Occurrence and Compositon

Hornblende and actinolite are both amphiboles. Hornblende is a widespread and common mineral in many types of igneous and metamorphic rocks. Actinolite is found in metamorphosed mafic to intermediate rocks.

Actinolite's composition is Ca2(Mg,Fe)5Si8O22(OH)2. Hornblende has a more complicated and more variable composition, approximated by (K,Na)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2.

Strong brown or green coloration and pleochroism, and 60o-120o cleavage angles identify hornblende. Actinolite is similar, but its color ranges from light green (Fe-poor varieties) to dark green (Fe-rich varieties).

Important properties
 ·Color - Hornblende is generally pleochroic in shades of brown and green, less commonly yellow or red-brown. Concentric or patchy color zonation may be present. Actinolite is similar but colors are restricted to greens.
 ·Cleavage - Amphiboles have diagnostic 60o-120o cleavage angles when viewed in end section. Other views may show only one cleavage.
 ·Interference colors - Maximum interference colors are middle second order but they may be masked by the color of the mineral.
 ·Form - Often sub- to anhedral but may be prismatic crystals forming lathes. End views may show diamond shaped cross-sections.
 ·Interference figure - Hornblende and actinolite are biaxial (-) with a large 2V.

Similar minerals
 ·Diagnostic 60o-120o amphibole cleavage distinguishes hornblende and actinolite from other sometimes dark colored minerals such as tourmaline, pyroxenes and micas.

Hornblende in Syenite from near Cuttingsville, Vermont

The photos above show a large grain of hornblende with several other hornblende grains around it. Calcite is present down and to the left of the largest hornblende grain. Plagioclase fills the space between grains. Typical amphibole cleavage can be seen.

The field of view is about 2.5 mm across.

Actinolite Schist from Near Chester, Vermont

The photos above show mostly the pleochroic green amphibole actinolite and clear quartz (PP). Also present is black opaque magnetite. In the XP view, the deep color of the actinolite partially masks the interference colors, making them somewhat anomalous. A few of the amphibole grains show the classic 60o-120o cleavage angles.

The field of view is about 2.5 mm across.


Epidote and Hornblende in a Mafic Schist from near Panoche Pass, California

The hornblende in this sample is pleochroic in various shades of green (PP). Its interference colors range up to high second order but are somewhat masked by the color of the mineral. Some of the hornblende shows a hint of a diamond shape and of amphibole's characteristic 60o-120o cleavage. Epidote (clear; PP) is also present as more equant grains. The epidote grains show multiple interference colors (XP), in some cases concentrically color zoned.

The field of view is about 2.5 mm.