ERTH 2002

1
ERTH 2002

• Mineral of the Week (MoW)
• olivine
• 2. pyroxene
• 3. feldspar (some review)
• 4. garnet ( Al2SiO5)
• 5. amphibole
• 6. mica
• 7. quartz
• 8. clays
• 9. carbonates (some review)
• 10. Fe-Ti oxides
• 11. sulphides (some review)

properties chemical composition crystal
structure phase diagrams occurrence use
2
MoW 4 Garnet (Nesse, Ch. 16, p. 310-314)
photos mindat.org
3
MoW 4 Garnet (Nesse, Ch. 16, p. 310-314)
an orthosilicate with the general formula
X3Y2Si3O12 where X Fe2,Mg,Ca,Mn Y
Al,Fe3,Cr3
extensive solid solution sometimes grouped for
convenience as follows
Y Al (Fe,Mg,Ca,Mn)3Al2Si3O12 Y Fe3 or
Cr3 (Ca,Mn)3(Fe3,Cr3)2Si3O12
end members
almandine (Alm) Fe3Al2Si3O12 pyrope (Pyr)
Mg3Al2Si3O12 spessartine (Sps) Mn3Al2Si3O12
(pyralspite series)
grossular (Grs) Ca3Al2Si3O12 andradite (And)
Ca3Fe32Si3O12 uvarovite (Uvr) Ca3Cr32Si3O12
(ugrandite series)
hydrogrossular Ca3Al2Si2O12(OH)4
4
MoW 4 Garnet (Nesse, Ch. 16, p. 310-314)
an orthosilicate with the general formula
X3Y2Si3O12 where X Fe2,Mg,Ca,Mn Y
Al,Fe3,Cr3
recognised in thin section by high
relief equant shape (6-sided to rounded) lack of
cleavage reddish colour isotropism
isometric isotropic
5
MoW 4 Garnet (Nesse, Ch. 16, p. 310-314)
an orthosilicate with the general formula
X3Y2Si3O12 where X Fe2,Mg,Ca,Mn Y
Al,Fe3,Cr3
crystal structure isolated SiO44-
tetrahedra linked to X-site cations (CN8) and
Y-site octahedra (CN6)
X complete solid solution (Fe, Mg, Ca, Mn) Y
either Al or Fe3 no substitution for Si
6
MoW 4 Garnet (Nesse, Ch. 16, p. 310-314)
PPL
PPL
MR-3 garnet in gneissic amphibolite (6.25 mm)
OM-6 garnet in sillimanite schist (4 mm)
XN
XN
7
MoW 4 Garnet (Nesse, Ch. 16, p. 310-314)
occurrence widespread in medium- to
high-grade metamorphic rocks (most bulk
compositions) common accessory mineral in
Al-rich felsic igneous rocks common detrital
mineral in sedimentary rocks main Al-bearing
phase in mantle at depths gt 75 km
(xenocrysts in kimberlites indicate mantle
source) in all these cases, garnet
composition reflects rock composition
P-T conditions of formation use
gemstone abrasive filter
8

MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)

andalusite

sillimanite
kyanite
photos mindat.org
9
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
orthosilicate group with 3 common
polymorphs general formula Al AlO SiO4 (2
different Al sites)
andalusite
sillimanite
kyanite
VIAl VAlO SiO4 one Al in 6-fold site one Al in
5-fold site
VIAl VIAlO SiO4 both Al in 6-fold sites
VIAl IVAlO SiO4 one Al in 6-fold site one Al in
4-fold site
triclinic
orthorhombic
orthorhombic
10
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
orthosilicate group with 3 common
polymorphs general formula Al AlO SiO4 (2
different Al sites)
var. chiastolite cruciform inclusions
(001) cross-section this orientation has low
birefringence!!!
var. fibrolite fine needles or fibres elongated
// c-axis (cat-hair texture)
11
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
orthosilicate group with 3 common
polymorphs general formula Al AlO SiO4 (2
different Al sites)
(001) cross-section this orientation has low
birefringence!!!
And biaxial ve parallel extinction low
birefringence
Sil biaxial ve parallel extinction mod
birefringence
And
Sil
interference colour range
12
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
orthosilicate group with 3 common
polymorphs general formula Al AlO SiO4 (2
different Al sites)
And biaxial ve parallel extinction low
birefringence
Ky biaxial -ve inclined extinction low-mod
birefringence
Ky
Sil biaxial ve parallel extinction mod
birefringence
And
Sil
interference colour range
13
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
phase diagram for Al2SiO5
G 3.53-3.67
G 3.23-3.27
G 3.13-3.16
Nesse, Fig. 5.3b
P-T stability of polymorphs reflects density
(specific gravity, G)
14
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
Al2SiO5 minerals in thin section, ERTH 2001
collection
PPL
PPL
PPL
XN
XN
XN
OM-13 andalusite (var. chiastolite) 2 mm
OM-4 kyanite 4 mm
OM-6 sillimanite 4 mm
15
MoW 4B Al2SiO5 (Nesse, Ch. 16, p. 314-319)
occurrence common minerals in Al-rich
metamorphic rocks (e.g., pelites) may also
occur in Al-rich felsic igneous rocks rarely
found in metabasites at unusual P-T conditions
locally found in hydrothermal veins
specific polymorph present depends on P-T
conditions use manufacture of refractory
materials (after conversion to mullite)
ceramics, abrasives, fillers kyanite,
andalusite, more widely used than sillimanite