Geology Reference
In-Depth Information
(a) - different stages of crystallisation 1
1
3
1mm
1mm
1mm
Early crystallisation of
phase 1 (euhedral)
Phase 2 starts to crystallise
in between phase 1
Further growth leads to euhedral (early)
and sub-anhedral (late) relationship
(b) - different stages of crystallisation 2 development of piokiolitic texture
oikocryst
1
2
3
1mm
1mm
1mm
chadocrysts
Early crystallisation of
phase 1 (euhedral)
Second later stage 2 pahse
forms single large crystals
Poikiolitic texture results with large
oikiocrysts encosing small chadocrysts
Figure 3.6
Schematic texture development to show: (a) Different stages of crys-
tallisation shown by euhedral-anhedral relationships. (b) Different stages of
crystallisation with large crystals enclosing older small crystals.
homogeneous but occasionally
flow banding
or
mineral lamination
are
encountered (Figure 3.7). Flow banding in medium and coarse-grained rocks
may be expressed through the visible alignment of elongate crystals (for
example Chapter 2; Figure 2.10) or, in fine-grained rocks, by a fine-ribbing or
colour banding (flow banding) which is often most noticeable on weathered
surfaces (for example, Chapter 2; Figure 2.9). As flow structures are developed
parallel to the direction of movement, measurement of their orientation will
act as a useful aid in mapping lava sequences. Mineral lamination, expressed
as alternating lighter and darker layers in coarse-grained rocks, may originate
through fractionation processes (as described in Chapter 2 and Figure 2.11) or,
perhaps, through high-grade metamorphism of otherwise homogeneous igneous
material. Subsequent interpretation of field data in all these cases is made a
great deal easier if careful notes are made in the field of the colour, texture,
orientation and scale of the layering, not only in hand samples, but also in the
outcrop as a whole.
The textures which have been introduced in this section are those frequently
encountered in igneous field studies; the list will serve as a general guide in
