EPSC210 Introductory Mineralogy

Inosilicates

Bowen’s reaction series

Bowen’s reaction series describe the typical order of crystallization from a basaltic magma.

The nesosilicate olivine and the inosilicates (chain silicates = pyroxenes and amphiboles) precipitate before phyllosilicates and quartz because they contain a lower proportion of SiO2.

They are also less polymerized (less corner sharing among SiO4) and generally denser than phyllosilicates and quartz. Cations must occupy a larger share of the space among tetrahedra to satisfy the valence of oxygen.

3 important groups among inosilicates

A) pyroxenes (single chain)

b) pyroxenoids (single chain, twisted)

c) amphiboles (double chain)

Being “chain silicates” gives all inosilicates a prismatic cleavage, prismatic habit and a moderate hardness (5.5).

The prismatic cleavage and growth habit are most pronounced in the amphiboles.

Nearly each pyroxene has a corresponding amphibole, i.e. one where the same type of metallic cations joins the chains together.

Amphiboles, with (OH)- groups, are stable at lower temperatures than the equivalent pyroxenes.

Common metamorphic reactions include

- the alteration of pyroxenes to amphiboles, if a rock is reheated (without melting it) and is brought in contact with hot water. H2O supplies the OH- groups to build an amphibole.

- amphiboles breaking down to pyroxene in rocks that are re-heated enough that the OH- groups break away from the structure.

The growth habit of inosilicates generally obeys the law of Bravais (lecture notes #13) i.e. their largest faces are found along lattice planes of highest node density.

Since the chains of SiO4 tetrahedra (the shortest and strongest bonds in the structure) are along the c axis, growth is expected to be fastest along that direction.

The slow-growing faces are therefore parallel to the c axis, and often end up being the largest ones on the crystals.

Pyroxenes have the general formula

X Y Z2 O6

Where…

X are cations in 6 to 8-fold coordination,

Y are cations in 6-fold coordination,

Z are cations in 4-fold coordination.

These sites are also given labels: M2, M1, T

The oxygen ions are further apart along the base of the tetrahedra than around the apex (tips) of the chains. This defines the two types of octahedral sites for cations, called M2 (larger, blue) and M1 (smaller, red). M2 corresponds to X, and M1 to Y in the general formula.

The M1 cations are bonded mostly to apical oxygens (at tips of tetrahedra ). They have exactly 6 oxygens around the cation arranged into a regular octahedron.

The M2 sites, at the base of tetrahedra, can house slightly larger divalent cations than the M1 sites. They are coordinated to the 6-8 nearest oxygen ions.

The shortest and strongest bonds are Si-O followed by M1-O bonds. In 3-D, they form strong chains sometimes described as “I-beams” because they give strength to the crystalline structure.

The M2-O bonds are the easiest ones to break. The cleavage runs between the base of tetrahedra, in a jagged pattern, making it rough, less than “perfect”.

Orthopyroxenes have an ortho-rhombic symmetry, and their M2 and M1 site are filled by cations fairly similar in size...

Clinopyroxenes are monoclinic when X and Y cations have very different sizes. The “beta” angle, between the a and c axes, is > 90 degrees. The parallelohedron {001} is inclined (but not perpendicular) at the end of the prism.

(left: orthopyroxene)See how the M1 octahedra alternate in orientation?

Below: clinopyroxenes are monoclinic (note shorter a axis and beta angle.)

Seen down their c axis (chains perpendicular to the screen), the orthopyroxenes and clino-pyroxenes show the same cleavage angles.But go back to the previous slide and see how the unit length along the a axis extends twice as far in an orthopyroxene than in a clinopyroxene…

sketch the axes and the cleavage for yourself

Seen down their c axis (chains perpendicular to the screen), the orthopyroxenes and clino-pyroxenes show the same cleavage angles.But go back to the previous slide and see how the unit length along the a axis extends twice as far in an orthopyroxene than in a clinopyroxene…

In opx, cleavage is indexed as{210} because the unit length along the a axis is about twice as long. Cleavage intercepts it at 1/2 its length.

cell of MgSiO3

a = 18.2b = 8.8c = 5.2

In cpx, cleavage is {110}.The cell of diopside: a = 9.73,b = 8.9, c = 5.3.

Pyroxenes: which ones are opx, cpx...

(opx) enstatite MgSiO3

(opx) enstatite-ferrosilite series (Mg, Fe)SiO3

(cpx) diopside CaMgSi2O6

(cpx) augite Ca(Mg, Fe)(Al, Si)2O6

(cpx) spodumene LiAlSi2O6

(cpx) jadeite NaAlSi2O6

(Compare the ionic radii of the first two ions…)

There is a fair amount of flexibility in the structure of single chain silicates.

The tetrahedra can twist in order to share corners with the octahedra surrounding M1 and M2 cations of various sizes.

view down (010)->>

view down (100): see how chains are slightly twisted

clinopyroxenes

Rhodonite and wollastonite are pyroxenoids.

Their divalent cations, Mn2+ or Ca2+, are all in 8-fold coordination, and the SiO4 chains are strongly twisted.

Above: MnSiO3

Right: CaSiO3

Both minerals show some compositional variation: Fe2+, Mn2+, Ca2+, Zn2+, and (to a lesser degree) Mg 2+ ...

Pyroxenoids have a twisted chain structure to accommodate all MeO8 polyhedra. This further lowers their symmetry to triclinic.

CaSiO3, wollastonite is increasingly used in industry as a filler in rubber, asphalt tiles, etc... Its fibrous texture can be a good substitute for asbestos.

Being triclinic, the pyroxenoids MnSiO3 or CaSiO3 do not form a solid solution with pyroxenes because their structure cannot mix largish (Ca2+, Mn2+) and smallish ions (Mg2+or Fe2+) in M1 sites.

There are no pyroxenes of intermediate compositions in this area of the diagram...

Some versions of this diagram show “tie-lines” connecting the pairs of minerals that would form from a melt of intermediate composition. Many igneous rocks contain both orthopyroxenes and clinopyroxenes as separate crystals because of this limited solid solution.

What is pigeonite?

At high temperature, orthopyroxenes and clinopyroxenes can tolerate mixing Ca 2+ and smaller Mg2+ and Fe2+ ions in the M2 sites. However, these ions tend to unmix during cooling.This chemical umixing of one mineral into two different species is called exsolution. It occurs in several types of magmatic minerals, but it is not necessarily visible to the naked eye.

The same process is responsible for the perthite in a feldspar:paler veins of NaAlSi3O8

unmixed from KAlSi3O8.

From this diagram, you see that an opx (Mg,Fe)SiO3 can contain more Ca at high temperature, but would unmix during slow cooling to exsolve lamellae of augite.

Pigeonite is rare, but it is a sensitive indicator of cooling history. See how it should not exist at lower temperatures according to the phase diagram?

It is only at high temperatures that M2 sites can hold appreciable amounts of cations of different sizes such as Ca2+ and Mg2+ .

If cooling is slow, pigeonite “unmixes” to a Ca-free orthopyroxene, (Mg,Fe)SiO3 containing small lamellae of augite, a more stable Ca-rich clinopyroxene with a M2 site filled mostly by Ca2+ (and possibly other large ions such as Na+).

The clinopyroxene pigeonite is only preserved if quick magmatic crystallization prevents diffusion and exsolution. This happens more often in in volcanic rocks than in intrusive rocks.

This meteorite (a shergottite, if you must know) is a piece of Martian basaltic lava that landed in the Libyan Sahara desert. Larger crystals are magnesian olivine (forsterite), and some of the smaller ones are pigeonite.

Pigeonite is common enough in basaltic flows that spilled out to form plateaus on the ocean floor and on continents.

Dar al Gani 476

Pigeonite grew in this basaltic rock. Being 2/m, it is prone to twinning along the plane (010) shown as a dotted line. During slow cooling, this pigeonite unmixed to an orthopyroxene (Ca-poor) and thin lamellae of clinopyroxene (Ca-rich augite). The cpx lamellae

form a herringbone pattern within the yellow opx crystal. But they first exsolved from pigeonite crystals (cpx) related by twinning. Pigeonite lost enough Ca to the lamellae and became an opx crystal.

Thin section of basalt under crossed polarizers.

In the crystal traced in orange, the lamellae are clearly mirrored in each part of the twinned crystal.

What substitutions relate a diopside to an augite?Composition substitutions?CaMgSi2O6

viiiCa2+ + viMg2+ = viAl3+ +viiiNa +

viMg2+ + ivSi4+ = ivAl3+ +viAl3+

(Na, Ca) (Mg,Fe,Al)(Al,Si)2O6

... two coupled substitutions(Note: Al3+ occurs in two different types of sites.)

Another rare inosilicate… Mt Saint Hilaire is world famous for the occurrence of large, euhedral serandite crystals, first found in 1963.

serandite viiiNaviMn2ivSi3O8(OH) is a quasi-

pyroxenoid which shows a twisted chain.

What substitutions relate wollastonite CaSiO3 to serandite viiiNaviMn2

ivSi3O8(OH) ?

(Hint... start from 3*CaSiO3 = Ca3Si3O9)Check the ionic radii to find largest ion...

... vi2Mn2+ for viii2Ca2+ -> CaMn2Si3O9

... viiiNa+ for viiiCa2+ (charge balance?)

... O2-H+ or (OH)- for O2- (charge balance?)... these last two substitutions must be combined in a single equation as the charge balance is solved by coupling them: Na+ + OH- = Ca2+ + O2- -> viiiNaviMn2

ivSi3O8)(OH)

Amphiboles are double-chainedsilicates.

Many more types of sites between the oxygen ions: M4, M3, M2, M1… but also a larger A site between the chains. OH groupsOH groups line up with tetrahedral tips.

General formula of amphiboles: W X2 Y5 Z8 O22 (OH)2

A 0-1 (M4 )2 (M1,2,3)5 T8 O22 (OH)2

where:A+ is a large cation (can be totally absent)M4 is a cation equivalent to M2 in a pyroxene

M1,2,3 are cations equivalent to M1 in a pyroxeneT is a small cation in tetrahedral coordination

The size difference between X, Y (M4 vs M1,2,3) cations also controls the overall symmetry.Those radii are close in orthoamphiboles; Radius in M4 >> than in M1, 2, 3 for clinoamphiboles.

The difference in cleavage angles among pyroxenes and amphiboles are obvious in thin section, under the microscope.

Pyroxene (left): angles of 87 & 93 degrees. Cleavage is coarser, less regular, parallel to smaller faces.

Amphibole (right): angles of 120 and 60 degrees. Cleavage is better developed, parallel to larger faces, more evenly spaced.

The cleavage breaks the weakest bonds, M4-O and A-O, along the bases of tetrahedra. It is better (nearly planar) than in pyroxenes.

What’s wrong with this picture? Angles are OK, but what bonds are being broken?

Tie-lines connect pairs of amphiboles that would form from a melt of intermediate composition. If, at high temperature, cummingtonite can take more Ca than is shown here, it will tend to exsolve (unmix) actinolite lamellae when it cools down...

The shape of some of the fields (solid solution) expands at higher temperature.

Tie-lines

• T (tetrahedra): Si, Al. The limit of Al substitution in these sites is about 2 out of 8.

• M2 (small octahedron): Al3+, Cr3+,Fe3+,Ti4+,Fe2+,Mg2+ • M1, M3 (medium octahedra): Fe2+, Mg2+, Mn2+.

• M4 (larger cation site): Ca2+, Na+, Mn2+, Fe2+, Mg2+.

• A: Na+, K+, or vacancies (i.e. can be left empty).

Common substitutions in amphiboles, written in a more compact notation…

Al2Mg-1Si-1 is the same as writing the following equation: 2 Al3+ = Mg2+ + Si4+

isomorphous: Fe2+Mg-1 , MnMg-1, MgCa-1

coupled: Al2Mg-1Si-1 Fe3+AlMg-1Si-1

TiAl2Mg -1Si-2

These coupled substitutions fill the A site. The “V” stands for a vacant (empty) site.

NaAlV-1Si-1 (equivalent to NaAl-1 Si-1)

KAlV-1Si-1 (equivalent to KAl -1Si-1)

NaAlCa-1Mg-1

Which ones of these amphiboles are ortho- or clino?

tremolite Ca2Mg5Si8O22(OH)2

actinolite Ca2(Mg,Fe)5Si8O22(OH)2

glaucophane Na2Mg3Al2Si8O22(OH)2

anthophyllite Mg7Si8O22(OH)2

hornblende (see why it’s called a “garbage can”?)(Na,K)0-1Ca2(Mg,Fe,Al,Ti)5(Si6-8Al0-2)8O22(OH)2

W X2 Y5 Z8 O22 (OH)2

A 0-1 (M4 )2 (M1, 2, 3)5 T8 O22 (OH)2

The difference in radii of X vs. Y cations determines which amphiboles are ortho- or clino...

tremolite Ca2Mg5Si8O22(OH)2

<<clino>>actinolite Ca2(Mg,Fe)5Si8O22(OH)2

<<clino>>glaucophane Na2Mg3Al2Si8O22(OH)2

<<clino>>anthophyllite Mg7Si8O22(OH)2

<<ortho>>

hornblende (it’s a clino “garbage can”...)(Na,K)0-1Ca2(Mg,Fe,Al,Ti)5(Si6-8Al0-2)8O22(OH)2

Is the A site filled in any of them?

W X2 Y5 Z8 O22 (OH)2

A 0-1 (M4 )2 (M1, 2, 3)5 T8 O22 (OH)2

The bad name of asbestos comes from amphiboles! Some amphiboles, including crocidolite, an iron-rich variety of glaucophane, Na2Mg3Al2Si8O22(OH)2, grow with a fibrous habit. Crocidolite has been used as “blue asbestos”. Over long periods of exposure, its fibers are far more damaging to lung tissues than chrysotile.

Ironically, the popular gemstone “tigereye” or “hawkeye” is a pseudomorphic replacement of crocidolite by quartz…