The Silicate Minerals



BackGround Into The Silicates

A silicate mineral is one containing silicon and oxygen. Of the 3000 mineral types known, 1000 are silicate minerals. The silicate group of minerals are divided into separate subgroups based on their internal structures, chemical makeup, and habit. The definitive character of each subgroup, however, is the arrangement of the polyhedra (silica tetrahedra and octahedra).

    The Silicate Subgroups include:
  • Tektosilicates/Tectosilicates: quartz, feldspars, feldspathoids, zeolites
  • Phyllosilicates: micas, serpentines, clays, chlorites
  • Inosilicates: amphiboles, pyroxenes, pyroxinoids
  • Nesosilicates: garnets, olivine, alumino-silicates
  • Sorosilicates: epidote, vesuvianite
  • Cyclosilicates: tourmalines, beryl
      Polyhedral Arrangements
  • Tektosilicates/Tectosilicates: "network" , each tetrahedron linked to four other tetrahedra in a 3D network , Si = 1 & O = 4
  • Phyllosilicates: "sheet", Si = 3 & O = 10
  • Inosilicates: "chain", Si = 1 & O = 3 or 5
  • Nesosilicates: "isolated", silicon tetrahedra do not share polyhedral elements, Si = 1 & O = 4
  • Sorosilicates: "bowties", Si = 2 & O = 7 per bowtie which in turn are linked by larger cations
  • Cyclosilicates: "ring", Si = 6 & O = 18
    Return to the top of page.

    Tektosilicates/Tectosilicates
      Quartz Group
    • Quartz- SiO , H=7 , massive to pseudo-hexagonal prisms, no good cleavage ... instead good conchoidal fracture when broken, density = 2.65
    • Coarse Crystalline Varieties of Quartz, same formula as listed above for quartz
    • Rock Crystal : colorless
    • Amethyst : purple
    • Rose Quartz : pink
    • Smoky Quartz : gray-black
    • Citrene : yellow-brown
    • Milky Quartz : white
    • Microcrystalline Varieties of Quartz, same formula as listed above for quartz
    • Chalcedony : brown to gray
    • Carnelian : red
    • Chrysoprase : apple-green
    • Agate : layered, brown to red
    • Flint : black
    • Chert : light gray
    • Tridymite- SiO
    • Cristobalite- SiO
    • Opal- SiO*nHO, H= 5.5-6.5, play of colors, vitreous to resinous luster
      Feldspar Group (Most abundant rock type in crust).
    • Sanidene- KAlSiO , only seen in volcanic rocks , can't be seen in hand sample
    • Orthoclase- KAlSiO , H=6, white color and streak, 1 perfect cleavage and 1 good cleavage at approximately 90 degrees, density = 2.54-2.57, lack of twin striations
    • Microcline- KAlSiO
    • Plagioclase -End Members and Intermediates: tabular crystals, 1 perfect cleavage and 1 good cleavage at approximately 90 degrees, twin striations; may be white, red or grey with white streak
    • Albite- NaAlSiO , white color
    • Oligoclase- Ab-An to Ab-An , gray color
    • Andesine- Ab-An to Ab-An , darker gray
    • Labradorite- Ab-An to Ab-An , has a blue flash
    • Bytonite- Ab-An to Ab-An , brown color
    • Anorthite- CaAlSiO , color varies from red to pink to gray
      Feldspathoid Group, never occur with the feldspars
    • Leucite- KAlSiO , trapezohedral crystals often embedded in volcanic rocks, white to grey color, H=5.5 - 6.5, chalky appearance
    • Nepheline- NaAlSiO , greasy luster, grey to white, H=5.5-6, generally massive, occurs with cancrinite and sodalite
    • Sodalite- Na(AlSiO)Cl , dark blue color, H=5.5-6, density low ~2.15-2.30, occurs with cancrinite and nepheline
    • Cancrinite- Na(AlSiO)(HCO) , yellow color, H=5-6, density low= 2.45, generally massive
    • Lazurite- (Na,Ca)(AlSiO)(SO,S,Cl), blue color, association with pyrite, H=5.5-6.5, density= 2.4-2.5, generally massive
      Scapolite Series (Group) , blocky prismatic crystals, imperfect cleavage on two faces, H=5-6, density is slightly high for a light colored mineral, white to pale lavender color
    • Marialite- Na(SiO)(Cl,CO)
    • Meionite- Ca(AlO )(Cl,CO)
      Zeolite Group
    • Heulandite- CaAlSiO*6HO , tomb-stoned shaped crystals: white, colorless, pink, yellowish, H= 3.5-4, density is low = 2.18-2.2, association with other zeolites
    • Stilbite- CaAlSiO*7HO , white to yellowish waxy broccoli-shaped crystals associated with other zeolites, pearly luster, H=3.5-4 , low density = 2.1-2.2
    • Laumontite- CaAlSiO*4HO , small white prismatic crystals, H=4 , density = 2.25-2.3 , 2 perfect cleavages
    • Chabazite- CaAlSiO*6HO , cubic to rhombohedral crystals , color may vary usually white or pink , H=4 , density low = 2.05-2.15 , association with other zeolites
    • Analcime- NaAlSiO*HO , trapezohedral crystals , colorless or white , H=5 , density = 2.3 , association with other zeolites
    • Natrolite- NaAlSiO*2H
      Phyllosilicates
        Serpentine Group
      • Antigorite- MgSiO(OH) , waxy luster , green to olive green color , H=3-5
      • Chrysotile- MgSiO(OH) , asbestos like , fibrous , silky luster
        Clay Group
      • Kaolinite- (AlSiO(OH)) , H=2 , dull earthy luster , white color , sticks to tongue
      • Talc- (MgSiO(OH)) , H=1 , pearly to greasy luster , 1 good cleavage , generally white
      • Montmorillonite- (Na,Ca)(Al,Mg)SiO(OH)*nHO , white to tan color , swells when water is added
        Mica Group
      • Muscovite- k Al(AlSiO)(OH) , colorless , one perfect cleavage , H=2 to 2.5
      • Phlogopite- k Mg(AlSiO)(OH) , light brown color , H=2.5 to 3 , 1 perfect cleavage , occurs in metamorphosed dolomites & Mg-rich limestones and peridotites , found with calcite
      • Biotite- k (Mg,Fe)(AlSiO)(OH) , H=2 to 3 , vitreous luster , a perfect cleavage , black to dark green in color
      • Lepidolite- k (LiAl)(AlSiO)(O,OH;F) , light pink/purple color , 1 good cleavage , small hexagonal habit , H=2.5 to 4
      • Margarite- CaAl(AlSiO)(OH) , pink color , H=3.5 to 5 , 1 perfect cleavage , vitreous luster
        Chlorite Group
      • Chlorite- (Mg,Fe)(Si,Al)O(OH)*(Mg,Fe)(OH) , dark green color and streak , massive foliated habit or sheets , 1 perfect cleavage , H=2 to 2.5
      • Glauconite- k (Fe,Mg,Al)(SiO)(OH) , green granules , H=2 , dull earthy luster , green streak , sedimentary , doesn't really fit profile of phyllosilicates
      • Apophyllite- k Ca(SiO)F*8HO , cubic to octahedral habit , colorless or white , H=5 , occurs with zeolites , it doesn't fit the phyllosilicate profile
      • Prehnite- CaAl(AlSiO)(OH) , pale green color , tabular crystals , but generally in globular encrustations or masses , H=6.5 , associated with zeolites , white streak , found growing on basalt
      • Chrysocolla- CuHSiO(OH) , green to green-blue color , displays conchoidal fracture , vitreous to earthy luster , H=2 to 4 , a product of weathering of Cu bearing rock , similar to turquoise

      Inosilicates
        Amphibole Group
      • Anthophyllite-Gedrite- (Mg,Fe)SiO(OH) , aggregates of prismatic crystals , sometimes fibrous , H=6 , white to grey to brown color , vitreous luster , white streak , 2 perfect cleavages , at 120o , occurence in magnesium rich metamorphic rocks , often with talc or cordierite , solid solution
      • Cummingtonite-Gruenerite- (Fe,Mg)SiO(OH) , aggregates of fibrous crystals often radiating , 2 perfect cleavages at 120o , H=6 , density=3.2 to 3.6 , pale to dark brown color , white streak , occurence in iron rich calcium poor metamorphic rocks , solid solution
      • Tremolite- Ca(Mg,Fe)SiO(OH) , long prismatic to acicular white crystals , 2 perfect cleavages , white streak , vitreous luster , H=6 , low to medium grade metamorphic rocks , especially metamorphosed dolomitic limestones , white in schist
      • Hexagonite- an uncommon mineral , a very distinctive lavender tremolite
      • Actinolite- Ca(Fe,Mg)SiO(OH) , prismatic crystals to a massive habit , green color , white streak , 2 cleavages , at 120o , H=6 , low to medium grade metamorphic rocks , common mineral
      • Hornblende- NaCa(Mg,Fe,Al)(Si,Al)O(OH) , prismatic crystals to massive , dark green to dark brown-black , white to grey streak , six sided cross section , 2 cleavages at 120o , H=6 , common in schists , the common amphibole of plutonic igneous rocks
      • Kaersutite- dark brown , Ti-rich variety of hornblende
      • Glaucophane-riebeckite- Na(Mg,Fe,Al)SiO(OH) , steel to grey blue color , blue grey to white streak , generally massive but sometimes acicular , 2 perfect cleavages at 120o when visible ; found in low to medium grade metamorphic rocks , sodium rich granites , rhyolites and granitic pegmatites
        Pyroxene Group
      • Enstatite- MgSiO , massive irregular blocky crystals , white to pale green , white streak , 2 good cleavages at 90o , H=6 , found in low calcium rocks : peridotites , pyroxinites , norites , basalts , andesite
      • Hypersthene- FeSiO , solid solution with enstatite therefore some similar habits and occurences , differences include brownish green to black color and grey streak
      • Diopside- CaMgSiO , white to pale green , white streak , H=6 , 2 good cleavages at 90o , occurence in medium to high grade metamorphic rocks: peridotites , calcite granulites (aka marbles)
      • Hedenbergite- CaFeSiO , solid solution with diopside , similar characteristics , dark greenish brown color , occurence in Ca-rich metamorphosed iron ore deposits , found with magnetite etc.
      • Augite- NaAlSiO , same characteristics as diopside but dark green color , the most common pyroxene of igneous rocks , especially basalts , gabbros and andesites
      • Jadeite- NaAlSiO , massive or fine-grained granular , usually green to white , H=6.5 , white streak , metamorphic rocks associated with serpentine
      • Spodumene- LiAlSiO , long prismatic crystals , flat platy masses , large crystals; 1 perfect cleavage and 1 parting , H=6.5 , white color and streak , vitreous luster; occurence in pegmatites with lepidolite , beryl , and tourmaline
        Pyroxinoid Group
      • Wollastonite- CaSiO , cleavable masses sometimes rectangular prismatic granular aggregates , silky to vitreous luster , H=5 , 1 perfect and 2 good cleavages , white color and streak , occurence in metamorphosed silica-rich limestones , often associated with diopside (green) and grossular (red) garnet
      • Pectolite- CaNaHSiO , aggregates of white needle-shaped grains , very sharp (can penetrate the skin) , H=5 , white streak , vitreous or silky luster , occurence with zeolites in cavities (as a secondary mineral) in basalts
      • Rhodonite- MnSiO , pink to rose red color , H=6 , generally massive ,white streak , (much harder than rhodochrosite which is sometimes confused with rhodonite)

      Nesosilicates
      Garnet Group
      - Hardness = 8 , soccer ball (geometric) habit , occurence (almost exclusively) in metamorphic rocks
        Pyralspite Group of Garnets
        - difference in color or occurence
      • Pyrope- MgAlSiO
      • Alamandine- FeAlSiO
      • Spessertine- MnAlSiO
        Ugrandite Group of Garnets
      • Uvarovite- CaCrSiO
      • Grossular- CaAlSiO
      • Andradite- CaFeSiO

        Olivine Group
        - soccerball shaped , green color , occurence in basalts , phenocrysts in aphanitic matrix
      • Forsterite- MgSiO , green color , round grains , occurence in basalt
      • Fayalite- FeSiO , dark green color , vireous luster , metamorphic
        Alumino-Silicate Group
        - the first three alumino-silicates listed below are polymorphs : they have the same chemical compostion but each has a unique outward appearance
      • Kyanite- AlSiO , blue color , hardness varies from 5 on the sides to 7 across , popsicle shaped bladed habit , occurence in metamorphic schists and phyllites , also can be found in eclogites
      • Andalusite- AlSiO , brown elongate squarish prisms , hardness=7.5 , occurence in metamorphic schists and phyllites
      • Sillimanite- AlSiO , aggregates of needle shaped brown to white grains (small fine grained clumps), occurence in metamorphic schists
      • Topaz- AlSiO(F,OH) , H=8 , colorless to pale green color , coarse prisms
      • Staurolite- FeAlSiO(OH) , brown color , elongate prisms , H=7.5 , occurence in schists and gneisses commonly associated with kyanite and garnet , penetration twins common giving a cross shape
      • Zircon- ZrSiO , tetrahedral pyramid habit , brown to colorless , H=7.5
      • Sphene (Titanite)- CaTiSiO , wedge-shaped brown crystals , H=5 to 5.5 , resinous (syrupy) luster, appearance similar to zircon but sphene isn't as hard

      Sorosilicates
      • Hemimorphite- Zn(SiO)(OH)*HO , tabular crystals generally in rounded masses or crusts, white to pale blue-grey color , secondary mineral of zinc ore deposits , associated with other zinc minerals sphalerite and smithsonite , ( weathering product appearing as a crust)
      • Epidote- CaAlSiO(OH) , elongate dark green crystals or fine grained pistachio green masses , striations present, H=6 to 7
      • Vesuvianite- Ca(Mg,Fe)Al(SiO)(SiO)(OH) , yellow brown square prisms , striations , resinous luster , occurence with grossular and andradite in granulites

      Cyclosilicates

      • Beryl- BeAlSiO , large coarse hexagonal prisms , white to pale green , H=7.5 to 8 , metamorphic , (emerald is a dark green beryl)
      • Cordierite- (Mg,Fe)AlSiO*nHO , short pseudo-hexagonal prisms , blue to blue grey color , H=7 to 7.5 , occurence in hornfels and argillaceous metamorphic rocks
      • Tourmaline- (Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)(BO)(SiO)(OH) , elongate triangular prisms , H=7 to 7.5 , striations ; color varies commonly black , browns ,greens ,blues


    • Return to the Mineral Reference