Phyllosilicates EENS 2110 Tulane University Mineralogy Prof Stephen A Nelson Phyllosilicates Micas Chlorite Talc Serpentine This document last updated on 29 Nov 2011 Phyllosilicates Sheet Silicates The phyllosilicates or sheet silicates are an important group of minerals that includes the micas chlorite serpentine talc and the clay minerals Because of the special importance of the clay minerals as one of the primary products of chemical weathering and one of the more abundant constituents of sedimentary rocks they will be discussed in more detail in the next lecture The basic structure of the phyllosilicates is based on interconnected six member rings of SiO4 4 tetrahedra that extend outward in infinite sheets Three out of the 4 oxygens from each tetrahedra are shared with other tetrahedra This leads to a basic structural unit of Si2O5 2 Most phyllosilicates contain hydroxyl ion OH with the OH located at the center of the 6 membered rings as shown here Thus the group becomes Si2O5 OH 3 When other cations are bonded to the SiO4 sheets they share the apical oxygens and the OH ions which bond to the other cations in octahedral coordination This forms a layer of cations usually Fe 2 Mg 2 or Al 3 that occur in octahedral coordination with the O and OH ions of the tetrahedral layer As shown here the triangles become the faces of the octahedral groups that can bind to the tetrahedral layers The octahedral layers take on the structure of either Brucite Mg OH 3 if the cations are 2 ions like Mg 2 or Fe 2 or Gibbsite Al OH 3 if the cations are 3 like Al 3 In the brucite structure all octahedral sites are occupied and all anions are OH 1 In the Gibbsite structure every 3rd cation site is unoccupied and all anions are OH 1 Page 1 of 7 11 29 2011 Phyllosilicates This gives rise to 2 groups of sheet silicates 1 The trioctahedral sheet silicates where each O or OH ion is surrounded by 3 divalent cations like Mg 2 or Fe 2 2 The dioctahedral sheet silicates where each O or OH ion is surrounded by 2 trivalent cations usually Al 3 We can build the structures of the various sheet silicates by starting with the octahedral layers similar to the structures of brucite or gibbsite as shown below The trioctahedral phyllosilicates are based on the structure where the octahedral layers are similar to brucite where Mg 2 occupies the cation position The dioctahedral phyllosilicates are based on the structure where the octahedral layers are similar to gibbsite where Al 3 occupies the cation position The octahedral sheets in both cases are held together by weak Van der Waals bonds If we start with the brucite and gibbsite structures shown above and replace 2 of the OH ions with O where the Oxygens are now the apical Oxygens of the tetrahedral sheets then we get the structure of the serpentine mineral Lizardite if the octahedral layer is trioctahedral containing Mg 2 If the octahedral layer is dioctahedral containing Al 3 the structure of the clay mineral Kaolinite is obtained This leads to a tetrahedral octahedral T O structure where each T O layer is bonded to the top or bottom of another T O layer by Van der Waals bonds If 2 more of the OH ions in the octahedral layer are replaced by O and these O become the apical Oxygens for another tetrahedral layer the this builds the trioctahedral phyllosilicate talc or the dioctahedral pyrophyllite This becomes a T O T layer that can bond to other T O T layers by weak Van der Waals bonds Page 2 of 7 11 29 2011 Phyllosilicates If an Al 3 is substituted for every 4th Si 4 in the tetrahedral layer this causes an excess 1 charge in each T O T layer To satisfy the charge K 1 or Na 1 can be bonded between 2 T OT sheets in 12 fold coordination For the trioctahedral sheet silicates this becomes Phlogopite Mg biotite and for the dioctahedral sheet silicates this becomes Muscovite This makes a T O T T O T layer that again can bind to another T O T T O T layer by weak Van der Waals bonds It is along these layers of weak bonding that the prominent 001 cleavage in the sheet silicates occurs Replacing 2 more Si 4 ions with Al 3 ions in the tetrahedral layer results in an excess 2 charge on a T O T layer which is satisfied by replacing the K 1 with Ca 2 This results in the trioctahedral sheet silicate Clintonite and the dioctahedral sheet silicate Margarite Because of the differences in charge balance between the trioctahedral and dioctahedral sheet silicates there is little solid solution between the two groups However within the trioctahedral sheet silicates there is complete substitution of Fe 2 for Mg 2 and limited substitution of Mn 2 into the octahedral sites Within the dioctahedral sheet silicates there is limited substitution of Fe 3 for Al 3 in octahedral sites In addition F or Cl can substitute for OH in the hydroxyl site As previously discussed substitution of F 1 stabilizes the mineral to higher pressures and temperatures Another group of phyllosilicates that is more of mixture of structural types is the chlorite group Although chlorite is complex in that the amount of Al that can substitute Mg and Si is Page 3 of 7 11 29 2011 Phyllosilicates variable one way of looking at the chlorite structure is shown below Here the chlorite structure is depicted as consisting of a brucite like layer with some Al sandwiched between tetrahedral layers that are similar to phlogopite Another important sheet silicate structure is that of vermiculite This is similar to the talc structure discussed above with layers of water molecules occurring between each T O T layer Similarly insertion of layers of water molecules between the T O T sheets of pyrophyllite produces the structure of smectite clays The vermiculite and smectite groups are therefore expanding type sheet silicates and as the water is incorporated into the structure the mineral increases its volume Although we have shown that the octahedral layers fit perfectly between the tetrahedral layers this is an oversimplification If the tetrahedral layers were stacked perfectly so that apical oxygens were to occur vertically aligned then the structure would have hexagonal symmetry But because this is not the case most of the phyllosilicates are monoclinic Serpentine Group The serpentine group of minerals has the formula Mg3Si2O5 OH 4 Three varieties of serpentine are known Antigorite and Lizardite are usually massive and fine grained while Chrisotile is fibrous As discussed above the imperfect fit of the octahedral layers and the