MICA-AMPHIBOLE-PYROXENE-BIOPYRIBOLE POLYSOMATIC SERIES

Polysomatic series: A modular view of mineral structures as illustrated by amphiboles, pyroxenes, and sheet silicates. (see p.491-492)

This approach is based on the idea that some series of minerals can be envisioned as constructed from two dimensional slabs of one or more types. Different members of the series can be generated by assembling the slabs in different ratios and in different patterns.

Up until now we have thought about mineral structures as being constructed by arrangement of polyhedral units (e.g., tetrahedra, octahedra, and larger coordination environments).

Thus, here we step back and consider how minerals can be viewed as constructed from larger structural pieces. This approach helps us to understand common defect microstructures in these minerals and propose mechansims by which one mineral may be converted to another.

LEGO BLOCK analogy !

Polysomes or polysomatic series are series of crystal structures that can be constructed by combining two or more slabs that may be different chemically and structurally.

Requirement: that the slabs fit together (e.g., Lego)

The end-members of a particular polysomatic series are then a mineral constructed simply from building block 'A' and one constructed from 'B':

  • Members of the polysomatic series would then include minerals such as

    AB AB AB; AAB AAB AAB; AABB AABB; ABB ABB; ABAAB ABAAB etc.

    Examples we will see:

    Example 1: Brucite and the sheet silicates:

    As we saw on Wednesday, it is possible to describe chlorite as a member of a polysomatic series based on brucite-like (B) layers and talc-like (T) layers.

    Brucite has a composition: Mg (OH)2 = Mg3 (OH)6. Talc has the composition: Mg3Si4O10(OH)2

    BTBTBT.... = B + T = Mg6Si4O10(OH)8 Thus, derive chlorite

    diagram and compare this to the movie we saw in the Sheet Silicate lecture.

    Example 2: Another example involving sheet silicate minerals:

    serpentine and chlorite possibilities

    ** Can also have disordered intergrowths : "random interstratification" - very common!

    There are other regularly interstratified sheet silicates such as

    talc-chlorite 1:1 interstratifications: kulkeite;

    1:1 dioctahedral mica - dioctahedral smectite = rectorite)

    Example 3: Relating the major mineralogical groups:

    Pyroxenes, amphiboles, (biopyriboles) and sheet silicates.

    Review the pyroxene structure:

    and Si:O stoichiometry

    Recall the structure of the 2:1 sheet silicates:

    Review of the amphibole structure:

    The critical observation is that the amphibole structure can be subdivided into pyroxene slabs and mica slabs

    Thus, we can describe amphibole as constructed from P and M-type slabs (P=pyroxene, M = mica)

    Thus amphibole = PMPMPMPMPMP.......

    Is there a compositional implication ?

    i.e., if we add a pyroxene to mica composition do we get amphibole ?

    If we construct the amphibole from orthopyroxene-type slabs we produce orthoamphibole, and from clinopyroxene-type slabs, clinoamphibole

    What if, instead of the sequence PMPMPM we include an extra mica slab ?

    Get the sequence PMMPMMPMM

    If we combine amphibole-type slabs (PMPMPM...) with slabs of jimthompsonite (PMMPMMPMM) we arrive at chesterite e.g. PMM PM PMMPM PM ..

    The same sequence from clino px-type slabs produces clinochesterite.

    The view of amphiboles and multiple chain silicates as structures derivative of pyroxene and mica provides a clear model for how reactions such as pyroxene to amphibole, amphibole to sheet silicate, and pyroxene or amphibole to triple chain silicate reaction mechanisms could occur and why these structures intergrow so readily.

    Many other examples in major mineral groups: tend to produce important but less abundant groups of minerals not covered in detail in this course:

    Pyroxenoids

    Pyroxenoids can be thought of a sequences based on pyroxene-type slabs (P)

    and wollastonite-type slabs (W)

    Humite group

    The humite group of minerals can be thought of being built from alternations of olivine-type (O) and norbergite-type (N) slabs: