The resulting mechanical colour zoning is therefore linked to
exsolution of these particles and also is epigenetic.
This excess pressure may be due to the formation of bubbles by gas
exsolution, or to the fact that heat loss from the magma to its cooler surroundings causes the growth of crystals that are less dense than the magmatic liquid and so occupy a larger volume.
The method is demonstrated with spectrum images of the [Fe.sup.3+]/[SIGMA]Fe changes occurring across mineral interfaces between ilmenite ([Fe.sup.2+]Ti[O.sub.3]) and magnetite ([Fe.sup.2+] [Fe.sup.3+.sub.2][O.sub.4]) in a sample with microscale
exsolution lamellae.
Fast cooling of volcanic ash avoids
exsolution of feldspar and favours the preservation of magmatic sanidine; therefore calculation of the feldspar composition according to the method proposed by Orville can be estimated as accurate.
Okorusu magnetites exhibit abundant
exsolutions of ulvospinel in a characteristic cloth texture, and oxidation
exsolution occurred subsequently.
First, [H.sub.2]O saturation occurs by
exsolution of an aqueous fluid to forma distinct phase in the silicate melt, at which point the fluid boils and gas bubbles form.
The assemblage titanite-magnetite-quartz-pyrrhotite may be a late auto-oxidation and sulphidation product of vapour
exsolution (Candela 1991), as it is not consistent with high Fe (-Ti) biotite (Wones 1981).
The copper appears to have formed by epigenetic
exsolution from elbaite.