What is optical sign in mineralogy ?
Optical sign : definition
The birefringent minerals (uniaxial and biaxial) are divided into two groups : positive and negative, according to the value of their optical sign.
A uniaxial mineral is said to be positive when the fastest ray in a crystal is of ordinary radius, and negative when it is the extraordinary radius. The speed of propagation being inversely proportional to the refractive index, the fastest ray moves in the direction of the lowest index, denoted Np, the slowest ray in the direction of the highest index, denoted Ng. The ellipsoid of the indices is thus a convenient means of visualizing the optical sign : the extraordinary ray (called epsilon) being figure according to the vertical axis of revolution, a flattened ellipsoid of revolution is uniaxial negative. In other words, the optical axis (axis of revolution around the main axis of the ellipsoid) is Ng in the case of positive uniaxis, Np in the case of negative uniaxis.
The case of biaxial minerals is different because they always have two extraordinary refracted rays and two optical axes. We say that a mineral is biaxial positive when the acute bisector of the two optical axes (the 2V angle) is Ng, and that it is biaxial negative when this bisector is Np.
The optical signs are determined in practice under an optical microscope using auxiliary slides : quarter-wave mica, quarter-wave gypsum or quartz sensitive tint for minerals with low birefringence.
A uniaxial mineral is said to be positive when the fastest ray in a crystal is of ordinary radius, and negative when it is the extraordinary radius. The speed of propagation being inversely proportional to the refractive index, the fastest ray moves in the direction of the lowest index, denoted Np, the slowest ray in the direction of the highest index, denoted Ng. The ellipsoid of the indices is thus a convenient means of visualizing the optical sign : the extraordinary ray (called epsilon) being figure according to the vertical axis of revolution, a flattened ellipsoid of revolution is uniaxial negative. In other words, the optical axis (axis of revolution around the main axis of the ellipsoid) is Ng in the case of positive uniaxis, Np in the case of negative uniaxis.
The case of biaxial minerals is different because they always have two extraordinary refracted rays and two optical axes. We say that a mineral is biaxial positive when the acute bisector of the two optical axes (the 2V angle) is Ng, and that it is biaxial negative when this bisector is Np.
The optical signs are determined in practice under an optical microscope using auxiliary slides : quarter-wave mica, quarter-wave gypsum or quartz sensitive tint for minerals with low birefringence.