-1-
TEXTURES OF THE IGNEOUS ROCKS*
Structure - large-scale features recognizable in the field, such as banding, lineation, jointing, and vesicularity.
Texture - refers to degree of crystallinity, grain size, and geometrical relationships between the constituents
of a rock (fabric).
I. Igneous Textures
A. Degree of Crystallinity
Holocrystalline - composed wholly of crystals
Hypocrystalline - contains both glass and crystals
Holohayline - consists entirely of glass
Microlites - minute incipient crystals which are birefringent
Crystallites - smaller than microlites, spherical, rod- and hair-like isotropic forms
B. Grain Size or Granularity
Cryptocrystalline - crystals cannot be distinguished even with a microscope
Aphanitic - crystals not visible to the unaided eye
Phaneritic - grains readily distinguished with the unaided eye
If the grains of the rock are roughly the same size:
Fine < 1 mm
Medium 1-5 mm
Coarse 5 mm-3 cm
Very coarse > 3 cm
C. Shape of Crystals
1. Euhedral (idiomorphic) - grains completely bounded by crystal faces
Subhedral (hypidiomorphic) - grains only partly bounded by crystal faces
Anhedral (allotriomorphic) - grains completely devoid of crystal boundaries
2. Habit - columnar, acicular, fibrous, tabular, prismatic, equant, and flaky
3. Order of crystallization
a. When one mineral is surrounded by another, the enclosing mineral is younger.
b. Early crystals are generally euhedral or at least more nearly so than later crystals.
c. If both large and small crystals occur together, the large ones are those that began to
develop first.
d. There are numerous exception to these rules.
D. Granular Texture - most of the minerals of a rock are approximately equidimensional or equant.
Panidiomorphic-granular (automorphic-granular) - the chief minerals are euhedral
Hypidiomorphic-granular (hypautomorphic-granular or granitic) - some constituents are euhedral,
some subhedra, and the rest anhedral
