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Silicate Minerals, Structure and Properties

ByAkram Jan Jaffar

Silicate Mineral Structure

The Silicate minerals group is of great importance because they constitute about 90% of the Earth’s crust. They are found in all the common rocks except limestone. To understand the differences between major silicate mineral groups, it is necessary to study their structure. Every silicate mineral contains oxygen and silicon, and all except quartz, contain one or more additional elements to complete their structure.

The basic unit in all silicate minerals is the “silicon-oxygen tetrahedron”. This structure is composed of four oxygen atoms with the silicon atom at its centre. These tetrahedra can occur in the silicate structures either as single units or joined into chains, sheets, and three-dimensional networks by sharing oxygen atoms. Depending upon the type of structure built by these tetrahedra, the slicates are classified into the following groups:

Silicate Minerals Structures showing Oxygen and Silicon ratio.
  1. Nesosilicates. Nesosilicates include those minerals which are built up from isolated SiO4 tetrahedra. The atomic packing of the neosilicate structure is generally dense, which causes the minerals of this to have relatively high specific gravity and hardness. The crystal habit of these minerals is generally equidimensional, and they have poor cleavage. Olivine, Zircon, and Gernets are examples of this class.
  2. Sorosilicate. Sorosilicates are characterised by a linked pair of SiO4 tetrahedra. In this structure, only one oxygen is shared, giving the ratio of Si : O =2:7. Hemimorphite [Zn4 (Si2O7) (OH)2 H2O] is an example of this class.
  3. Cyclosilicate. Cyclosilicate is also known as the ring silicate. It contains rings of linked SiO4 tetrahedra having a ratio of Si:O = 1:3. These rings may consist of groups of three, four or six linked tetrahedra. Cyclosilicates from extremely strong minerals, such as beryl and tourmaline.
  4. Inosilicate (Chain Silicate). In this group, SiO4 tetrahedra are linked by sharing oxygens to form straight chains of indefinite length. These chains may be single chains or a double-linked channel. In the single chain structure, two of the four oxygens in each SiO4 tetrahedron are shared, giving a ratio of Si:O = 1:3. In the double chain structure, half of the tetrahedra share three oxygens, while the other half share two oxygens yielding a ratio of Si:0 =4:11. Inosilicates split easily in one crystal direction because bonds within chains are strong but are weaker between them. These minerals commonly form needle-like crystals, such as asbestos. Pyroxenes are examples of single-chain minerals, and amphiboles are examples of double-chain minerals.
  5. Phylosilicates (Sheet Silicates). The phylosilicates form sheet structures in which there is the continuous linking of hexagonal groups of silica tetrahedra. In this structure, three of the four oxygens in each SiO4 tetrahedron are shared with neighbouring tetrahedra, giving a ratio of Si:O =2:5. As the atomic bonding perpendicular to the sheet structure is generally weak, these minerals split easily into thin sheets. Flaky minerals, such as micas, chlorite and kaolinite, are examples of this class.
  6. Tectosilicates. They are also known as the framework silicates. In tectosilicates, SiO4 tetrahedra are linked in a three-dimensional framework. All oxygens in each SiO4 tetrahedron are shared with neighbouring tetrahedra. This results in a strongly bonded structure in which the ratio Si:O is 1:2. The minerals belonging to the tectosilicate group possess uniform properties throughout. Quartz are felspars are the examples of this class.

Table showing Silicate Mineral Groups and Examples.

Major group Structure Chemical formula Example
Nesosilicates isolated silicon tetrahedra [SiO4]4− olivine, garnet, zircon…
Sorosilicates double tetrahedra [Si2O7]6− epidote, melilite group
Cyclosilicates rings [SinO3n]2n− beryl group, tourmaline group
Inosilicates single chain [SinO3n]2n− pyroxene group
Inosilicates double chain [Si4nO11n]6n− amphibole group
Phyllosilicates sheets [Si2nO5n]2n− micas and clays
Tectosilicates 3D framework [AlxSiyO(2x+2y)]x− quartz, feldspars, zeolites

List of Silicate Minerals

01. Nesosilicates or Orthosilicates

Nesosilicates (word derived from ancient Greek language named after “Nesos Island”), also known as Orthosilicates, have the orthosilicate ion, present as isolated (insular) [SiO4]4− tetrahedra connected only by interstitial cations. The Nickel–Strunz classification is 09.A. Following minerals and mineral groups are included in this category. Minerals of this group along with their chemical structure are given as under;

  • Phenakite group
    • Phenakite – Be2SiO4
    • Willemite – Zn2SiO4
  • Olivine group
    • Forsterite – Mg2SiO4
    • Fayalite – Fe2SiO4
    • Tephroite – Mn2SiO4
  • Garnet group
    • Pyrope – Mg3Al2(SiO4)3
    • Almandine – Fe3Al2(SiO4)3
    • Spessartine – Mn3Al2(SiO4)3
    • Grossular – Ca3Al2(SiO4)3
    • Andradite – Ca3Fe2(SiO4)3
    • Uvarovite – Ca3Cr2(SiO4)3
    • Hydrogrossular – Ca3Al2Si2O8(SiO4)3−m(OH)4m
  • Zircon group
    • Zircon – ZrSiO4
    • Thorite – (Th,U)SiO4
    • Hafnon – (Hf,Zr)SiO4
  • Al2SiO5 group
    • Andalusite – Al2SiO5
    • Kyanite – Al2SiO5
    • Sillimanite – Al2SiO5
    • Dumortierite – Al6.5–7BO3(SiO4)3(O,OH)3
    • Topaz – Al2SiO4(F,OH)2
    • Staurolite – Fe2Al9(SiO4)4(O,OH)2
  • Humite group – (Mg,Fe)7(SiO4)3(F,OH)2
    • Norbergite – Mg3(SiO4)(F,OH)2
    • Chondrodite – Mg5(SiO4)2(F,OH)2
    • Humite – Mg7(SiO4)3(F,OH)2
    • Clinohumite – Mg9(SiO4)4(F,OH)2
  • Datolite – CaBSiO4(OH)
  • Titanite – CaTiSiO5
  • Chloritoid – (Fe,Mg,Mn)2Al4Si2O10(OH)4
  • Mullite (aka Porcelainite) – Al6Si2O13

02. Sorosilicates

Sorosilicates (from the Greek word Sōros ‘heap, mound’) have isolated pyrosilicate anions Si2O6−7. It consists of double tetrahedra with a shared oxygen vertex—a silicon: oxygen ratio of 2:7. The Nickel–Strunz classification is 09.B.

Examples include:

  • Thortveitite (Sc,Y)2(Si2O7)
  • Hemimorphite (calamine) Zn4(Si2O7)(OH)2·H2O
  • Lawsonite  CaAl2(Si2O7)(OH)2·H2O
  • Axinite  (Ca,Fe,Mn)3Al2(BO3)(Si4O12)(OH)
  • Ilvaite  CaFeII2FeIIIO(Si2O7)(OH)
  • Epidote group (has both (SiO4)4− and (Si2O7)6− groups}
    • Epidote  Ca2(Al,Fe)3O(SiO4)(Si2O7)(OH)
    • Zoisite  Ca2Al3O(SiO4)(Si2O7)(OH)
      • Tanzanite – Ca2Al3O(SiO4)(Si2O7)(OH)
    • Clinozoisite – Ca2Al3O(SiO4)(Si2O7)(OH)
    • Allanite – Ca(Ce,La,Y,Ca)Al2(FeII,FeIII)O(SiO4)(Si2O7)(OH)
    • Dollaseite-(Ce) – CaCeMg2AlSi3O11F(OH)
  • Vesuvianite (idocrase) – Ca10(Mg,Fe)2Al4(SiO4)5(Si2O7)2(OH)4

03. Cyclosilicates

Cyclosilicates (from Greek word “Kýklos” ‘circle’), or ring silicates, have three or more tetrahedra linked in a ring. The general formula is (SixO3x)2x−, where one or more silicon atoms can be replaced by other 4-coordinated atom(s). The silicon: oxygen ratio is 1:3. Double rings have the formula (Si2xO5x)2x− or a 2:5 ratio. The Nickel–Strunz classification is 09.C. Possible ring sizes include:

Some example minerals are:

  • 3-member single ring
    • Benitoite – BaTi(Si3O9)
  • 4-member single ring
    • Papagoite – CaCuAlSi2O6(OH)3.
  • 6-member single ring
    • Beryl – Be3Al2(Si6O18)
    • Bazzite – Be3Sc2(Si6O18)
    • Sugilite – KNa2(Fe,Mn,Al)2Li3Si12O30
    • Tourmaline – (Na,Ca)(Al,Li,Mg)3–(Al,Fe,Mn)6(Si6O18)(BO3)3(OH)4
    • Pezzottaite – Cs(Be2Li)Al2Si6O18
    • Osumilite – (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30
    • Cordierite – (Mg,Fe)2Al4Si5O18
    • Sekaninaite – (Fe+2,Mg)2Al4Si5O18
  • 9-member single ring
    • Eudialyte – Na15Ca6(Fe,Mn)3Zr3SiO(O,OH,H2O)3(Si3O9)2(Si9O27)2(OH,Cl)2
  • 6-member double ring
    • Milarite – K2Ca4Al2Be4(Si24O60)H2O

The ring in axinite contains two B and four Si tetrahedra and is highly distorted compared to the other 6-member ring cyclosilicates.

04. Single Chaine Inosilicates

Single-chain inosilicates are a group of silicate minerals where silica tetrahedra (SiO4)(\text{SiO}_4) are linked together in continuous single chains. These are most commonly known as the pyroxene group.

  • Pyroxene group
    • Clinopyroxene subgroup
      • Aegirine (or acmite) – NaFe3+Si2O6
      • Augite – (Ca,Mg,Fe)2Si2O6
      • Diopside – CaMgSi2O6
      • Hedenbergite – CaFe2+Si2O6
      • Jadeite – Na(Al,Fe3+)Si2O6
      • Pigeonite – (CaxMgyFez)(Mgy1Fez1)Si2O6, where 0.1 ≤ x ≤ 0.4, x + y + z = 1 and y1 + z1 = 1
      • Spodumene – LiAlSi2O6
    • Orthopyroxene subgroup
      • Enstatite – Mg2Si2O6
      • Ferrosilite – Fe2+2Si2O6
  • Pyroxferroite – (Fe,Mn,Ca)SiO3
  • Rhodonite – CaMn3Mn(Si5O15)
  • Wollastonite group
    • Pectolite – NaCa2Si3O8(OH)
    • Wollastonite – Ca3(Si3O9)

05. Double Chain Inosilicates

Double-chain inosilicates are silicate minerals where silica tetrahedra (SiO4)(\text{SiO}_4) link together to form paired (double) chains. These are characteristic of the amphibole group.

  • Amphibole group
    • Anthophyllite – (Mg,Fe)7Si8O22(OH)2
    • Cummingtonite series
      • Cummingtonite – Fe2Mg5Si8O22(OH)2
      • Grunerite – Fe7Si8O22(OH)2
    • Tremolite series
      • Tremolite – Ca2Mg5Si8O22(OH)2
      • Actinolite – Ca2(Mg,Fe)5Si8O22(OH)2
    • Hornblende – (Ca,Na)2–3(Mg,Fe,Al)5Si6(Al,Si)2O22(OH)2
    • Sodium amphibole group
      • Glaucophane – Na2Mg3Al2Si8O22(OH)2
      • Riebeckite (asbestos) – Na2FeII3FeIII2Si8O22(OH)2
      • Arfvedsonite – Na3(Fe,Mg)4FeSi8O22(OH)2

06. Phyllosilicates

Phyllosilicates (also called sheet silicates) are minerals in which silica tetrahedra (SiO4)(\text{SiO}_4) are linked to form two-dimensional sheets. The name comes from the Greek “Phyllon”  meaning leaf, reflecting their layered structure. Basic structure;

Each tetrahedron shares three oxygen atoms with neighbouring tetrahedra, creating wide, flat sheets.

(Si2O5)n(Si_2O_5)_n

  • The repeating unit is (Si₂O₅)ₙ
  • The fourth oxygen points out of the sheet, allowing bonding with other layers (like octahedral sheets of Al, Mg, or Fe)

Minerals included in this group are:

  • Ajoite  (K,Na)Cu7AlSi9O24(OH)6·3H2O
  • Apophyllite group
    • Fluorapophyllite-(K)  KCa4(Si8O22)F·8H2O
  • Bannisterite  (Ca,K,Na)(Mn2+,Fe2+)10(Si,Al)16O38(OH)8·nH2O
  • Carletonite  KNa4Ca4Si8O18(CO3)4(OH,F)·H2O
  • Cavansite  Ca(VO)Si4O10·4H2O    (dimorph of pentagonite)
  • Chlorite group – (Al,Fe2+,Fe3+Li,Mg,Mn,Ni)5−6(Al,Fe3+,Si)4(O,OH)18    (2:1:1 clays)
    • Chamosite – (Fe2+,Mg,Al,Fe3+)6(Si,Al)4O10(OH,O)8    (Fe endmember)
    • Clinochlore – Mg5Al(AlSi3O10)(OH)8    (Mg endmember)
    • Cookeite – (LiAl4◻)[AlSi3O10](OH)8
  • Chrysocolla  Cu2−xAlx(H2−xSi2O5)(OH)4·nH2O, x < 1
  • Ekanite  Ca2ThSi8O20
  • Gyrolite  NaCa16Si23AlO60(OH)8·14H2O
  • Hisingerite  Fe3+2(Si2O5)(OH)4·2H2O
  • Imogolite  Al2SiO3(OH)4
  • Kaolinite-Serpentine group
    • Greenalite (Fe2+,Fe3+)2−3Si2O5(OH)4
    • Kaolinite subgroup    (1:1 clays)
      • Dickite – Al2(Si2O5)(OH)4
      • Kaolinite – Al2Si2O5(OH)4
      • Halloysite – Al2Si2O5(OH)4
    • Serpentine subgroup
      • Amesite  Mg2Al(AlSiO5)(OH)4
      • Antigorite  Mg3Si2O5(OH)4
      • Chrysotile  Mg3Si2O5(OH)4
      • Lizardite  Mg3Si2O5(OH)4
  • Mica group
    • Brittle mica group
      • Clintonite – CaAlMg2(SiAl3O10)(OH)2
      • Margarite – CaAl2(Al2Si2)O10(OH)2
    • Dioctahedral mica group
      • Celadonite subgroup
        • Celadonite – K(MgFe3+◻)(Si4O10)(OH)2
      • Glauconite  K0.60−0.85(Fe3+,Mg,Al)2(Si,Al)4O10](OH)2
      • Muscovite  KAl2(AlSi3)O10(OH)2
      • Paragonite  NaAl2(AlSi3O10)(OH)2
      • Roscoelite – K(V3+,Al)2(AlSi3O10)(OH)2
    • Trioctahedral mica group
      • Aspidolite – NaMg3(AlSi3O10)(OH)2
      • Biotite  subgroup  K(Fe2+,Mg)2(Al,Fe3+,Mg,Ti)([Si,Al,Fe]2Si2O10)(OH,F)2
        • Annite  KFe2+3(AlSi3O10)(OH)2    (Fe endmember)
        • Phlogopite  KMg3(AlSi3)O10(OH)2    (Mg endmember)
      • Lepidolite  (polylithionite-trilithionite series) – K(Li2,Li1.5Al1.5)AlSi3−4O10(F,OH)2
      • Zinnwaldite  series – KFe2+2Al(Al2Si2O10)(OH)2
  • Neptunite  KNa2Li(Fe2+)2Ti2[Si4O12]2
  • Okenite  Ca10Si18O46·18H2O
  • Palygorskite group    (2:1 clays)
    • Palygorskite  Al2Mg2◻2Si8O20(OH)2(H2O)4·4H2O
    • Tuperssuatsiaite  Fe3+Fe3+2(Na◻)◻2Si8O20(OH)2(H2O)4·2H2O
  • Pentagonite  Ca(VO)Si4O10·4H2O    (dimorph of cavansite)
  • Pyrophyllite-Talc group
    • Pyrophyllite  Al2Si4O10(OH)2
    • Talc  Mg3Si4O10(OH)2    (2:1 clay)
  • Sepiolite group
    • Sepiolite Mg4(Si6O15)(OH)2·6H2O    (2:1 clay)
    • Falcondoite  (Ni,Mg)4Si6O15(OH)2·6H2O    (Ni analogue of sepiolite)
  • Smectite group    (2:1 clays)
    • Hectorite  Na0.3(Mg,Li)3(Si4O10)(F,OH)2
    • Montmorillonite  (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O
    • Nontronite  Na0.3Fe2((Si,Al)4O10)(OH)2·nH2O
    • Saponite  Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2·nH2O
    • Stevensite  (Ca,Na)xMg3−x(Si4O10)(OH)2
  • Stilpnomelane group
    • Stilpnomelane  (K,Ca,Na)(Fe,Mg,Al)8(Si,Al)12(O,OH)36·nH2O
  • Vermiculite  Mg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4·8H2O    (2:1 clay)

 

07. Tectosilicates

Tectosilicates (also called framework silicates) are the most structurally complex silicate minerals. In this group, every silica tetrahedron (SiO4)(\text{SiO}_4) shares all four of its oxygen atoms with neighboring tetrahedra, forming a 3D interconnected framework.

 

Minerals included in this group are;

  • Quartz group (silica)  SiO2
    • Chalcedony – a cryptocrystalline variety of silica composed mostly of quartz with some moganite
    • Polymorphs of silica
      • α-quartz – trigonal, “normal” quartz under 573 °C (846 K; 1,063 °F)
      • β-quartz – hexagonal, high-temperature quartz
      • Coesite – monoclinic
      • Cristobalite – tetragonal
      • Melanophlogite – cubic or tetragonal, rare
      • Moganite – monoclinic
      • Stishovite – tetragonal, extremely hard and dense
      • Tridymite – orthorhombic
  • Feldspar group
    • Alkali feldspar series (potassium feldspars or K-spar)
      • Microcline  KAlSi3O8
      • Orthoclase  KAlSi3O8
      • Anorthoclase  (Na,K)AlSi3O8
      • Sanidine  KAlSi3O8
    • Plagioclase feldspar series
      • Albite  NaAlSi3O8    (Na endmember)
      • Oligoclase  (Na,Ca)Al(Si,Al)Si2O8    (Na:Ca 90:10 to 70:30)
      • Andesine  (Na,Ca)Al(Si,Al)Si2O8    (Na:Ca 50:50 to 70:30)
      • Labradorite  (Ca,Na)Al(Al,Si)Si2O8    (Na:Ca 30:70 to 50:50)
      • Bytownite  (Ca,Na)Al(Al,Si)Si2O8    (Na:Ca 10:90 to 30:70)
      • Anorthite  CaAl2Si2O8    (Ca endmember)
    • Other feldspars
      • Buddingtonite  NH4AlSi3O8
      • Celsian  BaAl2Si2O8
      • Hyalophane (K,Ba)[Al(Si,Al)Si2O8]
  • Feldspathoid group
    • Cancrinite subgroup
      • Afghanite  (Na,K)22Ca10[Si24Al24O96](SO4)6Cl6
      • Cancrinite  (Na,Ca,◻)8(Al6Si6O24)(CO3,SO4)2·2H2O
      • Sacrofanite  (Na61K19Ca32)(Si84Al84O336)(SO4)26Cl2F6·2H2O
    • Leucite  K(AlSi2O6)
    • Nepheline subgroup
      • Nepheline – Na3K(Al4Si4O16)
    • Sodalite subgroup
      • Hauyne  Na3Ca(Si3Al3)O12(SO4)
      • Lazurite  Na7Ca(Al6Si6O24)(SO4)(S3)·H2O
      • Nosean  Na8(Al6Si6O24)(SO4)·H2O
      • Sodalite  Na4(Si3Al3)O12Cl
      • Tugtupite  Na4(BeAlSi4O12)Cl
  • Scapolite group
    • Marialite  Na4Al3Si9O24Cl
    • Meionite  Ca4Al6Si6O24CO3
  • Zeolite group
    • Amicite  K2Na2Al4Si4O16·5H2O
    • Analcime  Na(AlSi2O6)·H2O
    • Brewsterite subgroup  (Ba,Sr,Ca)Al2Si6O16·5H2O
    • Chabazite-Lévyne subgroup
      • Chabazite – M[Al2Si4O12]·6H2O
      • Lévyne  (Ca1−2,Na1−2,K2)Al2Si4O12·6H2O
    • Clinoptilolite subgroup  (Na,Ca,K)3−6(Al6−7Si29−30O72)·20H2O
    • Erionite subgroup  (Na1−2,K1−2,Ca1−2)2Al4Si14O36·15H2O
    • Faujasite subgroup  (Na1−2,Ca1−2,Mg1−2)3.5[Al7Si17O48]·32H2O
    • Ferrierite subgroup  [Mg2(K,Na)2Ca0.5](Si29Al7)O72·18H2O    (Ferrierite-Mg)
    • Heulandite subgroup  (Na,Ca,K)5−6[Al8−9Si27−28O72]·nH2O
    • Laumontite  CaAl2Si4O12·4H2O
    • Mordenite  (Na2,Ca,K2)4(Al8Si40)O96·28H2O
    • Natrolite subgroup
      • Mesolite – Na2Ca2Si9Al6O30·8H2O
      • Natrolite – Na2Al2Si3O10·2H2O
      • Scolecite – CaAl2Si3O10·3H2O
    • Paulingite subgroup  (K2,Ca,Na2,Ba)5[Al10Si35O90]·45H2O    (Paulingite-K)
    • Phillipsite subgroup
      • Phillipsite – (Ca3(Si10Al6)O32·12H2O    (Phillipsite-Ca)
    • Pollucite  (Cs,Na)2(Al2Si4O12)·2H2O
    • Stilbite subgroup
      • Stellerite – Ca4(Si28Al8)O72·28H2O
      • Stilbite – (NaCa4,Na9)(Si27Al9)O72·28H2O
    • Thomsonite subgroup – NaCa2Al5Si5O20·6H2O    (Thomsonite-Ca)
    • Yugawaralite  CaAl2Si6O16·4H2O

 

 

By Akram Jan Jaffar

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