Determination of Extinction Angle of Plagioclase:

 The extinction angle of plagioclase is the angle between the cleavage plane of the mineral and the direction of the light passing through it. It is used as a diagnostic feature to identify the type of plagioclase feldspar.

                                                                Fig: Plagioclase grain(a)

The following steps are for the determination of extinction angle of Plagioclase:

1. Find a plagioclase grain and orient it to any homogenous position(No lamellae).
2. Noted the reading on the stage.
3. Now rotate the stage clockwise till one of the lamellae set becomes either bright or dark,noted the reading.
4. Being the stage to the original position(i.e;homogenous position as you took initially).
5. Again rotate the stage anticlockwise till the lamellae set show either become dark or bright.
6. Record the stage reading.
7. Find the difference between i.e;clockwise and anticlockwise.

• If the difference is within the range of [0,4] then the grain is suitable for measuring extinction.
• If the difference is more than 4 ,then choose another homogeneous position.

      8.If the difference is well within 4 ,then take the average of both stage(i.e; clockwise and      

          anticlockwise) which will be the extinction angle for that plagioclase.

Example:

If initial position (homogeneous)is =328
Clockwise position=337
Anticlockwise position=313
Now,
For clockwise 337 - 328=9
For anticlockwise 328-313=15
Again,
For selection of suitable grain 15-9=4 (So,it is suitable)
Then,
15+9=24/2=12
So it is the mineral of Albite

Determination of Plagioclase composition by using Michel-Levy Technique:
The Michel-Levy method for determinatining plagioclase composition from the extinction angle of
twins.

The extinction angle varies depending on the composition of the plagioclase,which ranges from pure sodium feldspar (albite) to pure calcium feldspar (anorthite). In general, the extinction angle increases as the calcium content of the plagioclase increases.

Fig:Graph used with Michel-Levy measurements to determine the abundance of Na and Ca.

Percentage:

Albite=0-10%

Oligoclase=10-30%
Andesine=30-50%
Labrodorite=50-70%
Bytonite=70-90%
Anorthite= 90-100%
For Plagioclase Curve;
Upper line for Volcanic rocks
Lower line for Plutonic rocks

Extinction angle for plagioclase composition:
Albite=(-20 to -12)°
Oligoclase=(-12 to 12)°
Andesine=(12 to 28)°
Labrodorite=(28 to 39)°
Bytonite=(40 to 52)°
Anorthite= (< 52)°

By measuring the extinction angle of plagioclase, geologists can identify the composition of the plagioclase in a rock sample. This information is important for understanding the history and formation of the rock, as well as for determining its potential uses in construction, industrial applications, or other
fields.