Mineralogy PPT
Mineralogy PPT
Mineralogy PPT
Each mineral has a crystallised structure and almost a definite chemical composition.
The properties which are related directly to the chemical composition of the minerals are:
1. ISOMORPHISM
2. POLYMORPHISM
3. PSEUDOMORPHISM
CHEMICAL PROPERTIES OF MINERALS
1. ISOMORPHISM:
When the minerals show continuous variation in their chemical composition, but their crystal structure
2. POLYMORPHISM:
When minerals have identical chemical composition but different atomic structure called polymorphs
Mineral pseudomorphs are formed when a mineral is replaced by another without any change in the
e.g: pieces of fossil wood where the wood fibres have been replaced by silica.
MINERAL IDENTIFICATION USING THE
PETROGRAPHIC MICROSCOPE
Optical Microscopy
• Study of how light passes through thin sections – rock cut and
polished to about 0.3 mm thickness
• Use properties of light absorption and propogation through a mineral
→ affected by atomic arrangement and composition
• Learn the properties of light associated with techniques governing the
use of a petrographic microscope
OPTICAL PROPERTIES OF MINERALS
composition
SOME TERMINOLOGIES:
• Ordinary light
it travels in straight lines with a transverse motion i.e. it vibrates in all directions at right angles to
the direction of propagation
• Polarised light
when the vibration of wave motion is confined to a single plane only the light is called polarized
light. The plane along which the vibrations take place is known as plane of vibration.
• Refractive index
Refractive Index of a mineral can be expressed as the ration of velocity of light in air to the
velocity of light in a mineral.
• Isotropic mineral
those minerals in which light travels in with same velocity in all directions.
• Anisotropic mineral
• when a ray of unpolarized light travels through anisotropic crystal it breaks into polarized rays by breaking
• one which obeys the laws of refraction is known as ordinary ray or the O-ray and the second ray which
does not obey the these laws is known as extraordinary ray or E-ray
• O-Ray travels with the same velocity or direction through the crystal. It has a constant refractive index
• in case of E-ray the velocity changes with the variation of its path to the crystal and therefore it
refractive index also changes.
OPTICAL PROPERTIES OF MINERALS
1. COLOUR
2. PLEOCHROISM
3. RELIEF
4. REFRACTIVE INDEX
5. FORM
6. CLEAVAGE
7. FRACTURE
8. BIREFRINGENCE
9. EXTINCTION
10. TWINNING
OPTICAL PROPERTIES OF MINERALS
1. COLOUR
• All anisotropic coloured minerals have Pleochroism in Biotite when the stage is rotated 90°
pleochroism.
4. FORM
If the grain shows irregular boundaries only, the grain is anhedral (xenomorphic).
If the grain has both regular and irregular boundaries, it is subhedral (hypidiomorphic).
5. CLEAVAGE
7. EXTINCTION:
• When the rotational direction of an anisotropic mineral coincides with those of the polarizer and analyser, the
mineral appears dark.
• Parallel Extinction: The mineral grain is extinct when the cleavage or length is aligned with one of the
crosshairs.
Example Orthopyroxene, Biotite
• Inclined Extinction: The mineral is extinct when the cleavage is at an angle to the crosshairs.
Example: Clinopyroxene, Hornblende
Wavy Extinction:
Example: quartz
Quartz
Clinopyroxene
8. TWINNING
If more than two crystals are twinned, having parallel twinning planes, the twinning is called
polysynthetic (sometimes also called lamellar twinning).
Plagioclase commonly shows lamellar twinning. Such twinning is one of the most diagnostic features
of plagioclase.
1. Solidification of Magma
3. Metamorphism