EPSC 210 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 Si. O 2. They are also less polymerized (less corner sharing among Si. O 4) 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. H 2 O supplies the OH- groups to build an amphibole. - amphiboles breaking down to pyroxene in rocks that are re-heated enough that the OHgroups 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 Si. O 4 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 Z 2 O 6 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: M 2, M 1, 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 M 2 (larger, blue) and M 1 (smaller, red). M 2 corresponds to X, and M 1 to Y in the general formula.
The M 1 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 M 2 sites, at the base of tetrahedra, can house slightly larger divalent cations than the M 1 sites. They are coordinated to the 6 -8 nearest oxygen ions.
The shortest and strongest bonds are Si-O followed by M 1 -O bonds. In 3 -D, they form strong chains sometimes described as “I-beams” because they give strength to the crystalline structure.
The M 2 -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”.
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. Orthopyroxenes have an orthorhombic symmetry, and their M 2 and M 1 site are filled by cations fairly similar in size. . .
(left: orthopyroxene) See how the M 1 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 clinopyroxenes 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 clinopyroxenes 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 Mg. Si. O 3 a = 18. 2 b = 8. 8 c = 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 Mg. Si. O 3 (opx) enstatite-ferrosilite series (Mg, Fe)Si. O 3 (cpx) diopside Ca. Mg. Si 2 O 6 (cpx) augite Ca(Mg, Fe)(Al, Si)2 O 6 (cpx) spodumene Li. Al. Si 2 O 6 (cpx) jadeite Na. Al. Si 2 O 6 (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 M 1 and M 2 cations of various sizes.
view down (100): see how chains are slightly twisted clinopyroxenes view down (010)->>
Rhodonite and wollastonite are pyroxenoids. Above: Mn. Si. O 3 Their divalent cations, Mn 2+ or Ca 2+, are all in 8 fold coordination, and the Si. O 4 chains are strongly twisted. Right: Ca. Si. O 3 Both minerals show some compositional variation: Fe 2+, Mn 2+, Ca 2+, Zn 2+, and (to a lesser degree) Mg 2+. . .
Pyroxenoids have a twisted chain structure to accommodate all Me. O 8 polyhedra. This further lowers their symmetry to triclinic. Ca. Si. O 3, 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 Mn. Si. O 3 or Ca. Si. O 3 do not form a solid solution with pyroxenes because their structure cannot mix largish (Ca 2+, Mn 2+) and smallish ions (Mg 2+or Fe 2+) in M 1 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 Mg 2+ and Fe 2+ ions in the M 2 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 Na. Al. Si 3 O 8 unmixed from KAl. Si 3 O 8.
From this diagram, you see that an opx (Mg, Fe)Si. O 3 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 M 2 sites can hold appreciable amounts of cations of different sizes such as Ca 2+ and Mg 2+. If cooling is slow, pigeonite “unmixes” to a Cafree orthopyroxene, (Mg, Fe)Si. O 3 containing small lamellae of augite, a more stable Ca-rich clinopyroxene with a M 2 site filled mostly by Ca 2+ (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.
Dar al Gani 476 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.
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? Ca. Mg. Si 2 O 6 viii. Ca 2+ + vi. Mg 2+ = vi. Al 3+ +viii. Na + vi. Mg 2+ + iv. Si 4+ = iv. Al 3+ +vi. Al 3+ (Na, Ca) (Mg, Fe, Al)(Al, Si)2 O 6. . . two coupled substitutions (Note: Al 3+ 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 viii. Navi. Mn 2 iv. Si 3 O 8(OH) is a quasipyroxenoid which shows a twisted chain.
What substitutions relate wollastonite Ca. Si. O 3 to serandite viii. Navi. Mn 2 iv. Si 3 O 8(OH) ? (Hint. . . start from 3*Ca. Si. O 3 = Ca 3 Si 3 O 9) Check the ionic radii to find largest ion. . . vi 2 Mn 2+ for viii 2 Ca 2+ -> Ca. Mn 2 Si 3 O 9. . . viii. Na+ for viii. Ca 2+ (charge balance? ). . . O 2 -H+ or (OH)- for O 2 - (charge balance? ). . . these last two substitutions must be combined in a single equation as the charge balance is solved by coupling them: Na+ + OH- = Ca 2+ + O 2 -> viii. Navi. Mn iv. Si O )(OH)
Amphiboles are double-chained silicates.
Many more types of sites between the oxygen ions: M 4, M 3, M 2, M 1… but also a larger A site between the chains. OH groups line up with tetrahedral tips.
General formula of amphiboles: W X 2 Y 5 Z 8 O 22 (OH)2 A 0 -1 (M 4 )2 (M 1, 2, 3)5 T 8 O 22 (OH)2 where: A+ is a large cation (can be totally absent) M 4 is a cation equivalent to M 2 in a pyroxene M 1, 2, 3 are cations equivalent to M 1 in a pyroxene T is a small cation in tetrahedral coordination The size difference between X, Y (M 4 vs M 1, 2, 3) cations also controls the overall symmetry. Those radii are close in orthoamphiboles; Radius in M 4 >> than in M 1, 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, M 4 -O and AO, 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?
The shape of some of the fields (solid solution) expands at higher temperature. 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. . .
• T (tetrahedra): Si, Al. The limit of Al substitution in these sites is about 2 out of 8. • M 2 (small octahedron): Al 3+, Cr 3+, Fe 3+, Ti 4+, Fe 2+, Mg 2+ • M 1, M 3 (medium octahedra): Fe 2+, Mg 2+, Mn 2+. • M 4 (larger cation site): Ca 2+, Na+, Mn 2+, Fe 2+, Mg 2+. • A: Na+, K+, or vacancies (i. e. can be left empty).
Common substitutions in amphiboles, written in a more compact notation… Al 2 Mg-1 Si-1 is the same as writing the following equation: 2 Al 3+ = Mg 2+ + Si 4+ isomorphous: Fe 2+Mg-1 , Mn. Mg-1, Mg. Ca-1 coupled: Al 2 Mg-1 Si-1 Fe 3+Al. Mg-1 Si-1 Ti. Al 2 Mg -1 Si-2 These coupled substitutions fill the A site. The “V” stands for a vacant (empty) site. Na. Al. V-1 Si-1 (equivalent to Na. Al -1 Si-1) KAl. V-1 Si-1 (equivalent to KAl -1 Si-1) Na. Al. Ca-1 Mg-1
Which ones of these amphiboles are ortho- or clino? tremolite Ca 2 Mg 5 Si 8 O 22(OH)2 actinolite Ca 2(Mg, Fe)5 Si 8 O 22(OH)2 glaucophane Na 2 Mg 3 Al 2 Si 8 O 22(OH)2 anthophyllite Mg 7 Si 8 O 22(OH)2 hornblende (see why it’s called a “garbage can”? ) (Na, K)0 -1 Ca 2(Mg, Fe, Al, Ti)5(Si 6 -8 Al 0 -2)8 O 22(OH)2 W A X 2 0 -1 Y 5 Z 8 O 22 (OH)2 (M 4 )2 (M 1, 2, 3)5 T 8 O 22 (OH)2
The difference in radii of X vs. Y cations determines which amphiboles are ortho- or clino. . . tremolite Ca 2 Mg 5 Si 8 O 22(OH)2 actinolite Ca 2(Mg, Fe)5 Si 8 O 22(OH)2 glaucophane Na 2 Mg 3 Al 2 Si 8 O 22(OH)2 anthophyllite Mg 7 Si 8 O 22(OH)2 <<clino>> <<ortho>> hornblende (it’s a clino “garbage can”. . . ) (Na, K)0 -1 Ca 2(Mg, Fe, Al, Ti)5(Si 6 -8 Al 0 -2)8 O 22(OH)2 Is the A site filled in any of them? W A X 2 0 -1 Y 5 Z 8 O 22 (OH)2 (M 4 )2 (M 1, 2, 3)5 T 8 O 22 (OH)2
The bad name of asbestos comes from amphiboles! Some amphiboles, including crocidolite, an iron-rich variety of glaucophane, Na 2 Mg 3 Al 2 Si 8 O 22(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…
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