Page 168 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES OXIDES

ments typically contain assemblages of almost pure magnetite ± fer- The spinel group
rianilmenite (possibly with exsolved hematite) ± titanohematite (poss- The general unit cell formula of the spinels is R~+Ri60Jz. where Rz+ and
ibly with exsolved ilmenite) ± rutile. Greenschist facies rocks may con- RH stand for divalent and trivalent cations respectively but the formula
tain a magnetite + rutile assemblage which gives way to a is usually simplified to R 0 4 • All spinels are cubic b~t there are two
3
titanohematite + ferrianilmenite assemblage in amphibolite facies. structural types with differing distributions of the cations:
The following are noted for Figure 3.8:
Normal spinels
R~ + in fourfold tetrahedral co-ordination with oxygen
(a) The Ti0 2 polymorphs are:
Ri6 in sixfold octahedral co-ordination with oxygen
rutile, tetragonal cia < 1
Inverse spinels
anatase, tetragonal cia > 1 (metastable?) ) found in low Ri+ in fourfold tetrahedral co-ordination with oxygen
temperature R~+ and Ri+ in sixfold octahedral co-ordination with oxygen
brookite, orthorhombic (metastable?) hydrothermal
environment Most natural spinels have an intermediate structure (see Fig. 3.9).In the
spinel structure, oxygen is oz- in tetrahedral co-ordination. .
(b) Ferropseudobrookite is only stable above 1100 °C and is very rare. The spinels are normal valence compounds in that the . total. catiOn
(c) Pseudobrookite is only stable above 585 °C and is rare (in high charge balances the total anion charge. Divalent R + catwns mclude
2
temperature contact metamorphosed rocks). Mgz +, Fez+, znz+, Mnz+ and Ni +, and RH cations induct: AI~+, F: + an.d
3
2
(d) The ilmenite-hematite solid solution series is complete above
CrH. One way of representing the extens1ve solid solutiOn m spmels IS
about 800 °C.
shown in Figure 3.10.
(e) The ulvospinel-magnetite solid solution series is complete above It is possible for Ti 4 + (and y•+) to enter the structure due to.a coupled
about 600 °C. substitution of the type 2FeH ~ FeZ+ + Ti +. The inverse spmel ~tru~­
4
(f) Magnetite and rutile can coexist only below about 400 °C.
ture of maghemite y-Fe 2 0 3 supports a cation site vacancy wh1ch IS
(g) From 1100 octo 600°C Ti-rich ferrianilmenite coexists with Ti-
3
2
produced by the substitution 3Fe + ~ 2Fe + + [ ] ; the formula may be
poor titanomagnetite; the exact composition of the coexisting pair
written
depends on oxygen fugacity as well as temperature (the Budding-
ton and Lindsley (1964) magnetite-il~T~enite geothermometer Fe~ +(tetr.)Fe~ +( oct.)Fe~+ [ ] ( oct.)O 12
oxygen barometer). The dotted lines show how the compositions
(approximate) of coexisting pairs depend on temperature and
oxygen fugacity. Key
(h) Oxidation of titanomagnetite at relatively high temperatures
Q oxygen
results in exsolution lamellae of ferrianilmenite in the ( 111) orien-
tation in magnetite, and this oxidation can result from cooling Q octahedral cations
alone. Similarly, reduction of ferrianilmenite results in
titanomagnetite lamellae in the (0001) orientation of ilmenite. • tetrahedral cations
(i) Titanomaghemites form at low temperatures ( < 600 oq by non-
equilibrium oxidation of titanomagnetites; they are cation
deficient and have a wide range in composition.
(j) Hemo-ilmenite is a ferrianilmenite host with titanohematite lamel-
lae.
(k) Ilmeno-hematite is a titanohematite host with ferrianilmenite
lamellae.
(!) The bulk composition of coexisting hemo-ilmenite and ilmeno-
hematite grains depends on temperature. Figure 3.9 The spinel unit cell, orientated so as to emphasise the (111) planes.
(m) Wtistite is cation deficient relative to FeO. It is very rare as it is Atoms are not drawn to scale; the circles simply represent the centres of atoms
stable only above 570 oc at low oxygen fugacities. (after Lindsley, in Rumble 1976).
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