6. IGNEOUS PETROLOGY
Magma & Lava
LIGHT
IN
COLOR
• Specific gravity-
low
• High proportion
of mineral-
• Quartz & alkali
feldspars
DARK
IN
COLOR
• Specific gravity-
High
• High proportion
of mineral-
Olivin, pyroxene,
hornblende,
biotite
Basic Igneous Rock
10. TEXTURE OF IGNEOUS ROCK
Crystallinity
A) Degree of
Crystallization
• Holocrystalline
• Holohyaline
• Mero-Crystalline
Granularity
B) Size of
Grain
•Phaneric
•Aphanitic
Fabric
C) Shape of crystal
• Euhedral
• SubHedral
• Unhedral
Mutual relations of grains
• Equigranular Textures
• Inequigranular Texture
• Porphyritic Texture
• Poikilitic texture
• Ophitic Texture
• Inter-granular & Intersertal
Texture
11. A) Crystallinity
The main factor that determines the Crystallinity of an igneous rock are-
Rate of cooling of magma
Viscosity of magma
Rapid cooling and high viscosity favor the formation of Glass
Slow cooling and low viscosity promote the formation of crystal
13. B) GRANULARITY GRAIN SIZE
PHANERIC
COARSE
GRAIN
MEDIUM
GRAIN
FINE
GRAIN
APHANITIC
MICRO-
CRYSTALLINE CRYPTO-
CRYSTALLINE
TEXTURE
14. C) Fabric
i) Shape of the grains
Crystals forms are described with references of the development of their faces.
I. Euhedral: Crystals are bounded by faces
III. Subhedral: Crystals are partly bounded by faces
II. Anhedral: Crystals are not bounded by faces
15. b. Mutual Relationship of grains
The fabric of a rock is influenced not only by
the shape of the crystals, but their relative
size and their mutual arrangement.
Depending id classified as
i.Equigranular
ii.Inequigranular
iii.Intergranular
iv.Directive Texture
16. i. Equigranular Texture:
Panidiomorphic: Most of the crystals are euhedral
Hypidiomorphic: Most of the crystals are subhedral
Allodiomorphic: Most of the crystals are anhedral