What is biaxial interference figure?

What is biaxial interference figure?

Biaxial interference figures are similar to uniaxial figures but show more variety in the shape and movement of isogyres. Biaxial figures often contain only a single isogyre, which is not always parallel to a crosshair. One new term must be introduced: a melatope is the point of emergence of an optic axis.

What is an interference figure?

Definition of interference figure : a figure observed with a conoscope when a section of a doubly refracting crystal is in the path traversed by convergent plane-polarized light (as when a centered black cross is superimposed over a black spot at the center of a series of concentric colored rings)

What is acute Bisectrix figure?

Acute Bisectrix Figure. The Acute Bisectrix (Bxa) figure results when the Bxa is perpendicular to the microscope stage. If the 2V angle is < 60°, both Melatopes (M), marking the point of emergence of the Optic Axes, lie within the FOV.

What is Bxa and BxO?

Biaxial Negative. Acute Bisectrix (Bxa) = X. Obtuse Bisectrix (Bxo) = Z. 2VX = acute angle between OAs.

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.

How do you calculate 2V?

Precise determination of 2V can only be made by determining the 3 principal refractive indices of the mineral. But, 2V can be estimated from Acute Bisectrix figures and Optic Axis figures using the diagrams shown here. Recall that for a BXA figure the distance between the melatopes is proportional to the 2V angle.

How do you find interference figures?

To view the figure, the light rays leaving the microscope must emerge more or less in parallel. This is typically achieved either by pulling out the eyepiece altogether (if possible), or by placing a Bertrand lens (Emile Bertrand, 1878) between the objective lens and the eyepiece.

What is the difference between Conoscopic and orthoscopic observations?

The orthoscopic image is one where the mineral is viewed perpendicular to the path of light, whereas under conoscopic light the mineral is viewed simultaneously from different angles in a cone of light using the maximum aperture.

What is 2V angle?

The acute angle between the optic axes is called the 2V angle. Just like in uniaxial minerals, if one is looking down one of the optic axes, light traveling along the optic axis will be vibrating in the β direction, and thus the mineral would be extinct for all rotation positions.

What causes interference colors?

Interference colors and birefringence: Anisotropic minerals, unless viewed down an optic axis, cause polarized light to be split into two rays as it travels through a grain. When the rays emerge from the grain, they combine to produce interference colors. Interference colors are only seen in XP light!

What is optic axis figure?

Illuminator. Uniaxial Interference Figures. There are two types of uniaxial interference figures: optic-axis figures and off-center optic-axis figures. Optic-axis figures occur when the c-axis is perpendicular to the stage. Isochromes.

How do you determine if a mineral is uniaxial or biaxial?

If there is any curvature to the isogyre, the mineral is biaxial. If the isogyre is straight, then the mineral is either biaxial with a low 2V or is uniaxial.

What is a biaxial interference figure?

Biaxial interference figures are most useful for the determination of optic sign and estimation of the 2V angle, both of which are useful diagnostic properties of biaxial minerals. The two most useful are the BXAfigure and OA figure.

What is the meaning of BXA in chemistry?

M= melatope; Bxa = acute bisectrix; Bxo = obtuse bisectrix. Figure 2.8.12.B. Different types of interference figures produced by biaxial minerals. For simplicity, isochromes are not shown. The lower part of each image shows the orientation of the indicatrix for the mineral.

What does the middle part of the interference figure mean?

The middle part of the diagram shows the interference figure relative to the thin section and the upper image shows the view through the microscope ocular. M= melatope; Bxa = acute bisectrix; Bxo = obtuse bisectrix. Figure 2.8.12.B. Different types of interference figures produced by biaxial minerals.

What is the difference between a BXA and optic normal figure?

A BXOfigure will be similar to the BXAfigure, except that the melatopes will be outside of the field of view most of the time during a 360orotation. Still, every 90othe broad cross will form as the OAP becomes parallel to one of the cross hairs. Optic Normal Figure (O.N.)