Biaxial Minerals

Biaxial Minerals - Biaxial Minerals Definition: Are...

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Biaxial Minerals Definition : Are minerals with 2 optic axes; i.e. 2 directions along which the light shows no birefringence and vibrates in a circular section with a unique constant refractive index. (known as β ). Biaxial minerals are ones that crystallize in the orthorhombic, monoclinic and triclinic systems. Biaxial minerals have 3 indeces of refraction: α , β , and γ , listed in order of increasing values (i.e. γ is always > β > α ). The maximum birefringence of a biaxial mineral will be: γ - α . Light incident along one of the two optic axes will vibrate in one direction only with a refractive index value given by the radius of the circular section ( β ) to which it is perpendicular. If β has a value closer to α than to γ , the mineral is biaxial positive, and vice versa. A light ray incident at any angle to the optic axes will still split into 2 rays. However, unlike in the case of uniaxial minerals, both rays are extraordinary . One of these extraordinary rays will vibrate with a refractive index of a value between α and β (called α ’), the other between β and γ (called γ ’). The Biaxial indicatrix A triaxial ellipsoid, with axes X, Y and Z coinciding with, and proportional to α , β , and γ , respectively (Fig. 1). The XZ plane = optic plane , Y is known as the optic norm . All sections through this ellipsoid are ellipses, except 2 circular sections, each of which is perpendicular to one of the optic axes. The angle between the two optic axes is known as the optic angle “2V” (Fig. 1). The axis of the indicatrix which bisects the acute angle 2V is known as the acute

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This note was uploaded on 02/27/2012 for the course GLY 314 taught by Professor Staff during the Spring '09 term at Marshall.

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Biaxial Minerals - Biaxial Minerals Definition: Are...

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