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Part 3: Crystal shapes
•Two kinds of term used to describe crystal shapes:
1.Those relating to the quality of the development of faces on crystals
2.Those specifying the three-dimensional shapes of individual crystals
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1. Terms indicating the quality of the development of faces on crystals
- Unfortunately, three sets of words are in use to describe the same ideas, the most
   commonly used set being that in the first column of the following table.
Preferred terms         Synonymous terms         Synonymous terms           Meaning
Euhedral                      Idiomorphyc                     Automorphic
Subhedral                    Hypidiomorphic               Hypautomorphic
Anhedral                      Allotriomorphic               Xenomorphic
Crystal completely bounded by its characteristic faces.
Crystal bounded by only some of its characteristic faces.
Crystal lacks any of its characteristic faces.

Euhedral olivine in olivine basalt
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Subhedral olivine in picritic basalt
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Anhedral olivine phenocryst in basalt
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2. Terms indicating three-dimensional crystal shape
In hand specimens of coarse-grained rocks is often possible to see the three-dimensional shape of a
crystal on a broken surface. For finer-grained rocks, however,  the crystals have to be examined in
thin section and two-dimensional shapes of several crystals of different oriantations used to
deduce the tree-dimensional shapes of the crystals in general.

General tree-dimensional terms:
- The shape either be an equidimensional (syn. equant) or an inequidimensional one, as
  illustrated in figs. A and B where the names applied to the various shapes are shown.
Fig. A Examples of equidimensional crystal shapes:
The words grain and granule are often used for equidimensional crystals, and drop and bleb for
particularly small examples.
Fig. B Examples of inequidimensional shapes:
Although these are euhedral examples, they could besubhedral or anhedral.
*Bleded feldspars crystals by common usage are frequently described as ´lath-shaped´or  as ´laths
of feldspar´, in allusion to the slats (laths) in a Venetian blind.
 *

Specific three-dimensional terms:
Skeletal, dendritic and embayed crystals:
Skeletal crystals are those which have hollows and gaps, possibly regularly developed, and usually
with particular crystallographic orientations. In thin section these spaces appear as
embayments1 and holes in the crystal, filled with groundmass crystals or glass. Dendritic crystals
consist of a regular array of fibres sharing a common optical orientation (i.e. all part of a
single crystal) and having braching pattern resembling that of a tree or veins in a leaf or a
feather. In practice, many crystals can be described as either skeletal or dendritic because they
have characteristics of both.
1  A common mistake among petrologists is the terms ´embayment´and ´embayed´imply resorption of
crystal by reaction with liquid. While this may be true of some crystals (see below fig. 29),
others (see fig. 26,27) have embayments which probably formed during growth.

Skeletal olivines in picritic basalt
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Skeletal olivine



Dendritic olivines
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Embayment in augite phenocryst
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Embayment quartz
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Parallel-growth crystals:
The term is applied to an aggregate of elongate crystals of the same mineral whose crystalographic
axes are mutually parallel, or almost so. Although in thin section the individual parts of the
aggregate may be isolated from one another, in the third dimension they are probably conected. A
parallel-growth crystals is therefore a single, incomplete crystal formed by a particular style of
skeletal growth.
Olivine parallel growth

Parallel growth in very coarse-grained rock



Sieve-textured crystals:
These contain abundant, small, interconnected, box-shaped glass inclusion, giving  the crystals a
spongy, or porous, appearance.
Sieve-textured feldspar

Elongate, curved, branching crystals:
These are rarely genuinely bent, rather the curveture is caused by development of branches along
the length of the crystals, each branch having a slightly different crystallographic orientation to
its neighbours (Fig. 34-36).
Curved branching augite

Branching augite in lamprophyre dyke



Curved and branching plagioclase crystals in dolerite



Composite branching augite crystal
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Pseudomorphs:
It may be found that crystals in thin section, althoug having the characteristic shape of a
particular mineral, prove to be of another mineral, or an aggregate of crystals of another
mineral.  The name pseudomorph is used for such a crystal. If the pseudomorph has the same
composition as the original mineral (e.g. ´quartz´in place of tridymite) it is known as a
paramorph.
Carbinate pseudomorphs after olivine