The Phyllosilicates

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Basic Units

Symbols

In Sheets:

• s Silicon
• o Oxygen or hydroxyl (OH)
• * Magnesium
• x Aluminum
• % Ferrous (+2) Iron
• # Ferric (+3) Iron
• L Lithium

Between Sheets

• (K), (Ca), etc.: Specific Cations
• (W) Water
• (+) General Cations

The Silica Sheet Si2O5

Schematic edge-on view of Si2O5 sheet

o---o---o---o---o---o---o---o---o---o---o---o---o---o---o---o
 \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o

Seen from face-on, a silica sheet has been traditionally drawn as looking like this. The Si atoms are in the middle of each tetrahedron, hidden by the central oxygen atom.

Silica sheets are actually represented more accurately as shown below.

Strictly, phyllosilicate sheets are somewhere between these two configurations, but nearer the second one. The sides of the enclosed triangles are actually bent outward slightly. Note that this arrangement leads to tighter packing. The rear oxygens actually form a close-packed sheet without holes.

Small mismatches between this configuration and the octahedral sheets to be described next are accommodated by slight rotations of the tetrahedra, so real minerals are somewhere between the two configurations above, but are closer to the trigonal form.

The Brucite Sheet Mg(OH)2

In the brucite and gibbsite sheets, there are two layers of close-packed OH units with cations in the octahedral interstices. Viewed edge-on to the sheets, the octahedra are schematically represented by boxes.

o---o---o---o---o---o---o---o---o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * | * | * | * | * | * | * | * | * |   
o---o---o---o---o---o---o---o---o---o---o---o---o---o---o---o

Brucite can be a mineral in its own right. It usually occurs in weathered or hydrothermally altered serpentine bodies. All of the octahedral sites are filled, hence this sheet is termed trioctahedral (3/3 of the octahedra are occupied).

Seen from face-on, a trioctahedral sheet looks like this. o's represent hydroxyls in the upper layer, v's those in the lower layer, and *'s represent magnesium atoms. Each hexagon of alternating upper and lower hydroxyls represents the vertices of an octahedron, with the magnesium in the center between the two layers. The upper faces of several octahedra are outlined in the upper left.

Between the octahedral interstices are tetrahedral interstices. Each v surrounded by three o's is a downward-pointing tetrahedron. Each o surrounded by three v's is an upward-pointing tetrahedron.

o---o---o---o   o   o   o   o   o   o   o   o   o   o   o   o
v\*/v\*/v\*/v\* v * v * v * v * v * v * v * v * v * v * v * v *
--o---o---o---o   o   o   o   o   o   o   o   o   o   o   o   o
*/v\*/v\*/v\*/v * v * v * v * v * v * v * v * v * v * v * v * v 
o---o---o---o   o   o   o   o   o   o   o   o   o   o   o   o
v * v * v * v * v * v * v * v * v * v * v * v * v * v * v * v *
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
* v * v * v * v * v * v * v * v * v * v * v * v * v * v * v * v 
o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
v * v * v * v * v * v * v * v * v * v * v * v * v * v * v * v *
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
* v * v * v * v * v * v * v * v * v * v * v * v * v * v * v * v 
o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
v * v * v * v * v * v * v * v * v * v * v * v * v * v * v * v *
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
* v * v * v * v * v * v * v * v * v * v * v * v * v * v * v * v 
o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o

The Gibbsite Sheet Al(OH)3

o---o---o---o---o---o---o---o---o---o---o---o---o---o---o---o
| x | x | x | x | x | x | x | x | x | x | x | x | x | x | x |   
o---o---o---o---o---o---o---o---o---o---o---o---o---o---o---o

Gibbsite is one of the bauxite minerals. There are several others, so usually gibbsite is not distinguishable and the group term bauxite is used instead. Because of aluminum's +3 charge, only 2/3 of the octahedral sites are filled, hence this sheet is termed dioctahedral (2/3 of the octahedra are occupied).

Seen from face-on, a dioctahedral sheet looks like this. Note that:

• Only two thirds of the octahedra are filled. The filled octahedra outline a hexagonal net.
• The filled octahedra form a continuous network of polyhedra (they must).
• Even though some octahedra are empty, all the hydroxyls are still present.

o---o---o---o   o   o   o   o   o   o   o   o   o   o   o   o
v\x/v\x/v\ /v\x v x v   v x v x v   v x v x v   v x v x v   v x
--o---o---o---o   o   o   o   o   o   o   o   o   o   o   o   o
x/v\ /v\x/v\x/v   v x v x v   v x v x v   v x v x v   v x v x v 
o---o---o---o   o   o   o   o   o   o   o   o   o   o   o   o
v x v x v   v x v x v   v x v x v   v x v x v   v x v x v   v x
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
x v   v x v x v   v x v x v   v x v x v   v x v x v   v x v x v 
o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
v x v x v   v x v x v   v x v x v   v x v x v   v x v x v   v x
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
x v   v x v x v   v x v x v   v x v x v   v x v x v   v x v x v 
o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
v x v x v   v x v x v   v x v x v   v x v x v   v x v x v   v x
  o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o
x v   v x v x v   v x v x v   v x v x v   v x v x v   v x v x v 
o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o

Phyllosilicate Structures

o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * |
o---o---o---o---o---o---o---o
o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * |
o---o---o---o---o---o---o---o

o---o---o---o---o---o---o---o
| x | x | x | x | x | x | x |
o---o---o---o---o---o---o---o
o---o---o---o---o---o---o---o
| x | x | x | x | x | x | x |
o---o---o---o---o---o---o---o

--o---o---o---o---o---o---o---o
s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ 
o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * |
o---o---o---o---o---o---o---o
--o---o---o---o---o---o---o---o
s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ 
o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * |
o---o---o---o---o---o---o---o

o---o---o---o---o---o---o---o--
 \s/ \s/ \s/ \s/ \s/ \s/ \s/ \o 
  o---o---o---o---o---o---o---o
  | x | x | x | x | x | x | x |
  o---o---o---o---o---o---o---o
o---o---o---o---o---o---o---o--
 \s/ \s/ \s/ \s/ \s/ \s/ \s/ \o 
  o---o---o---o---o---o---o---o
  | x | x | x | x | x | x | x |
  o---o---o---o---o---o---o---o

--o---o---o---o---o---o---o---o
s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ 
o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * |
o---o---o---o---o---o---o---o
s\ /s\ /s\ /s\ /s\ /s\ /s\ /s\ 
--o---o---o---o---o---o---o---o
--o---o---o---o---o---o---o---o
s/ \s/ \s/ \s/ \s/ \s/ \s/ \s/ 
o---o---o---o---o---o---o---o
| * | * | * | * | * | * | * |
o---o---o---o---o---o---o---o
s\ /s\ /s\ /s\ /s\ /s\ /s\ /s\ 
--o---o---o---o---o---o---o---o

o---o---o---o---o---o---o---o--
 \s/ \s/ \s/ \s/ \s/ \s/ \s/ \o 
  o---o---o---o---o---o---o---o
  | x | x | x | x | x | x | x |
  o---o---o---o---o---o---o---o
 /s\ /s\ /s\ /s\ /s\ /s\ /s\ /o
o---o---o---o---o---o---o---o-- 
o---o---o---o---o---o---o---o--
 \s/ \s/ \s/ \s/ \s/ \s/ \s/ \o 
  o---o---o---o---o---o---o---o
  | x | x | x | x | x | x | x |
  o---o---o---o---o---o---o---o
 /s\ /s\ /s\ /s\ /s\ /s\ /s\ /o
o---o---o---o---o---o---o---o-- 

o---o---o
 \s/ \s/  
--o---o--
* | * | *
--o---o--
 /s\ /s\  
o---o---o
--o---o--
* | * | *
--o---o--
o---o---o
 \s/ \s/  
--o---o--
* | * | *
--o---o--
 /s\ /s\  
o---o---o

o---o---o
 \s/ \s/  
--o---o--
* | * | *
--o---o--
 /s\ /s\  
o---o---o
 (W) (+) 
o---o---o
 \s/ \s/  
--o---o--
* | * | *
--o---o--
 /s\ /s\  
o---o---o

o---o---o
 \s/ \s/  
--o---o--
* | * | *
--o---o--
 /s\ /s\  
o---o---o
 (K) (K)
o---o---o
 \s/ \s/  
--o---o--
* | * | *
--o---o--
 /s\ /s\  
o---o---o

o---o---o
 \s/ \s/  
--o---o--
x | x | x
--o---o--
 /s\ /s\  
o---o---o
 (K) (K)
o---o---o
 \s/ \s/  
--o---o--
x | x | x
--o---o--
 /s\ /s\  
o---o---o

o---o---o
 \s/ \s/  
--o---o--
x | x | x
--o---o--
 /s\ /s\  
o---o---o
 (W) (K) 
o---o---o
 \s/ \s/  
--o---o--
x | x | x
--o---o--
 /s\ /s\  
o---o---o

o---o---o
 \s/ \s/  
--o---o--
x | x | x
--o---o--
 /s\ /s\  
o---o---o
 (W) (Ca)
o---o---o
 \s/ \s/  
--o---o--
x | x | x
--o---o--
 /s\ /s\  
o---o---o

Other Important Phyllosilicates

o---o---o---o
 \s/ \s/ \s/  
--o---o---o--
* | % | * | %
--o---o---o--
 /s\ /s\ /s\ 
o---o---o---o
 (K) (K) (K)
o---o---o---o
 \s/ \s/ \s/  
--o---o---o--
* | % | * | %
--o---o---o--
 /s\ /s\ /s\ 
o---o---o---o

o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
x | x | x | x
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o
 (Na)(Na)(Na)
o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
x | x | x | x
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o

o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
L | L | L | L
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o
 (K) (K) (K)
o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
L | L | L | L
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o

o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
# | x | # | x
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o
 (K) (Ca)(K)
o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
# | x | # | x
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o

o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
x | x | x | x
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o
 (Ca)(Ca)(Ca)
o---o---o---o
 \s/ \s/ \s/ 
--o---o---o--
x | x | x | x
--o---o---o--
 /s\ /s\ /s\  
o---o---o---o

• Biotite has phlogopite (trioctahedral) structure with Fe(+2) substituting for Mg
• Paragonite is muscovite with Na in place of K. It is rare because sodium tends to occur almost exclusively in albite.
• Lepidolite is muscovite with Li in place of Al. To offset the charge difference (+1 for Li, +3 for Al), the lithium sheet is trioctahedral instead of dioctahedral.
• Glauconite is basically muscovite structure, substituting Ca for K and Fe(+3) for Al.
• Margarite is basically muscovite structure, substituting Ca for K.

The common metamorphic mineral chloritoid is not a true phyllosilicate (it has isolated tetrahedra) but its structure is clearly related, and it contains Mg and Al sheets.

o---o---o---o---o---o---o---o---o---o--
| * | * | * | * | * | * | * | * | * | *    Brucite sheet
o---o---o---o---o---o---o---o---o---o--
 \s/   /s\   \s/   /s\   \s/   /s\         Layer of isolated tetrahedra
--o---o---o---o---o---o---o---o---o---o
x | x | x | x | x | x | x | x | x | x |    Corundum (Al2O3) sheet
--o---o---o---o---o---o---o---o---o---o
   \s/   /s\   \s/   /s\   \s/   /s\  
o---o---o---o---o---o---o---o---o---o--
| * | * | * | * | * | * | * | * | * | * 
o---o---o---o---o---o---o---o---o---o--
 \s/   /s\   \s/   /s\   \s/   /s\  
--o---o---o---o---o---o---o---o---o---o
x | x | x | x | x | x | x | x | x | x | 
--o---o---o---o---o---o---o---o---o---o

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Created 27 Oct 1997, Last 31 December 2002

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