Geology Reference
In-Depth Information
(a)
(b)
(c)
1cm
Term
Euhedral
Subhedral
Anhedral
Other
terms
(Idiomorphic,
Automorphic)
(Hypidiomorphic,
Hypautomorphic)
(Allotriomorphic,
Xenomorphic)
Description Crystal completely
bounded by its
characteristic faces
Crystal bounded by only
some of its characteristic
faces
Crystal lacks any
of its characteristic
faces
Figure 3.5
Euhedral, subhedral, anhedral with descriptions. Sketches of (a)
euhedral, (b) subhedral and (c) anhedral amphibole crystals (cleavage traces
at 60
◦
and 120
◦
, cf. Table 3.6).
sets of terms are used to describe the quality of development of crystal faces
,
Euhedral, Subhedral
and
Anhedral
(see Figure 3.5).
In ideal circumstances, where the grain shapes of a coarse-grained rock are
extremely clear, it may be possible to classify the grain shape fabric of the
entire rock (for example,
granular texture
if all crystals are of uniform size).
Furthermore, a rock may not fit neatly into a single category: thus, one in which
<
50% anhedral might best be described
as having a mixed euhedral and anhedral granular texture. In some examples it
is possible to note the relative timing of crystallisation, buy careful examination
of the crystal population with a hand lense on both weathered and fresh surfaces.
Euhedral crystals often grow first when they are relatively free in the melt, and
subsequent crystallising phases have sub to anhedral shapes as they are forced to
occupy the space between existing euhedral crystals (for example, Figure 3.6a).
Where a new crystallising phase engulfs crystals of a pre-existing growing phase
this leads to a texture known as
poikilitic
, with the large
oikiocrysts
engulfing
the smaller
chadocrysts
(for example, Figure 3.6b).
After describing the individual crystals and their inter-relationships, the
remaining textural characteristics to notice concern the overall
homogeneity
of the rock. At the scale of a hand specimen, most igneous rocks are fairly
50% of the crystals are euhedral and
<
