Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Olivine - Mg
2
SiO
4
– Fe
2
SiO
4
Orthrhombic; pale yellow to olive green; vitreous luster
Usually as granular masses; Peridot – gemstone, nodules in basalt
Conchoidal fracture; H = 6.5 - 7, S.G. = 3.3-4.4
Structure – octahedra linked to isolated (SiO
4
)
4-
tetrahera
Occurrence: mafic to ultramafic igneous rocks, basalt to peridotite
Dunite – all olivine rock
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Garnet Group – A
3
B
2
(SiO
4
)
3
(general formula)
Isometric; usually red, may be pink to green; vitreous luster
Dodecahedral xstals and massive; Conchoidal fracture;
H = 6.5 – 7.5, S.G. = 3.5 - 4.3
Structure – A site = large divalent cations, Mg2+, Fe2+, Mn2+;
B site = smaller trivalent cations, Al3+, Fe3+, Cr3+
Pyrope (Mg-rich), Almandine (Fe-rich), Spessartine (Mn-rich),
Uvarovite (Cr-rich), Grossular (Ca-rich), Andradite (CaFe-rich)
Occurrence: metamorphic rocks, high P and high T
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Zircon – ZrSiO
4
Tetragonal; colorless to pink or brown; adamantine luster
Elongate xstals w/dipyramidal ends;
H = 7.5, S.G. = 4.7
Zircon contains significant U and Th, radioactive, metamict
Used in radioactive age dating in U-Pb method; found in heavy
mineral fraction of sandstones, useful for provenance
Occurrence: widely distributed accessory mineral in all rock types,
especially common in granite
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Andalusite – Al
2
SiO
5
Group
Orthorhombic; white to pink; vitreous luster
Coarse square prisms w/chiastolite crosses – dark colored carbon
inclusions
H = 7.5, S.G. = 3.2
Occurrence: contact metamorphic rocks (pelitic schists) –
conditions of High T and Low P
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Kyanite – Al
2
SiO
5
Group
Triclinic; dark blue; vitreous luster
Long, bladed xstals; perfect cleavage in one direction
H = 5 - 7, different along length and width; S.G. = 3.5
Occurrence: High P metamorphic rocks (blueschists)
Used as an index mineral in defining metamorphic zones
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Sillimanite – Al
2
SiO
5
Group
Orthorhombic; white to gray; vitreous luster
Long, slender xstals in parallel groups; frequently fibrous called
fibrolite
H = 6 - 7; S.G. = 3.2; one direction of cleavage
Occurrence: High P and High T metamorphic rocks (pelitic
schists) - regional metamorphism
Used as an index mineral in defining metamorphic zones
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Staurolite – Fe
2
Al
9
(Si
4
O
23
)(OH)
Monoclinic; brown; resinous to vitreous luster
Prismatic xstals with cruciform twins
H = 7 – 7.5; S.G. = 3.7;
Occurrence: Formed during regional metamorphismof Al-rich
rocks; found in gneisses and schists
Used as an index mineral medium grade metamorphic rocks
Silicates - major rock-forming minerals
Isolated Tetrahedral Silicates – (SiO
4
)
4-
Sphene (titanite) – CaTiO(SiO
4
)
Monoclinic; honey yellow to brown; resinous to adamantine luster
Wedged-shaped xstals
H = 5 – 5.5; S.G. = 3.4
Occurrence: Common in granites, gneisses, and chlorite schists
Used in radioactive age dating
Silicates - major rock-forming minerals
Paired Tetrahedral Silicates – (Si
2
O
7
)
6-
Epidote – Ca
2
(Al,Fe)Al
2
O(SiO
4
)(Si
2
O
7
)(OH)
Monoclinic; pistachio-green to black; vitreous luster
Elongate, prismatic xstals w/striations parallel to length; also
granular
H = 6 – 7; S.G. = 3.3-3.4
Occurrence: Common and widespread mineral; characteristic of
low to medium-grained metabasites and marbles
Associated w/actinolite, chlorite and albite in mafic rocks
Silicates - major rock-forming minerals
Ring Tetrahedral Silicates – (Si
6
O
18
)
12-
Beryl – Be
3
Al
2
(Si
6
O
18
)
Hexagonal; blue-green to light yellow to green; vitreous luster
Prismatic xstals w/hexagonal cross-sections vertically striated
H = 7.5 – 8; S.G. = 2.65 -2.8
Occurrence: Common and widely distributed in granitic rocks
and pegmatites
Gem beryl – emerald, aquamarine, morganite (pink), golden beryl
Harder than apatite; softer than corundum
Silicates - major rock-forming minerals
Ring Tetrahedral Silicates – (Si
6
O
18
)
12-
Tourmaline –
(Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)
6
(BO
3
)
3
(Si
6
O
18
)(OH)
4
Hexagonal; vitreous luster; color varies w/composition
Prismatic xstals w/trigonal cross-sections vertically striated
H = 7 -7.5; S.G. = 3
Occurrence: Common accessory in igneous and metamorphic
rocks and granite pegmatites
Schorl- black, Fe-rich form, most common
Dravite – brown; rubellite – pink, elbaite - watermelon
Silicates - major rock-forming minerals
Single Chain Silicates – Pyroxenes – (Si
2
O
6
)
4-
Tourmaline –
(Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)
6
(BO
3
)
3
(Si
6
O
18
)(OH)
4
Hexagonal; vitreous luster; color varies w/composition
Prismatic xstals w/trigonal cross-sections vertically striated
H = 7 -7.5; S.G. = 3
Occurrence: Common accessory in igneous and metamorphic
rocks and granite pegmatites
Schorl- black, Fe-rich form, most common
Dravite – brown; rubellite – pink, elbaite - watermelon
Silicates - major rock-forming minerals
Single Chain Silicates – Pyroxenes – (Si
2
O
6
)
4-
Orthopyroxene – Opx -
orthorhombic
Enstatite – Ferrosilite (Mg,Fe)SiO
3
;
Hypersthene- int. composition
Stout prisms, 90° cleavage, no H
2
O in structure
Color varies w/composition; more Fe-rich = darker color
Occurrence: Gabbros, peridotite, basalt, andesite; very high
grade metamorphic rocks – granulite facies
Enstatite
Hypersthene
Silicates - major rock-forming minerals
Single Chain Silicates – Pyroxenes – (Si
2
O
6
)
4-
Clinopyroxene – Cpx -
monoclinic
Diopside – Hedenbergite: CaMgSi
2
O
6
-CaFeSi
2
O
6
Augite- most common; Stout prisms, 90° cleavage, no H
2
O
Color varies w/composition; more Fe-rich = darker color
Occurrence: Gabbros, basalt, andesite, marble; very high
grade metamorphic rocks – granulite facies
Diopside
Augite
Augite
Silicates - major rock-forming minerals
Double Chain Silicates - Amphiboles – (Si
4
O
11
)
6-
Tremolite – Actinolite:
Ca
2
Mg
5
Si
8
O
22
(OH)
2
-
Ca
2
Fe
5
Si
8
O
22
(OH)
2
Monoclinic; contain H2O, elongate xstals, often acicular,
color varies w/composition; white to green
2 cleavages @ 60° & 120°
Occurrence: Tremolite – metamorphosed limestones
Actinolite – greenschist facies, metamorphosed igneous rocks
Tremolite
Actinolite
Silicates - major rock-forming minerals
Double Chain Silicates - Amphiboles – (Si
4
O
11
)
6-
Hornblende: (
Ca,Na)
2-3
(Mg,Fe,Al)
5
Si
6
(Si,Al)
2
O
22
(OH)
2
Monoclinic; contain H2O, black prismatic xstals
most common amphibole; 2 cleavages @ 60° & 120°
Occurrence: Important mineral in both igneous and metamorphic
rocks
Silicates - major rock-forming minerals
SIngle Chain Silicates – Pyroxenoids– (SiO
3
)
2-
Wollastonite –
CaSiO
3
Triclinic; vitreous luster, white
Bladed xstals w/high aspect ratio (length to width)
Twisted tetrahedral chains – lower symmetry than pyroxenes
Occurrence: Contact metamorphic mineral formed by fluid
Metamorphism (metasomatism)
CaCO
3
+ SiO
2
= CaSiO
3
+ CO
2