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The fibrillar growth habit and 100} twinning may produce anomalous optical properties in fibrous amphiboles. The optical properties of 16 fibrous tremolite, actinolite, and ferro-actinolite samples from Verkouteren and Wylie (2000) are described. One sample displays uniaxial-like properties, 9 display partial development of uniaxial-like properties, two contain fibers with both orthorhombic and monoclinic optical properties, and 4 contain fibers that do not go to extinction on (010) under crossed polars. A range in chemical composition may also contribute to variable optical properties. Anomalous optical properties may confound the identification of fibrous amphiboles.