Double Chain Silicates

DOUBLE CHAIN SILICATES: AMPHIBOLES, CHAIN WIDTH DEFECTS, and TRIPLE CHAIN SILICATES

Further depolymerization of the silicate anion (4 corners shared = framework, 3 corner shared = sheet) -> sheet fragments: double chains = basis for amphiboles

Double chain 'strands' with the apical oxygen atoms pointing toward each other -> module known as an I-beam. These are stacked back to back and tiled in a characteristic pattern:

Model Amphibole (glaucophane): (207K) Na2Mg3Al2Si8O22(OH)2

Si:O ratio ? count up repeat in double chain:

or two corners shared = Si2O6, three corners shared = Si2O5 ->Si4O11 6-

As in sheet silicates - octahedral sites involve apical O and OH groups

NOTE: AMPHIBOLES ARE HYRDOUS MINERALS, AS ARE SHEET SILICATES! - implications for geological environments?

Using two opposing chains and 2 OH to make I-beam -> [ ]Si8O22(OH)2

Charge needed to satisfy excess -ve = ??
14- :
How might this be achieved ? 7 x 2+ is likely!!
HOW MANY SITES and of what type?

See overhead: (M1)2 + (M2)2 + (M3) + (M4)2 = 7 sites!!

These sites are not all the same...
can take different elements to different extents: partitioning of elements into special cystallographic sites!
e.g., M1 - M3 are relatively conventional octahedral sites, M4 is a large site-
What 2+ cation might it accommodate ?
Where might we put large monovalent cations such as Na and K ?
Extra sites ! Remember micas ..?

What is the amphibole most similar to talc ?

Talc: Mg3Si4O10(OH)2
Three corners shared part:
Two corners shared part would be Mg4Si4O12
= Mg7 Si8 O 22 (OH)2 .... This is Anthophyllite!
Could also use Fe2+ ....This is Grunerite !
The above cases, Mg and Fe fill all sites - singly and combination.
How about use the larger nature of M4 sites to put in Ca
Result = Ca2Mg5Si8O22(OH)2 or Ca2Fe5Si8O22(OH)2

The Ca,Mg amphibole is tremolite (446K)!
If Fe, the amphibole is called Ferroactinolite!

Site flexibility: M4 either likes to talk all Ca or none (very little tollerance for Mg in Ca dominated M4 or Ca in Mg dominated M4

More at higher temperature!
Sketch binary join!
This is true regardless of whether Mg or Fe are in the M1-M3 sites. THUS, there is a compositonal gap!
Note: M1-M3 are too small for Ca, so never get Ca7...
THUS: amphibole quadrilateral and possible compositions.
THUS: exsolution!!
note: Na can substitute of Ca in M4.

What about those large sites similar to interlayer sites in micas?

If place M+ in these, then must charge balance substitution? Any ideas how?
Example: Na in 'A' site:
Na Ca2Mg5 (AlSi7) O22 (OH)2 = Edenite!
if K, then K-edenite

GENERAL AMPHIBOLE FORMULA:

A 0-1 [M4]2 [M1,M2,M3]5 T8 O22 (OH,F,Cl)2

A = nothing, Na, K
M4 = Ca, Na, Li, Mg, Fe, Mn
M1 + M2 + M3 = Mg, Fe, Mn, Al, Ti
T = Si, Al

HORNBLENE: complex intermediate composition amphibole - mixtures of many or all of the above (and more) in the various sites!

NOMENCLATURE: Based on chemistry: individual varietal names AND group names:

Fe-Mg-Mn group
Calcic amphiboles
solid calcic amphiboles

check reference books for Ca content, etc. to define these groups.

WHICH AMPHIBOLE WHERE ?

Dependent upon bulk rock chemistry, T, P.

e.g., Metamorphosed siliceous dolomite (Ca,Mg, Si-rich rocks) -> tremolite

e.g., Meta-iron formations -> riebeckite: Na2(Fe++,Fe+++)5Si8 O22 (OH)2

e.g., high pressure rocks: glaucophane: Na2 [Mg3Al2] Si8 O22 (OH)2

NOTE on RIEBECITE: Fibrous amphibole = asbestiform = dangerous form of asbestos! crocidolite

DETAILS OF CRYSTAL STRUCTURES

Stagger on 2:1 layer in layer silicates AND amphiboles

SYMMETRY?. One layer structure: stack slanted slab -> monoclinic symmetry
monoclinic symmetry found in Ca(Mg,Fe) amphiboles: CLINOAMPHIBOLES
how can we stack these monoclinic units to make an orthorhombic unit cell? -> ORTHOAMPHIBOLES

Above covers chemistry, structure, relationship between these. Where they form in detail covered in petrology.

Other: will cover more complex structures involving triple chains next lecture

Other: MICROSTRUCTURES IN AMPHIBOLES

. These were created on the CrystalMaker 1.1.4 interactive crystallography program . If you would like more information on the program, please contact:

David Palmer

(Technical information)