Optical Properties of Quartz

Quartz is a uniaxial hexagonal-rhombohedral mineral with extremely low birefringence. It is easy to determine on account of its lack of alteration, absence of cleavage or twinning. It is also one of the most common rock-forming minerals.

Refractive Index
The Refractive Index of quartz is 1.552, so in order to observe the birefringence of the mineral, we made a slide of the mineral in a 1.538 oil. Since this is below that of the the mineral the Becke lines should be inside of the mineral.



We then picked an oil with a higher refractive index than quartz, and the Becke lines are outside the mineral:

Bringing the Becke lines into focus, the white halo goes along the mineral boundary:

In order to match the epsilon ray of the quartz, we then made a new slide with 1.552 oil, which should match the refractive index:



Optic Sign
The optic sign is determined by the orientation of the colors which result when a quartz plate is slid into the microscope. Before the quarz plate is inserted, the quartz has a cross without color and no shading:

When the quartz plate is inserted, yellow appears in the upper left, and lower right corners, and blue appears in the upper right and lower left,

This signifies that quartz is positive.



Sign of Elongation
In order to find the Sign of Elongation of Quartz, we used a thin section and the quartz plate to determine whether quartz is a length-slow or -fast mineral. Without the quartz plate, the quartz appears as such:

when it is the stage is rotated 45 degrees to the right, the color of the quartz will change to a second order blue. This indicates that the slide is in an additive position. If the quartz plate were added without rotating the stage at all, the color of the quartz slide would become a first-order red. Although the color in this picture of the slide in the additive position is not true, the slide should be the bright blue.

When the stage is rotated 45 degrees to the left from it's original position, the color of the quartz plate changes to a straw yellow. This indicates that this is a subtractive position for the quartz slide.

When the quartz plate is pushed in, the slow axis is perpendicular to the direction it comes in at. Therefore, in order for the position to be additive, the slow direction of the slide must line up with that of the quartz plate. Given this, Quartz must be length-slow since the slow-direction is parallel to the long axis of the grain.

Extinction Angle
The Extinction Angle of Quartz is zero, which can be seen in the following graphics. In the first, quartz is fully visible, and the second has it completely extinct. these both happen at exactly ninety degrees from eachother, and along the axis.



Home