major rock-forming minerals Sheet Silicates - Faculty web pages

Silicates - major rock-forming minerals
Sheet Silicates – (Si 2 O 5 ) 4-
Serpentine Group – Mg 3 Si 2 O 5 (OH) 4
Monoclinic; green; greasy, waxy, silky luster
Antigorite- flaky, Lizardite*– fine-grained, Chrysotile*– asbestiform
*true polymorphs
Chrysotile – principal asbestos mineral, mined in southern Quebec
Occurrence: Forms as alteration product of Mg silicates such as ol,
px, amph; low grade metamorphism of sea floor basalts
Used as ornamental and building stone

Silicates - major rock-forming minerals
Sheet Silicates – (Si 2 O 5 ) 4-
Clay Mineral Group – Kaolinite - Al 2 Si 2 O 5 (OH) 4
Triclinic; common mineral
Chief constituent of kaolin or clay
Occurrence: secondary mineral formed by weathering or
hydrothermal alteration of Al silicates
Used in brick making, fine pottery, china

Silicates - major rock-forming minerals
Sheet Silicates – (Si 2 O 5 ) 4-
Clay Mineral Group – Talc – Mg 3 Si 4 O 10 (OH) 2
Monoclinic; apple-green to white; greasy luster and feel
Compact and massive variety called soapstone
Occurrence: secondary mineral formed by alteration of Mg silicates
Low grade metamorphic rocks such as schists

Silicates - major rock-forming minerals
Sheet Silicates – (Si 2 O 5 ) 4-
Mica Group – Muscovite – KAl 2 (AlSi 3 O 10 )(OH) 2
Monoclinic; light-colored, vitreous luster
Tabular crystals w/prominent basal planes and diamond-shaped or
hexagonal cross-section; perfect (001) cleavage
Occurrence: granites, pegmatites, metamorphic rocks
Used as insulating material, called isinglass, in furnaces, stove
doors

Silicates - major rock-forming minerals
Sheet Silicates – (Si 2 O 5 ) 4-
Mica Group – Biotite – K(Mg,Fe) 3 (AlSi 3 O 10 )(OH) 2
Monoclinic; dark-colored, vitreous luster
Tabular crystals w/prominent basal planes and diamond-shaped or
hexagonal cross-section; perfect (001) cleavage
Occurrence: granites, pegmatites, gabbros,metamorphic rocks

Silicates - major rock-forming minerals
Sheet Silicates – (Si 2 O 5 ) 4-
Chlorite – (Mg,Fe) 3 (Si,Al) 4 O 10 (OH) 2 (Mg,Fe) 3 (OH) 6
Triclinic or Monoclinic; green, micaceous habit but not a mica
Folia not elastic
Composition of chlorite is t-o-t layers interlayered w/”brucite”
layers
Occurrence: common in metamorphic rocks, greenschists
Associated w/actinolite and epidote

Silicates - major rock-forming minerals
Framework Silicates – (SiO 2 ) Group
Quartz – SiO 2
Hexagonal; crystals commonly prismatic,w/striated faces
Conchoidal fracture, vitreous luster
Color variable
Occurrence: common,ubiquitous abundant mineral in great
variety of geologic environments
All rock types, highly resistant to weathering

Silicates - major rock-forming minerals
Framework Silicates – (SiO 2 ) Group
Quartz – SiO 2 Varieties:
Amethyst – purple; rose quartz, smoky quartz, milky quartz,
citrine
Microcrystalline varieties:
Fibrous – chalcedony, agate, onyx
Granular – flint, chert, jasper

Silicates - major rock-forming minerals
Framework Silicates – (SiO 2 ) Group
Quartz – SiO 2 Varieties:
Amethyst – purple; rose quartz, smoky quartz, milky quartz,
citrine
Microcrystalline varieties:
Fibrous – chalcedony, agate, onyx
Granular – flint, chert, jasper

Silicates - major rock-forming minerals
Framework Silicates – Feldspar Group
K-Feldspar Group: KAlSi 3 O 8
Microcline – Triclinic; 2 cleavages @ 90°; white to pale pink
Amazonite – aqua colored, Pb 2+ substitutes for K + creating color
Center
Characteristic grid twinning – albite and pericline twins, may show
perthite –exsolution bands
Occurrence: slowly cooled igneous rocks, granites, pegmatites,
metamorphic rocks - gneisses

Silicates - major rock-forming minerals
Framework Silicates – Feldspar Group
K-Feldspar Group: KAlSi 3 O 8
Orthoclase – Monoclinic; 2 cleavages @ 90°; white to pale pink
Short prismatic crystals
Carlsbad penetration twins common, also Baveno and Manebach
twins
May show perthite –exsolution bands
Occurrence: moderately cooled igneous rocks, granites,
pegmatites, high grade metamorphic rocks - gneisses

Silicates - major rock-forming minerals
Framework Silicates – Feldspar Group
K-Feldspar Group: KAlSi 3 O 8
Sanidine – Monoclinic; 2 cleavages @ 90°; white to pale pink
tabular crystals; Carlsbad penetration twins common
Difficult to impossible to tell from orthoclase in hand specimen
Need optical or X-ray techniques
High T form, rapid cooling in volcanics
Occurrence: volcanic igneous rocks, rhyolites, dacites, SiO 2 -rich
volcanics

Silicates - major rock-forming minerals
Framework Silicates – Feldspar Group
Cleavlandite
Albite
Plagioclase Group: NaAlSi 3 O 8 –CaAl 2 Si 2 O 8
Solid solution series – albite to anorthite end members
Triclinic; 2 cleavages @ 90°; white to gray
Tabular crystals; Albite polysynthetic twins common
Labradorite may have play of colors
Occurrence: mafic and felsic igneous rocks, metamorphic rocks,
sedimentary rocks

Silicates - major rock-forming minerals
Framework Silicates – Feldspar Group
Labradorite
Labradorite
Plagioclase Group: NaAlSi 3 O 8 –CaAl 2 Si 2 O 8
Solid solution series – albite to anorthite end members
Triclinic; 2 cleavages @ 90°; white to gray
Tabular crystals; Albite polysynthetic twins common
Labradorite may have play of colors
Occurrence: mafic and felsic igneous rocks, metamorphic rocks,
sedimentary rocks

Silicates - major rock-forming minerals
Framework Silicates – Feldspathoid Group
Feldspathoid Group: Framework silicates similar to feldspars
Contain 2/3 as much SiO 2 as feldspars, lower SG – 2.1-2.5
Nepheline – (Na,K)AlSiO 4 , hexagonal
Usually massive, compact, greasy luster, colorless to white
Occurrence: SiO 2 -poor, alkali-rich igneous rocks
NEVER FOUND WITH QUARTZ

Silicates - major rock-forming minerals
Framework Silicates – Feldspathoid Group
Feldspathoid Group:
Sodalite –Na 8 AlSiO 4 Cl 2 , isometric
Usually massive, blue, vitreous luster
Rare mineral, associated w/nepheline
Occurrence: SiO 2 -poor, alkali-rich igneous rocks
NEVER FOUND WITH QUARTZ

Silicates - major rock-forming minerals
Framework Silicates – Feldspathoid Group
Feldspathoid Group:
Leucite – KAlSi 2 O 6 , isometric to tetragonal depending on T
Trapezohedron crystals, white to gray, vitreous luster,
Rare mineral
Occurrence: Certain K-rich volcanics and shallow intrusives
NEVER FOUND WITH QUARTZ

Silicates - major rock-forming minerals
Framework Silicates – Zeolite Group
Chabazite Chabazite
Zeolite Group: Hydrous aluminosilicates w/Na, Ca, K and
Variable amounts of water in large voids of framework which are
interconnected spaces or channels
Occurrence: Found in cavities in basalt, alterations of volcanic
glass and tuff
Defines low grade regional metamorphic facies – zeolite facies

Heulandite
Silicates - major rock-forming minerals
Framework Silicates – Zeolite Group
Natrolite
Zeolite Group:
Natrolite – Na 2 Al 2 Si 3 O 10 2H 2 O
Chabazite – Ca 2 Al 2 Si 4 O 12 2H 2 O
Heulandite – CaAl 2 Si 7 O 18 6H 2 O
Stilbite – NaCa 2 Al 5 Si 13 O 36 14H 2 O
Stilbite

Heulandite
Silicates - major rock-forming minerals
Framework Silicates – Zeolite Group
Natrolite
Stilbite
Used commercially as molecular sieves – molecules too large to
pass through channels excluded
Catalytic cracking of hydrocarbon molecules
Water softeners –cation exchange properties – hard water w/high
Ca made soft by exchanging Ca 2+ for Na +