Earth Materials Lecture 10: Isotropic, Opaque and Accessory minerals

Question 1

List the isotropic but transparent minerals

Answer 1

− Garnets – aluminosilicates of Fe, Mg, Ca, Ti, Cr etc..

− The Sodalite Group – hydrous aluminosilicates of Na, Ca etc.

− Analcime - Na[Al,Si2O6].H2O.

− ~Leucite – K[Al,Si2O6] tetragonal (pseudocubic).

− Halides, e.g. fluorite – CaF2
.
− Sphalerite* – ZnS (occasionally uniaxial).

− Perovskite* – (Ca,Na,Fe2+,Ce,Sr)(Ti,Nb)O3 – orthorhombic.

Question 2

Describe the structure of garnet

Answer 2

Orthosilicate of general formula X3Y2Si3O12
X = divalent cation, Fe, Mg, Ca, Mn in 8-fold;
Y = trivalent cation, Al, Fe3+, Cr3+ in 6-fold.

Alternate SiO4 tetrahedra and YO6 octahedra, produce framework with voids in which occur the divalent X cations bordered by 8 O.

Habit varies with cation ratio and unit size:

CaAl - Dodecahedron
FeAl, MnAl - trapezohedron
CaFe - Dodecahedron/trapezohedron (combination of the two)

Question 3

Describe the chemical composition of garnets

Answer 3

A3B2(SiO4)3

A
Represents a divalent (2+) metal ion, such as calcium (Ca++), magnesium (Mg++), ferrous iron (Fe++), or manganese (Mn++)

B
Represents a trivalent (3+) metal ion, such as aluminum (Al+++), ferric iron (Fe+++), or chromium (Cr+++), silicon (Si), titanium (Ti), vanadium (V), or zirconium (Zr

Question 4

What are the 5 most common garnets?

Answer 4

Pyrope
Mg3Al2Si3O12
RI 1.74
Density 3.5

Almandine
Fe32+Al2Si3O12
RI 1.83
Density 4.3

Spessartine
Mn3Al2Si3O12
RI 1.80
Density 4.2

Grossular
Ca3Al2Si3O12
RI 1.73
Density 3.6

Uvarovite
Ca3Cr2Si3O12
RI 1.86
Density 3.9

Question 5

Describe the occurance and paragenesis of garnet

Answer 5

Mostly associated with metamorphic rocks.

Some igneous rocks (granite pegmatites, kimberlites etc).

Detrital grains in sediments.

➢ Pyrope in upper mantle peridotite; eclogite; metasediments.

➢ Almandine in metamorphosed mudrocks.

➢ Spessartine in some granite pegmatites.

➢ Grossular and andradite in metamorphosed Ca rocks.

➢ Uvarovite in metamorphosed ultrabasic rocks

Question 6

list and describe the opaque minerals in PPL/XPL

Answer 6

− Magnetite – Fe3O4 – cubic.
− Ilmenite - FeTiO3 – trigonal.
− Pyrite - FeS2 - cubic.
− Chalcopyrite - CuFeS2 – tetragonal.
− Galena – PbS - cubic.
− Gold – Au, silver – Ag – cubic.
− Stibnite – Sb2S3 – orthorhombic.
− Arsenopyrite – FeAsS – monoclinic.
− Bornite – Cu5FeS4 – tetragonal.
− Tetrahedrite – Cu12SbS13 – cubic.
− Wolframite – (Fe,Mn)WO4 – monoclinic

Question 7

Describe Opaque minerals: reflected light microscopy

Answer 7

Uses light shone vertically onto polished sample.
⚫ Observe in PPL and XPL with similar properties and results to
transmitted light.

Question 8

Define reflectance in reflected light microscopy

Answer 8

Measured as difference in light intensity between incident and
reflected light. Record as %.

Question 9

Define bireflectance in reflected light microscopy

Answer 9

The change in reflectance in PPL as stage rotated. Measured as difference between max and min. Record as weak, moderate, strong etc.

Question 10

Define reflectance pleochroism in reflected light microscopy

Answer 10

(Basically pleochroism)
The colour change observed during stage rotation in PPL. Applies to anisotropic minerals only.

Question 11

Define anisotropism in reflected light microscopy

Answer 11

(Basically birefringence)
Change in colour intensity as stage is rotated in XPL. Applies to anisotropic minerals only. Weak, strong, moderate plus colour(s).

Question 12

Describe Apatite

Answer 12

Apatite - Ca5
(PO4)3(F,Cl,OH) – fluor/ chlor/ hydroxylapatite; francolite (CO2)

Hexagonal.

Columns of Ca polyhedra parallel to c-axis, with Ca in 7- and 9-fold coordination.

6 columns of 9-fold form circle in which F / OH
reside.
PO4 tetrahedra linked to 7-fold polyhedra.

Yellow, bluish hexagonal prisms.

Cleavage poor {0001} and {10T0}.
Twins on {1121} or rarely {1013}

Question 13

Describe the optical properties of apatite

Answer 13

Usually colourless in thin section.

Coloured varieties may be weakly
pleochroic.

Moderate relief;
RIs ~1.6.
Low birefringence, 1st order; varies with Cl, F, OH – dark grey to pale grey.

Extinction straight against prism edge.

Crystals usually small so little else seen.

Question 14

Describe the occurrence of Apatite

Answer 14

Most abundant phosphorous mineral.

Accessory phase in various igneous and metamorphic rocks, and hydrothermal veins:
➢ Granites and pegmatites; carbonatites – alkaline complexes;
➢ E.g. Phalaborwa, South Africa, 2060 Ma (phoscorite).

Most abundant in sedimentary rocks (bone beds).
➢ Largest is the Permian Phosphoria Fmn, NW USA

Question 15

Describe Zircon

Answer 15

Zr[SiO4] – Hf, U, Pb; tetragonal
Endmember of the Zircon Group:
Zircon ZrSiO4
Hafnon HfSiO4
Thorite ThSiO4
Coffinite USiO4

Structural similarities with other minerals,
e.g. scheelite, monazite, rutile and garnet.

Orthosilicate of isolated SiO4
tetrahedra sharing corners and edges with ZrO8 polyhedra.
The polyhedra share edges with each other to form chains parallel to (100).

Produces (110) cleavage, prismatic habit and
high birefringence.

Browns etc in hand specimen. Yellow in UV.

Question 16

Describe the optical properties of Zircon

Answer 16

Crystals usually tiny, micron scale.

Colourless to pale brown in PPL.
Surface often appears highly fractured.

High relief;
RIs 1.9-2.0
High birefringence,
3rd order colours.

Cleavage {110} imperfect, {111} poor.

In XPL often appears like a stained glass window.

Twinning rare {111}.
Zoning common.

Straight extinction.

Question 17

Describe Titanite

Answer 17

Ca,Ti,FeSiO4

Monoclinic orthosilicate.

Distinct lozenge shape

(sphenoidal - old
name is sphene).

Brown/black in hand specimen. Rarely green.

Zig-zag chains of TiO6 octahedra
lie parallel to a-axis, cross-linked by isolated SiO4 tetrahedra.

Produces large cavities in which
occurs Ca in 7-fold coordination.
OH, F at some SiO4 apices.

Various substitutions: Al, Fe3+ octahedral.
REEs, Y, Nb, Ta, Zr, V, U, Th for Ca etc.

Question 18

Describe the optical properties of Titanite

Answer 18

Usually brown in PPL, can be weakly pleochroic.
Varies with Ti / Fe content.

High relief; RIs 1.8 – 2.1.

Cleavage good on {110}.

Occasionally lamellar twins on {221}.

Extreme birefringence, 5/6th order.

Extinction difficult to determine.

Question 19

Describe the occurences of Zircon and Titanite

Answer 19

Zircon:
a variety of rocks:
* primary in igneous and metamorphic rocks;
* detrital in sediments – very robust, survives recycling.

Titanite:
*Common in intermediate and some acid igneous rocks; schists
and gneisses;
*detrital in sediments.