# How do you determine the optic sign of a uniaxial mineral?

## How do you determine the optic sign of a uniaxial mineral?

Recall that uniaxial minerals can be divided into 2 classes based on the optic sign of the mineral.

1. If ω > ε, the optic sign is negative and the uniaxial indicatrix would take the form of an oblate spheroid.
2. If ε > ω, the optic sign is positive and the uniaxial indicatrix would take the form of a prolate spheroid.

What is optical indicatrix in geology?

The optical indicatrix is a geometrical solid relating the refractive index of a mineral to the mineral’s structure. The surface of the indicatrix represents the different refractive indices in the crystal.

### How many optic axes does a uniaxial mineral have?

2 optic axes
All minerals that crystallize in the orthorhombic, monoclinic, or triclinic crystal systems are biaxial. Biaxial crystals have 2 optic axes, and this distinguishes biaxial crystals from uniaxial crystals….

EENS 2110 Mineralogy
Tulane University Prof. Stephen A. Nelson
Biaxial Minerals

What are the optical properties of a material?

Optical property of a material is defined as its interaction with electro-magnetic radiation in the visible. Electromagnetic spectrum of radiation spans the wide range from γ-rays with wavelength as 10-12 m, through x-rays, ultraviolet, visible, infrared, and finally radio waves with wavelengths as along as 105 m.

## How do you identify the optical sign of a mineral?

As can be seen, the optic sign can be determined in the position where the isogyres form a cross, or at 45º, when the cross breaks into two hyperbolic bands. When the cross is formed , the colouring of the various quadrants is the same as for uniaxial minerals.

What is accessory plate used for?

accessory plate (sensitive tint) In optical microscopy, a plate used to determine the optical properties of minerals. Quartz, mica, and gypsum are the common minerals used to determine the slow and fast vibration directions that relate to the two refractive indices of an anisotropic mineral.

### What is the difference between uniaxial and biaxial minerals?

The key difference between uniaxial and biaxial crystals is that uniaxial crystals have a single optic axis whereas biaxial crystals have two optic axes. The optic axis of a crystal is the direction that the light propagates through the crystal without facing double refraction.

What are the four optical properties?

Among the optical properties, refraction, absorption, reflection, and scattering of light are the most important.

## How is the uniaxial indicatrix constructed?

The uniaxial indicatrix is constructed by first orienting a crystal with its c-axis vertical. Since the c-axis is also the optic axis in uniaxial crystals, light traveling along the c-axis will vibrate perpendicular to the c-axis and parallel to the ωrefractive index direction.

Where is the uniaxial indicatrix of a mineral?

In this case the optic sign of the mineral is positive, and the uniaxial indicatrix is shown at the center of the crystal.

### What is the principal section of the uniaxial indicator?

Such an ellipse with the ε direction as one of its axes and the ωdirection as its other axis is called the Principal Sectionof the uniaxial indicatrix.

What is the refractive index of uniaxial crystals?

Like all anisotropic substances, the refractive indices of uniaxial crystals varies between two extreme values. For uniaxial minerals these two extreme values of refractive index are defined as ω(or No) and ε(or Ne). Values between ωand eare referred to as ε’.