Sheetsilicates There are many minerals with a sheet silicate structure, including mica and clay minerals. They share the same building units: (i) a tetrahedral sheet [Si2O5]2- and one of two different kinds of octahedral sheets: (ii) trioctahedral sheet – an infinite sheet of edge-sharing octahedra, with all the octahedra occupied by a divalent ion (e.g. Mg2+). The formula of a trioctahedral sheet is Mg3(OH)6. (iii) dioctahedral sheet – an infinite sheet of edge-sharing octahedra, with two thirds (2/3) of the octahedra occupied by a trivalent ion (e.g. Al3+). The formula of a dioctahedral sheet is Al2(OH)6.

A dioctahedral sheet – contains trivalent ions (e.g. Al3+)

Tetrahedral sheet linked to a trioctahedral sheet

Tetrahedral sheet linked to an octahedral sheet - side view a T-O layer

T-O layers stacked one on the other – a 1:1 layer silicate

The three main groups of layer silicates The 1:1 layer silicates T–O The 2:1 layer silicates T–O–T The 2:1:1 layer silicates T-O-T (O) dioctahedral trioctahedral dioctahedral trioctahedral

The 1:1 (T-O) layer silicates 7Å Sheets are weakly bonded by van der Waals bonds . kaolinite (dioctahedral) serpentine (trioctahedral) Al2Si2O5(OH)4 Mg3Si2O5(OH)4

Compositions of 1:1 Layer silicates   e.g. kaolinite: 1 T sheet + 1 O sheet = TO layer + Hydroxyl dioctahedral Si2O52- + Al2(OH)6 - 2(OH)- = Al2Si2O5(OH)4 e.g serpentine 1 T sheet + 1 O sheet = TO layer + Hydroxyl trioctahedral Si2O52- + Mg3(OH)6 - 2(OH)- = Mg3Si2O5(OH)4 Kaolinite and serpentine are actually group names, as each contains three polymorphs. Kaolinite Serpentine dickite antigorite halloysite chrysotile nacrite lizardite

Kaolinite Al2(OH)4[Si2O5]

Fitting the tetrahedral sheet onto the octahedral sheet Unless the spacings between the tetrahedral apices match the repeat in the octahedral sheet, the two sheets cannot fit together. The tetrahedral sheet can change its spacing by rotating the individual tetrahedra. The fit with the octahedral sheet can be improved by curving the two sheets.

The serpentine group minerals, with general formula Mg3Si2O5(OH)4 have three different forms : lizardite, chrysotile and antigorite. In lizardite, the sheets are flat, but the tetrahedra have to rotate so that their apices can connect to the octahedral sheet, just as in kaolinite.

In chrysotile, the tetrahedra are tilted and the layers are curved to accommodate the longer repeat in the octahedral layer. The sheets end up being rolled up like a carpet. Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999). Electron microscope image

Chrysotile C = chrysotile T = talc The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates

In antigorite, the tetrahedra are tilted and the layers are curved In antigorite, the tetrahedra are tilted and the layers are curved. This is accompanied by periodic switching of the direction of the tetrahedral layer. High-resolution transmission electron microscope image

Serpentine group Antigorite, Lizardite Chrysotile Mg3(OH)4[Si2O5] Antigorite Chrysotile

Sheets are weakly bonded The 2:1 (T-O-T) layer silicates interlayer Sheets are weakly bonded by van der Waals bonds (i) no interlayer ions e.g pyrophyllite (dioctahedral) talc (trioctahedral) Al2Si4O10(OH)2 Mg3Si4O10(OH)2

Compositions of 2:1 Layer silicates (i) with no interlayer cations   (i) with no interlayer cations e.g pyrophyllite 2 T sheets + 1 O sheet = TOT layer + Hydroxyl dioctahedral 2(Si2O52-) + Al2(OH)6 - 4(OH)- = Al2Si4O10(OH)2 e.g talc 2 T sheets + 1 O sheet = TOT layer + Hydroxyl trioctahedral 2(Si2O52-) + Mg3(OH)6 - 4(OH)- = Mg3Si4O10(OH)2 The TOT layers are electrically neutral and so don’t need any additional interlayer cations to hold them together. They are very weakly bonded by van der Waals bonding, and this is why both are very soft and feel waxy.

Talc Mg3(OH)2[Si4O10]

Pyrophyllite, Al2(OH)2[Si4O10]

The 2:1 (T-O-T) layer silicates interlayer (ii) with interlayer ions e.g. muscovite (dioctahedral) phlogopite (trioctahedral) KAl2(AlSi3O10)(OH)2 KMg3(AlSi3O10)(OH)2

Compositions of 2:1 layer silicates II (ii) with interlayer cations   One quarter of the Si4+ in the tetrahedral sites is replaced by Al3+ giving the layer a net negative charge. This is compensated by interlayer cations. e.g. muscovite dioctahedral TOT layer + interlayer cation Al2(Si4O10)(OH)2- + K+ - Si4+ + Al3+ = K Al2(AlSi3O10)(OH)2 e.g. phlogopite trioctahedral Mg3(Si4O10)(OH)2- + K+- Si4+ + Al3+ = K Mg3(AlSi3O10)(OH)2 When some Fe2+ substitutes for Mg the result is the common mica mineral biotite - K (Mg,Fe)3(AlSi3O10)(OH)2

Muscovite KAl2(OH)2[AlSi3O10]

Biotite, K(Mg,Fe)3(AlSi3O10)(OH)2

Lepidolite, K(Li,Al)2-3(O,OH,F)2[AlSi3O10] Phlogopite, KMg3(OH)2[AlSi3O10]

If one half of the Si4+ in the tetrahedral sites is replaced by Al3+ giving the layer a net negative charge. This is compensated by divalent interlayer cations, yielding the brittle micas e.g. margarite dioctahedral TOT layer + interlayer cation Al2(Si4O10)(OH)2- + Ca2+ - 2Si4+ + 2Al3+ = CaAl2(Al2Si2O10)(OH)2   e.g. xanthophyllite trioctahedral TOT layer + interlayer cation Mg3(Si4O10)(OH)2- + Ca2+- 2Si4+ + 2Al3+ = CaMg3(Al2Si2O10)(OH)2

Margarite, CaAl2(OH)2[Al2Si2O10]

The 2:1 (T-O-T) layer silicates interlayer (iii) with interlayer ions and H2O Trioctahedral Dioctahedral Smectite group Vermiculite group

The 2:1:1 (T-O-T-O) layer silicates interlayer with an octahedral sheet between the T-O-T layers e.g. chlorite

Compositions of 2:1:1 Layer silicates   The most common mineral in this group is chlorite which can be thought of in its simplest form as a TOT talc layer with a brucite sheet in between, i.e. Chlorite - trioctahedral - Mg3Si4O10(OH)2•Mg3(OH)6 The formula above is idealized as chlorites generally contain Al substituting for Si, and other octahedral cations such as Fe, Ni and Cr form various individual species. A general formula for chlorite group minerals is (Mg,Fe,Al)6(Si,Al)4O10(OH)8. The only purely dioctahedral chlorite is donbassite, Al4+x(Si3Al)O10(OH)8.

Chlorite (Mg,Fe)3(Si,Al)4O10(OH)2 •(Mg,Fe)3(OH)6

Other Sheet Silicates There are other sheet silicates that do not follow the structural pattern outlined. Their tetrahedral layers have irregularities. Examples are: Apophyllite KCa4Si8O20F•8H2O Prehnite CaAl(AlSi3O10)(OH)2

Prehnite CaAl(AlSi3O10)(OH)2

Apophyllite KCa4Si8O20F•8H2O