE:  Indistinct on {010},{100} Pleochroism : Weak, pale green pleochroism in thin section. ORIENTATION: Orientation diagram of Forsterite, Section parallel to (100). OPTIC ORIENTATION: X=b ,Y=c,Z=a  REFRACTIVE INDICES  : Alpha = 1.635 to 1.640 Beta = 1.651 to 1.660 Gamma= 1.670 to 1.680

14. (UNDER CROSSED NICOLS) Extinction: P arallel to the crystal outlines and cleavage traces.  Birefringence: Strong OPTIC SIGN: Biaxial +ve Max Birefringence: δ = 0.033 - 0.042 Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.

15. FAYALITE (UNDER PLANE POLARIZED LIGHT) COMPOSITION: Fe 2+ 2 SiO 4 CRYSTAL SYSTEM: ORTHORHOMBIC COLOR: Colourless to yellowish or neutral PLEOCHROISM : Weak, pale green pleochroism in thin section. CRYSTAL HABIT: Granular masses or rounded grains. CLEAVAGE: Poor cleavage on (010) and (110).These cleavages are rarely seen in thin section and do not control fragment orientation of grain mounts.

16. FORM: In cavities fayalite occur as euhedral crstals . RELIEF: Very high,n>balsam OPTIC ORIENTATION: Crystal showing cleavage are lenght slow . a=z,b=x,c=y REFRACTIVE INDICES  : Alpha = 1.805 to 1.835 Beta = 1.838 to 1.877 Gamma= 1.847to 1.886

17. UNDER CROSSED NICOLS EXTINCTION: Parallel to cleavage traces. BIREFRINGENCE: Strong OPTIC SIGN: Biaxial -ve Max Birefringence: δ = 0.042 - 0.051 Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.

18. MICA GROUP Micas constitute an important group of rock forming minerals,as ferromagnesian silictes. They form a link between feldspar and feldspathoids i.e, the light coloured constituents of igneous rocks and the dark coloured minerals. Of the 28 known species only 6 are common

19. CLASSIFICATION Chemically, Micas can be given the general formula X 2 Y 4-6 Z 8 O 20 (OH,F) 4 in which X=K, Na, Ba, Ca, Cs, (H 3 O), (NH 4 ) Y=Al, Mg, Fe 2+ , Li, Cr, Mn, V, Zn Z=Si, Al, Fe 3+ , Be, Ti Structurally micas can be classed as ‘dioctahedral’ Y=4 and ‘trioctahedral’ Y=6

20. DIOCTAHEDRAL MICA Muscovite -  KAl 2 (AlSi 3 O 10 )(F,O H ) The  dioctahedral  sheet silicates where each O or OH ion is surrounded by 2 trivalent cations, usually Al +3 .

21. TRIOCTAHEDRAL GROUP The  trioctahedral  sheet silicates where each O or OH ion is surrounded by 3 divalent cations, like Mg +2  or Fe +2 . Biotite-K(Mg,Fe) 3 AlSi 3 O 10 (F,OH) 2 Lepidolite-K(Li,Al,Rb) 2 (Al,Si) 4 O 10 (F,OH) 2 Phlogopite-KMg 3 (Al,Si 3 O 10 )(F,OH) 2 Zinnwaldite-KLiFeAl(AlSi 3 )O 10 (OH,F) 2 If the X ion is Ca, the mica is called as Brittle mica. ex- Clintonite: Ca(Mg,Al) 3 (Al 3 Si)O 10 (OH) 2

22. There is an another classification based according to the composition, they are Muscovite series These are alumino silicates of alkali metals without Mg or Fe and are colourless . The members are:- Muscovite -  KAl 2 (AlSi 3 O 10 )(F,O H ) Paragonite- NaAl 2 [(OH) 2 |AlSi 3 O 10 ] Lepidolite- K(Li,Al,Rb) 2 (Al,Si) 4 O 10 (F,OH) 2 Biotite series The series contain alkali metals which contain Mg,Fe and are dark in color. Biotite- K(Mg,Fe) 3 AlSi 3 O 10 (F,OH) 2 Phlogopite- KMg 3 (Al,Si 3 O 10 )(F,OH) 2 Zinnwaldite- KLiFeAl(AlSi 3 )O 10 (OH,F) 2

23. CRYSTAL STRUCTURE: Mica have sheet structure whose basic consists of two polymerized sheets silica . Two such sheets are juxtaposed with the vertices of their tetrahedrons pointing towards each other ; the sheets are cross linked with cations.

24. Micas are appear to be pseudohexagonal although they crystallize in monoclinic system , this is due to it sheet silicate structure. The sturcture of all micas is based on two opposite silica sheets in which upto one-fourth of the silicon ions have been replaced by aluminium ions. The net negative charge on the double sheet occasioned by the proxy of Al for Si in the tetrahedral sites is neutralized by the K located between the double sheets.

25. T-O-T STRUCTURE OF MICA The trioctahedral phyllosilicates are based on the structure where the octahedral layers are occupied by Mg +2  in the cation position. The dioctahedral phyllosilicates are based on the structure where the octahedral layers are occupied by Al +3  in the cation position. If 2 more of the OH ions in the octahedral layer are replaced by O, and these O become the apical Oxygens for another tetrahedral layer, this becomes a T-O-T layer that can bond to other T-O-T layers by weak Van der Waals bonds.

26. If an Al +3  is substituted for every 4 th  Si +4  in the tetrahedral layer, this causes an excess -1 charge in each T-O-T layer.  To satisfy the charge, K +1  or Na +1  can be bonded between 2 T-O-T sheets in 12-fold coordination .

27. Replacing 2 more Si +4  ions with Al +3  ions in the tetrahedral layer results in an excess -2 charge on a T-O-T layer, which is satisfied by replacing the K +1  with Ca +2 .   If the tetrahedral layers were stacked perfectly so that apical oxygens were to occur vertically aligned, then the structure would have hexagonal symmetry.  But, because this is not the case, most of the phyllosilicates are monoclinic

28. CRYSTALLOGRAPHY: The mica family often crystallize in monoclinic system with tabular forms and pseudohexagonal prisms. Contact twins united on (001) with twin axis [310] are fairly common. HABIT: They are typically tabular or foliated habits predominate . Discrete flakes,foliated masses;plumose,stellate,or globular. CLEAVAGE: The mica family of minerals shows perfect basal cleavage. HARDNESS: 2.5-3 LUSTER: Vitreous or silky luster

29. OCCURENCE: Muscovite has a widespread occurance and is characteristic of sedimentary, igneous and metamorphic rocks. Sediments eroded from igneous and metamorphic rocks often carry muscovite, accounting for its presence in sedimentary rocks. Igneous occurances include granite, grandorite, aplite, pegmatite and related felsic rocks. is common in granites and granitic pegmatites. Muscovite is very common in large variety of metamorphic rocks including slate, schist, phyllite, gneiss, hornfels and quartzite

30. OPTICAL PROPERTIES OF MAJOR MINERALS:

31. MUSCOVITE (under plane polarised light) CHEMICAL FORMULA : KAl 2 (AlSi 3 O 10 )(F,O H ) SYSTEM : Monoclinic COLOR : Colorless or shades of light green, red, or brown in hand sample; colorless in thin section. Pleochroism: No pleochroism. Crystal Habit : Well formed crystals are tabular and have pseudohexagonal outlines. More often found as micaceous flakes or tablets with irregular outlines. RELIEF: Fairly high

32. Under Crossed Nicols Extinction :Parallel to cleavage in all orientations. Birefringence : Strong OPTICAL ORIENTATION: The direction of cleavage traces is always the slower ray. b=z, X^c = +1 o  to +4 o  , Y^a = +1 o  to +3

33. REFRACTIVE INDICES  : Alpha = 1.556 to 1.570 Beta = 1.587 to 1.607 Gamma= 1.593to 1.611 OPTIC SIGN: Biaxial (-) Max Birefringence: δ =0.035 - 0.042 Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.

34. PHLOGOPITE (UNDER PLANE POLARIZED LIGHT) CHEMICAL FORMULA : KMg 3 (Al,Si 3 O 10 )(F,OH) 2 SYSTEM : Monoclinic COLOR : Pale brown tp colorless in thin sectin. Pleochroism: Slightly pleochroic. Crystal Habit : Well formed crystals are tabular and have pseudohexagonal outlines. More often found as micaceous flakes or tablets with irregular outlines. RELIEF: Fairly high

35. UNDER CROSSED NICOLS Extinction :Parallel to cleavage in all orientations. Birefringence : Strong OPTICAL ORIENTATION: The direction of cleavage traces is always parallel to the slower ray. b=y

36. REFRACTIVE INDICES  : Alpha = 1.551 to 1.562 Beta = 1.598 to 1.606 Gamma= 1.598to 1.606 OPTIC SIGN: Biaxial (-) Max Birefringence: δ =0.035 - 0.042 Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.

37. BIOTITE (UNDER PLANE POLARIZED LIGHT) CHEMICALFORMULA : K(Mg,Fe) 3 AlSi 3 O 10 (F,OH) 2 SYSTEM : Monoclinic COLOR : Brown to colorless in thin sectin.The absorption is stronger when the cleavage traces are parallel to the vibration plane. Pleochroism: Strongly pleochroic. Crystal Habit : Well formed crystals are tabular and have pseudohexagonal outlines. More often found as micaceous flakes or tablets with irregular outlines. RELIEF: High

38. UNDER CROSSED NICOLS Extinction :Parallel to cleavage in all orientations. Birefringence : Strong OPTICAL ORIENTATION: The direction of cleavage traces is always parallel to the slower ray. b=y

39. REFRACTIVE INDICES  : Alpha = 1.541 to 1.579 Beta = 1.574 to 1.638 Gamma= 1.574 to 1.638 OPTIC SIGN: Biaxial (-) Max Birefringence: δ =0.035 - 0.042 Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.

40. REFERENCE OPTICAL MINERALOGY ; PAUL F KERR MINDAT.Org Principles of Mineralogy ; William H. Blackburn & William H.Dennen Rock forming Minerals ; Deer,Howie,and Zussman

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