Textures of Basalts and Gabbros


Introduction

Basalts and gabbros are volcanic rocks that are common to the Earth, Moon, Mars, many large asteroids, and probably to Mercury and Venus. From the Apollo Missions and meteorites that we have received from the Moon, Mars, and asteroids, we have learned that the general characteristics of all basaltic samples are very similar and for the most part we can apply the textural characteristics of Earth rocks to those from extraterrestrial sources. With some limitations, we can also constrain the physical and chemical conditions under which these extraterrestrial rocks crystallized.

Textures

Texture refers to the degree of crystallinity, grain size, and fabric (geometrical relationships) among the constituents of a rock. Textural features are probably the most important aspect of an igneous rock because they are a necessary aid in understanding the conditions under which igneous rocks crystallized (e.g., cooling and nucleation rates and order of crystallization) that in turn depend on initial composition, temperature, pressure, gaseous contents and the viscosity of the magma.

Degree of crystallinity - Rocks composed entirely of crystals are called holocrystalline; those composed entirely of glass are holohyaline; rocks that contain both crystals and glass are hypocrystalline.

Grain size - Overall, there is a distinction between the grain size of rocks that have crystallized at depth and are medium to coarse grained (e. g., gabbros) and those that crystallized at shallow depth as dikes or were effusive are finer grained (e. g., basalts). Grain sizes of terrestrial and meteorite igneous rocks are defined as follows:

TermTerrestrial RocksMeteorites
Fine-grained < 1 mm< 0.5 mm
Medium-grained1 - 5 mm0.5 - 2 mm
Coarse-grained5 mm - 3 cm2 - 5 mm
Very coarse-grained> 3 cm> 5 mm

The cut-off grain size between basalt and gabbro for both sets of rocks is 2 mm.

Rock fabric - Fabric is the shape and mutual relationships among rock constituents:

  1. Euhedral, idiomorphic, or automorphic refer to grains that are bounded by crystal faces
  2. Subhedral or hypidiomorphic are grains that are bounded by some crystal faces
  3. Anhedral, allotrimorphic, or xenomorphic grains are devoid of crystal faces

Terms

Essential textural terms (those that may be encounter in reading meteorite classifications and research papers):