Not required for EPS2 students.
Single chain silicates: two tetrahedral oxygens shared -> Si:O = 1:3
Structures are analogous to amphiboles
Pyroxenes differ from amphiboles because
Opposing chains create octrahedral sites of two types:
These are rather regular sized 6-coordinated sites:
Accommodate: Mg, Fe (2+ and 3+), Al, Mn, etc.
These sites are known as M1 sites, analogous to the M1,M2,M3 sites in amphiboles.
Structure movies: pyroxenes and their structural components
Also, back to back chains provide cations to complete second strip of sites, these are octahedral, but may be large and distorted. These sites can accommodate:
Mg, Fe, Al, [Mn] Li, Ca, Na, etc.
This second set of sites, known as M2 sites, is analogous to the M4 sites in amphiboles.
Mn is larger and can not be accommodated in these sites; when it goes into the single chain silicate structure it forms a slightly different structural type known as a PYROXENOID
The pyroxene I-beam and pyroxene cleavage!
How do we deal with 3+ cations and 1+ cations ?
Rewrite: [M]4+Si2 O6
M+ M3+ Si2 O6
Also, tetrahedral sites can accommodate some Al3+ and Fe3+
THUS: three groups to be considered:
Consider common cations: Ca2+, Mg2+, Fe2+
Ca -> M2 site, like amphibole M4 site, wants either almost all Ca or almost no Ca, so
Ca (Mg,Fe) Si2 O6
and (Mg,Fe)(Mg,Fe) Si2 O6 e.g., Mg2 Si2 O6, Fe2 Si2 O6 and (Mg,Fe)2 Si2 O6
i.e., *Mg2-------------------range of Mg, Fe-------------------Fe2*
complete range of solid solution!
likewise *CaMg ---------------range of Mg,Fe-------------------CaFe*
known as the pyroxene quadrilateral. (see Fig. 13.47 in text).
MUST KNOW 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!) actually restricts the potential size of the M2 sites - the stagger is not possible if the M2 site is to accommodate Ca!! THUS- the Ca-rich pyroxenes are monoclinic.
Conversely, Ca rich pyroxenes do not have the alternating stagger and so they are monoclinic. (+,+,+...)
EXSOLUTION, note the shape and location of the miscibility gap.
note exsolution mechanisms as described for amphiboles
Common monoclinic pyroxenes (clinopyroxenes) e.g.,
JADEITE: NaAlSi2 O6 (recall nepheline NaAlSiO4 and albite NaAlSi3O8)
Forms at high pressures. Gem material Jade]
AEGERINE: NaFe3+Si2 O6 (also known as Acmite). Often Ca(Mg,Fe) pyroxenes contain some NaFe3+ <=> Ca(Mg,Fe) component. Aegerine is a relatively rare mineral - found in low Si rocks. (monoclinic)
SPODUMENE: LiAlSi2 O6: Recall the spodumene in the pegmatite on the field trip! (see the courtyard sample). (monoclinic)
Consider composition Ca2 Si2 O6. This can not be a pyroxene, as Ca will not go into M1. However, it is possible to accommodate Ca in an M1-type site if create a modified chain: 3 repeat, instead of 2 (7.1 A repeat rather than 5.2 A).
Chain has lower symmetry -> crystals are triclinic!
CaSiO3 = (Ca2 Si2 O6) = WOLLASTONITE
what kind of rocks ?
Note the chain geometry.
This possibility raises the case 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
This mineral has a 5-repeat pyroxenoid chain!
(Ca-someMn)SiO3 = PYROXMANGITE - 7 repeat chain.