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ACTINOLITE and HORNBLENDE Amphibole: A family of silicate minerals forming prism or needlelike crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Amphibolite: A rock made up mostly amphibole and plagioclase feldspar. Although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too. Actinolite: Actinolite has no aluminum; it and is needle-shaped and light green. Blue Amphibole: Blue amphibole contains sodium and, of course, is bluish in color. Feldspar: Family of silicate minerals containing varying amounts of potassium, sodium and calcium along with aluminum, silicon and oxygen. Potassium feldspars contain considerable potassium. Plagioclase feldspars contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white, gray, or pink. Hornblende: Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. -- Excerpts from: USGS/NPS Geology in the Parks Website, 2002 |