Day 21


These pages are under construction, and contian class notes....

Phyllosilicates (p. 498-524)

Serpentine Group
Antigorite Mg3Si2O5(OH)4 Monoclinic
Chrysotile Mg3Si2O5(OH)4 Monoclinic

Clay Group
Kaolinite Al2Si2O5(OH)4 Triclinic
Pyrophyllite Al2Si4O10(OH)2 Monoclinic
Talc Mg3Si4O10(OH)2 Monoclinic
Glauconite (K,Na,Ca)0.5-1(Fe,Al,Fe,Mg)2(Si,Al)O10¥nH2O Monoclinic

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

Mica Group
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic
Paragonite NaAl2(AlSi3O10)(OH) 2 Monoclinic
Margerite CaAl2(Al2Si2O10)(OH) 2 Monoclinic
Lepidolite K(Li,Al)2-3(AlSi3O10)(OH)2 Monoclinic
Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Biotite K(Fe,Mg)3(AlSi3O10)(OH)2 Monoclinic

Structural and Chemical Development of the Micas

The name of the group is derived from Greek phylon, or leaf, as in phyllo pastry. Structure consists of an infinite sheet of SiO4 tetrahedra, in which three of the oxygen are shared with adjacent tetrahedra (down 1 from the four of tectosilicates) and the apical oxygen of the sheets have a net charge of -1; sharing of oxygen leads to a 2:5 ratio of Si:O in the tetrahedral sheets, and thus the formula for the sheets if (Si2O5)-2. Each sheet, if undistorted, has a 6-fold symmetry.

Most phyllosilicates are hydroxyl bearing, with the (OH)-1 group located in the center and the same height of the rings of unshared apical oxygens. When ions, external to the (Si2O5OH)3- sheets, are bonded to the sheets they coordinate with two oxygen and the single (OH)-; the size of the triangle formed is close to that of an ideal XO6 octahedron (with X commonly Fe or Mg - more discussion of the consequences of misfit will follow). This means that each tetrahedral (Si2O5OH)3- sheet will coordiante with a sheet of regular octahedra, in which each octahedron is tilted onto one of it's triangular sides. When such tetrahedral and octrahedral sheets are joined , the general geometry of the kaolinite and antigorite structures are formed. (An 'open-faced' t-o sandwhich).

The phyllosilicates are divided into two major groups: trioctahedral and dioctahedral. The trioctahedral micas have divalent cations (mainly Fe+2 or Mg+2) in the octrahedral sites and each octahedral site is occupied. Each oxygen of the trioctahedral micas is surrounded by and coordinated to three cations in adjacent, filled octahedral sites. For the dioctahderal micas, trivalent cations in the octahedral sheets (generally Al, Fe+3, or Cr+3) are also bound to three oxygen or (OH)- groups, but to maintain charge balance one third of the octahedral sites are left empty (as for the corundum-ilmenite-hematite structures), thus each oxygen or (OH)- group has cations in two adjoining octahedra.

Brucite, Mg(OH)2, consists of two (OH)- planes between which Mg octahedra are coordinated. The sheets of the brucite structure can be noted as Mg3(OH)6. If we replace two of the (OH)- on one side with the apical oxygen of an Si2O5 sheet, then we obtain Mg3Si2O5(OH)4, which is the formula and structure for the trioctahedral mica antigorite. In short, the antigorite and chrysotile structures are built from one tetrahedral sheet and one octahedral sheet, giving them t-o layers that are electrically neutral and held together by van der Waals bonds. The equivalent structure of dioctahedral micas is kaolinite, Al2Si2O5(OH)4 (built from Gibbsite, Al(OH)3, note the missing cation to maintain charge balance).

Trioctahedral Micas Oct. Brucite Layer Tet. Si-O layer Antigorite 3Mg(OH)2 + (Si2O5) -2 = Mg3Si2O5(OH)4 + 2(OH)-

Dioctahedral Micas Oct. Gibbsite Layer Tet. Si-O layer Kaolinite 2Al(OH)3 + (Si2O5) -2 = Al2Si2O5(OH)4 + 2(OH)-

If we bind Si2O5 sheets to both sides of the octahedral layers we can derive additional phyllosilicates which each have t-o-t layers: as for talc, Mg3Si4O10(OH)2, and phyrohyllite, Al2Si4O10(OH)2. In each case it can begin with either brucite or gibbsite, and replace two (OH)- with two apical oxygen of Si2O5 groups.

If some Al+3 substitutes for Si+4 in the tetrahedral sheets, then we can carry the evolution of phyllosilicates. This substitution causes a charge to be located along the surfaces of the t-o-t layers. An ÔidealÕ substitution of Al for Si occurs when one of every four Si is replaced by Al: this results in a sufficient charge to bind a cation in 12-fold coordination between the bases of SiO5 sheets. This binding of t-o-t layers increases hardness, etc., relative to other micas.

Phlogopite KMg3(AlSi3O10)(OH)2 Monoclinic
Muscovite KAl2 (AlSi3O10)(OH) 2 Monoclinic

If half the Si are replaced by Al, then two charges per t-o-t layer become available. Then such ions as Ba and Ca may enter between and bind the t-o-t layers. The ionic bonds are stronger, hardness is increased, forming the "brittle micas"

Margerite CaAl2(Al2Si2O10)(OH)2 Monoclinic

Addional members of the micas can be developed by considering the layers of their structures as different "minerals": Chlorite can be considered as two layers of talc , Mg3Si4O10(OH)2, separated by a layer of brucite.

Mismatch between the octahedral and tetrahedral layers causes distortion of structures. This misfit is accommodated by bending the larger octahedral layers around the tetrahedral layers.

Note also that the octahedral sheets are staggered relative to the tetrahedral sheets. This gives rise to the monoclinic symmetry of most micas. Polytypism results from changing the stacking and staggering of micas: because of the three-fold symmetry of Si2O5 sheets, there are three alternate directions that octahedra can be staggered.

Class Discussisons:

Survey of Hydrous Phyllosilicates

Structure of Clay Minerals and resulting uses - expandable structure - perfect basal cleavage, low hardness - fine grain size

Chemistry of Clay Minerals and resulting uses: - aluminum-rich clay minerals - bonding with polar molecules - serpentine producing reactions - kaolinite producing reactions

Geologic Occurrences of select clay and serpentine group minerals. - Crysotile-Antigorite - Vermiculite - Kaolinite


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