Does biotite have birefringence?
Optical properties In thin section, biotite exhibits moderate relief and a pale to deep greenish brown or brown color, with moderate to strong pleochroism. Biotite has a high birefringence which can be partially masked by its deep intrinsic color.
What is the interference Colour of biotite?
It is light brown (PP). In XP light it shows a pebbly texture and mostly second order green interference color (XP). Most of the clear mineral matter beneath the biotite is anthophyllite, an amphibole.
How do you know if a biotite is thin?
Under cross-polarized light, the biotite thin section appears darker brown in areas as well as a pink-yellow color in others. Biotite, usually dark green, brown, or black, has a pale brown color under plane-polarized light in this sample.
What is birefringence in thin section?
A value, termed birefringence, describes the difference in velocity of the two rays. When the rays emerge from the grain, they combine to produce interference colors.
Is biotite intrusive or extrusive?
Intrusive and Extrusive Igneous Rocks Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy). Igneous rocks are called intrusive when they cool and solidify beneath the surface.
What causes birefringence?
Modified proteins such as immunoglobulin light chains abnormally accumulate between cells, forming fibrils. Multiple folds of these fibers line up and take on a beta-pleated sheet conformation. Congo red dye intercalates between the folds and, when observed under polarized light, causes birefringence.
Is quartz a birefringence?
Quartz is a slightly birefringent crystal, which make Quartz an important material for making optical waveplate. Large size, high quality optical grade Quartz is synthesized in an autoclave via the hydrothermal process.
How do you identify apatite in the thin section?
Due to the phosphate content, apatite crystals are usually small and can be hard to detect in thin section. The apatite crystal is very small and elongated. Its high relief allows it to stick out from the other colorless minerals. Under cross-polars apatite exhibits a first order gray color.
How is birefringence calculated?
Birefringence can be determined as it is the difference in the optical path (OPD) between the O- and E-rays, also known as the retardation, divided by the thickness t of the polymer part.
What is the birefringence of biotite?
Biotite has moderately high birefringence. In the XP view, the biotite interference colors range up to 2nd order red (with just a hint of 2nd order blue). The quartz is clear in the PP view, and shows maximum interference colors of 1st order gray in the XP view. Note the opaque graphite — it is black in both views.
What are the inclusions of accessory phases of brown biotite?
Comments – alters to green chlorite, often contains inclusions of accessory phases c-axis parallel section (left) and subhedral basal section (right) of brown biotite. Note the presence of the one perfect cleavage on the c-axis parallel section and its absence on the basal section. The colourless inclusions in the basal section are apatite.
How do you find apatite in brown biotite?
c-axis parallel section (left) and subhedral basal section (right) of brown biotite. Note the presence of the one perfect cleavage on the c-axis parallel section and its absence on the basal section. The colourless inclusions in the basal section are apatite. crystals of biotite showing colour variation by pleochroism.
What is the color of biotite?
The garnet is isotropic and so appears black in the XP view. The biotite doesn’t show easily identified interference colors because the brown color of the grain masks them. Still, with a little imagination you can see biotite may have second order colors. Here we see mostly brown biotite flakes.