What is Hornblende Group?
Hornblende is a field and collector name for a complex group of calcic amphiboles, typically dark green to black and common in igneous and metamorphic rocks.
Mineralogy and structure
Reference formula: A0-1B2C5T8O22W2. Hornblende is a group or root-name concept rather than one fixed composition. Amphiboles have double-chain silicate structures and prominent cleavage at about 60° and 120°; the model shown is representative. Hornblende Group belongs to the silicates and has a monoclinic crystal system. Typical hardness is 5–6 on the Mohs scale and specific gravity is approximately 3.0–3.5.
Colour and appearance
Typical colours include black, green-black, brown, dark green. The usual prismatic, fibrous, granular habit and vitreous, pearly lustre are useful descriptive features, but colour alone is not diagnostic.
Formation and localities
Hornblende Group forms in or is associated with intermediate to felsic igneous rocks, amphibolites, gneisses, skarns and other metamorphic assemblages. Important documented localities include Norway, Austria, Italy, Canada, the United States, Russia and Brazil.
Collecting and care
Preserve the host rock and locality; avoid disturbing fibrous aggregates and do not assign a precise amphibole species from colour alone.
