GLG 333 -- IGNEOUS TEXTURES AND STRUCTURES
The following is a reorganization and amplification of Blatt, Tracy, and Owens, 2006, pages 29-35.
(You may also want to refer to Barker, 1983, pages 106-120; Hyndman, 1985, pages 49-61; or
Jackson, 1970, pages 255-273 and 317-323. You will also definitely want to look at MacKenzie et
al., 1982, pages 4-73, to see what these igneous textures look like when viewed in thin-section under a
petrographic microscope.)
I. SHAPE OF INDIVIDUAL CRYSTALS (indicates sequence of crystallization)
A. Euhedral. Crystal completely bounded by its own crystal faces. Indicates early
crystallization from the magma, i.e. before enough other crystals were present to
cause interference for space.
B. Anhedral. Crystal not bounded by any of its own crystal faces; rather, its form is imposed on
it by the adjacent crystals. Indicates late crystallization from the magma, i.e. after most of the
available space was already occupied by earlier-formed crystals.
C. Subhedral. Intermediate between euhedral and anhedral.
II. GENERAL ROCK TEXTURES
A. Based on Degree of Crystallinity
1. Holohyaline. Rock composed entirely of glass; no crystals visible even with
magnification. Indicates cooling so rapid that no crystal growth could occur,
i.e. quenching.
a. Obsidian Massive volcanic glass.
b. Pumice Frothy (micro-vesicular) volcanic glass.
2. Hypocrystalline (or Hyalocrystalline). Rock composed of both crystals and glass.
a. Pitchstone Volcanic glass with a resinous luster or streaky appearance caused
by microscopic crystallites.
b. Vitrophyre Volcanic glass with macroscopic phenocrysts.
c. Perlite Volcanic glass with a cloudy or milky appearance caused by devitrification.
Often exfoliates in thin, concentric layers (i.e., onion-skin devitrification to
produce “Apache tears”)
3. Holocrystalline. Rock composed entirely of crystals (which may or may not be visible
without magnification). Indicates cooling that was sufficiently slow to allow complete
crystallization to occur.
