DOUBLE CHAIN SILICATES: 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 we will see in sheet silicates - octahedral sites involve apical O and OH groups

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

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

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

See figure: (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 these when we do micas on Wednesday.

What is the amphibole most similar to the single chain silicate enstatite ?

Enstatite: Mg₂Si₂O₆
= Mg₇Si₈O₂₂(OH)₂ .... This is Anthophyllite
Could also use Fe⁺² ....This is Grunerite !
The above cases, Mg and Fe fill all sites - singly and combination.
How about using the larger nature of M4 sites to put in Ca?
Result = Ca₂Mg₅Si₈O₂₂(OH)₂ or Ca₂Fe₅Si₈O₂₂(OH)₂

The Ca,Mg amphibole is tremolite
If Fe, the amphibole is called Ferroactinolite!

Site flexibility: M4 either likes to take all Ca or none (very little tolerance 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. See Figure 13.65, p. 489.
THUS: exsolution!!
note: Na can substitute for Ca in M4.

What about those large sites similar to the 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 (446K) (What's wrong with this movie??)
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

HORNBLENDE: 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

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: MICROSTRUCTURES IN AMPHIBOLES