PYROXENES and PYROXENOIDS

Silica tetrahedra may form simple chains by sharing 2 of the 4 oxygens. These simple single chain silicates have -> Si:O = 1:3

Pyroxene Chemistry

The simplest possible formula would be: [M]²⁺SiO₃

Opposing chains (whose apical oxygens point at one another) create octrahedral sites of two types:

[1] Octahedral sites form a chain between the apical oxygens of the opposed tetrahedral chains.

These are rather regular sized 6-coordinated sites.
Accommodate: Mg, Fe (2+ and 3+), Al, Mn, etc.
These sites are known as M1 sites (which you will see are analogous to the M1, M2, and M3 sites in amphiboles).

Structure Movies:

[2] Also, back to back chains provide cation sites to complete a second strip of sites.

These are large and distorted nominally 6-fold sites but 8-fold coordination is common.
These sites can accommodate: Mg, Fe, Al, [Mn], Li, Ca, Na, etc.
This second set of sites are designated the M2 sites (analogous to the M4 sites in amphiboles).

Significant amounts of the larger 2+ cations such as Ca and Mn can not be accommodated in the M1 sites; when such cations are present they force the structure to change and another single chain silicate structure is formed with a different c-cell dimension - see the PYROXENOID discussion below.

Note the relationship between the pyroxene I-beam and pyroxene cleavage (Fig. 13.49).

How do we deal with 3+ cations and 1+ cations ?

Rewrite the formula as: M1 M2 Si₂O₆

THUS: three groups to be considered:

M²⁺ M²⁺ Si₂O₆
M¹⁺ M³⁺ Si₂O₆
Pyroxenoids

Also, tetrahedral sites can accommodate some Al³⁺ and Fe³⁺

Common Pyroxenes with [M²⁺]₂ Si₂ O₆

Consider common cations: Ca²⁺, Mg²⁺, Fe²⁺

Ca -> M2 site which, (like amphibole M4 site), wants either almost all Ca or almost no Ca, so:

₂

₆

and (Mg,Fe)(Mg,Fe) Si₂O₆ e.g., Mg₂ Si₂O₆, Fe₂ Si₂O₆ and (Mg,Fe)₂ Si₂O₆

i.e., *MgMg-------------------range of Mg, Fe-------------------FeFe*

complete range of solid solution!

likewise *CaMg ---------------range of Mg,Fe-------------------CaFe*

Names:

Mg₂ Si₂O₆ = ENSTATITE
Fe₂ Si₂O₆ = FERROSILITE (rarely completely Mg-free)
CaMg Si₂O₆ = DIOPSIDE
CaFe Si₂O₆ = HEDENBERGITE

As we will see for the amphiboles, there is a quadrilateral diagram that summarizes the above which is known as the pyroxene quadrilateral. (see Fig. 13.47 in text).

MUST BE FAMILIAR WITH BOTH THE AMPHIBOLE AND PYROXENE QUADRILATERAL DIAGRAMS!!

Symmetry and Crystal Structures

As in layer silicates and amphiboles, formation of octahedral sites through opposing apical oxygens requires that the apical O do not superimpose exactly, but are offset.

RESULT: stagger in the I beam.

Note the stagger is reflected in the octahedral tilt. If I beams are stacked one upon the next -> what symmetry ?

Stagger reversal (+,+,-,-,+,+, -,-,...diagram) gives an orthorhombic structure (with longer a-cell dimension) and actually restricts the size of the M2 sites. The orthorhombic pyroxenes have only small +2 cations in the M1 and M2 sites.

The stagger reversal is not possible if the M2 site is to accommodate Ca; THUS- the Ca-rich pyroxenes are monoclinic. (+,+,+...)

Phase relationships in Ca,Mg,Fe-pyroxenes

EXSOLUTION, note the shape and location of the miscibility gap (Fig. 13.55, p. 481). Common monoclinic pyroxenes (clinopyroxenes) e.g.,

AUGITE: Doesn't really plot in the quadrilateral because it contains Al
PIGEONITE: a pyroxene slightly more Ca-rich than normal orthopyroxene. Generally found in volcanic rocks. Pigeonite is monoclinic (due to higher Ca).
Orthopyroxenes from enstatite - ~ 50:50 Mg:Fe

Other Common Pyroxenes with M⁺¹M³⁺

JADEITE: NaAl Si₂O₆ recall (although we haven't talked about them yet) nepheline NaAlSiO₄ and albite NaAlSi₃O₈)

Jadeite forms at high pressures (Fig. 13.58 p. 483). Gem material Jade.

AEGIRINE: NaFe³⁺Si₂O₆ (also known as Acmite). Often Ca(Mg,Fe) pyroxenes contain some NaFe³⁺ <=> Ca(Mg,Fe) component. Aegirine is a relatively rare mineral - found in low Si rocks. (monoclinic)

SPODUMENE: LiAlSi₂O₆: Recall the spodumene in the pegmatite on the field trip and see the courtyard sample. (monoclinic)

PYROXENOIDS

Consider the composition Ca₂ Si₂O₆. This can not be a pyroxene structure, because Ca will not fit into M1. However, it is possible to accommodate Ca in an M1-type site if we create a modified chain: a 3 tetrahedron repeat, instead of 2 (7.1 A repeat rather than 5.2 A).

This chain has lower symmetry -> crystals are triclinic.

CaSiO₃ = (Ca₂ Si₂O₆) = WOLLASTONITE

what kind of rocks ? Note the chain geometry. (See Fig. 13.59, p. 485)

This case raises the possibility of chain repeats longer than 3.

What about 4, 5, 6, 7, etc ?

Examples: 5-repeat, 7-repeat.

The other element not accommodated in high concentration in pyroxenes is Mn.

MnSiO₃ = RHODONITE almost always contains some Ca. (See Fig. 13.62, p.487)

This mineral has a 5-repeat pyroxenoid chain and its formula is best reported as CaMn₄ Si₅O₁₅.

(Mn-some Mg and Fe)₇Si₇O₂₁ = PYROXMANGITE - 7 repeat chain.

Pyroxene - Amphibole Comparison:

Two chains joined side by side form a double chain silicate (the amphiboles) which, because half the tetrahedra share 2 oxygens and half share 3 oxygens, have a Si:O ratio is 4:11.
Pyroxenes commonly alter to amphiboles under retrograde hydrous conditions.
Pyroxenes are anhydrous (no structural OH or H₂O) whereas amphiboles have structural OH, Cl, F.
Most members of each group are monoclinic but orthorhombic examples occur.
The c-cell dimensions of both pyroxenes and amphiboles are roughly the same at 5.2 A (2 tetrahedra). The a-dimensions are also analogous but the double wide chain leads to a commensurate doubling of the b-cell dimension in the amphiboles.
Pyroxenes tend to form short blocky crystals while amphiboles tend to elongate crystals.
Pyroxenes form at higher temperatures than do amphiboles.