Because there is little or no obstruction to the development of faces, the growth of ore minerals in silicate melts generally results in the development of euhedral crystals.

Thus, the refractory phases viz., primary chromite, magnetite, ilmenite and platinum minerals often occur as well developed equant euhedra interspersed in plagioclase, olivine and pyroxene of the host rocks.

In rapidly cooled basalts, unobstructed growth results in the formation of skeletal crystals which may be wholly or partly enclosed in subsequently crystallizing silicates.

Poikilitic development of silicates in oxides or oxides in silicates is commonly observed.

In oxide rich layers, the simultaneous crystallization of mutually interfering grains results in subhedral crystals with widely variable interfacial angles.

In contrast, the interfacial angles at triple grain junctions of monominerallic aggregates that have been annealed during slow cooling approach 120o.

Iron-sulfur melts from which iron-nickel-copper ores form generally crystallize later than the enclosing silicates.

The magnetite present in these ores crystallizes while the iron sulfides are wholly or partly molten and thus tends to be euhedral or skeletal.

Primary iron-sulfur melts also result in the formation of small (<100 um) round droplets trapped in rapidly cooled basalts and basaltic glasses.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals



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