This document summarizes information about the olivine and mica mineral groups. It discusses the crystal structure, composition, optical properties, and occurrence of common olivine minerals like forsterite and fayalite. It also summarizes the classification, crystal structure, optical properties, and common members of the mica group like muscovite, biotite, and phlogopite. Diagrams are included showing the crystal structure and optical orientation of these minerals.

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 Sio4 divalent
atoms in six fold co-ordination.
 The seris have two end members Forsterite
(Mg2SiO4) and Fayalite(Fe2SiO4).

3. Members of Olivine
family
 The general formula of olivine series of
minerals is R2SiO4 , 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 (Fo63Fa37) .

4. Diagrammatical representation of
Olivine series
Fayalite
0
20
40
60
80
100
90 70 50 30 10
10
30
50
70
90
Fayalite
Fayalite

5. STRUCTURE OF OLIVINE
SERIES
Figure 1: The atomic scale structure of olivine
looking along the aaxis. 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: Mg2SiO4
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: Parallel 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: Fe2+
2SiO4
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
X2Y4-6Z8O20(OH,F)4 in which
X=K, Na, Ba, Ca, Cs, (H3O), (NH4)
Y=Al, Mg, Fe2+, Li, Cr, Mn, V, Zn
Z=Si, Al, Fe3+, Be, Ti
Structurally micas can be classed as
‘dioctahedral’ Y=4 and ‘trioctahedral’
Y=6

20. DIOCTAHEDRAL MICA
 Muscovite - KAl2(AlSi3O10)(F,OH)
 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)3AlSi3O10(F,OH)2
 Lepidolite-K(Li,Al,Rb)2(Al,Si)4O10(F,OH)2
 Phlogopite-KMg3(Al,Si3O10)(F,OH)2
 Zinnwaldite-KLiFeAl(AlSi3)O10(OH,F)2
 If the X ion is Ca, the mica is called as
Brittle mica.
 ex- Clintonite: Ca(Mg,Al)3(Al3Si)O10(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- KAl2(AlSi3O10)(F,OH)
• Paragonite-NaAl2[(OH)2|AlSi3O10]
• Lepidolite-K(Li,Al,Rb)2(Al,Si)4O10(F,OH)2
Biotite series
The series contain alkali metals which
contain Mg,Fe and are dark in color.
• Biotite- K(Mg,Fe)3AlSi3O10(F,OH)2
• Phlogopite-KMg3(Al,Si3O10)(F,OH)2
• Zinnwaldite-KLiFeAl(AlSi3)O10(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 4th 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: KAl2(AlSi3O10)(F,OH)
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 = +1o to +4o , Y^a = +1o 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:
KMg3(Al,Si3O10)(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)3AlSi3O10(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.

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