Plagioclase thin section

Relief: Low-moderate negative/positive (Albite-Anorthite series)
Habit/Form: Plagioclase commonly occurs both as euhedral and anhedral grains. Crystals are usually tabular parallel to (010) and elongate parallel to the c or a axis. Cross sections are more or less rectangular.
Color: Colorless, frequently cloudy due to incipient alteration
Pleochroism: –
Cleavage: Sometimes after 1 or 2 directions; but are difficult to see in thin section due to very low relief; perfect on {001} and good/distinct on {010}, intersecting essentially at right angles.

Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray to white; feldspars are very useful in determining the thickness of any slide in which it occurs.
Extinction angle: The extinction is inclined in almost all orientations.
Twins: Is often characterized by a distinctive polysynthetic twinning (it looks like jail bars |||| 🙂 ), and it’s called ‘Albite law’ or ‘chessboard’ albite; and contains alternating twin lamellae somewhat like the pattern of squares on a chessboard, except the units tend to be elongate and lath shaped rather than square.
Also, Albite-Anorthite series (plagioclase) is characterized by a distinctive zoned structure. This chemical zoning is common, particularly in volcanic and hypabyssal intrusive rocks, and is expressed as a variation in the extinction angle from one zone to another. The zoning is considered normal if it changes smoothly from a more calcic core to a more sodic rim. The core usually has a larger extinction angle. Reverse zoning is just the opposite. Oscillatory zoning shows alternation between calcic and sodic zones, but the overall trend is usually from a more calcic core to a more sodic rim.
The combination of polysynthetic twinning and zoned structure are very common.
Uniaxial/Biaxial: Biaxial (+/-)
Optic axial angle (2V): “Albite 2V measured: 45°, calculated: 76 – 82°
Anorthite 2V measured: 78 – 83°, calculated: 78°”