CLASSIFICATION OF IGNEOUS ROCKS
Igneous rocks classified on the basis of CRYSTAL SIZE and SILICA
CONTENT
CRYSTAL SIZE
Glassy
Fine
No xls
under 1mm
microscope
Obsidian
Rhyolite
Andesite
Basalt
Medium
1-2mm
handlens
Microgranite
Microdiorite
Dolerite
Coarse
over 2mm
naked eye
Granite
Diorite
Gabbro
SILICA CONTENT / MINERALOGY (and colour)
'ACID' - rich in silica 60-70%
Light coloured = LEUCOCRATIC
Low density
Rich in FELSIC/SILICIC minerals
30% QUARTZ, 45% ORTHOCLASE FELDSPAR,
15% PLAGIOCLASE FELDSPAR (ALBITE; Na rich)
10% MICAS + AMPHIBOLES (eg Hornblende)
eg Granite, Microgranite, Rhyolite
'INTERMEDIATE' - 56-59% silica
Medium coloured MESOCRATIC
70% PLAGIOCLASE FELDSPAR (ANDESINE roughly equal Na/Ca)
30% AMPHIBOLE + PYROXENE + BIOTITE MICA
eg Andesite, Microdiorite, Diorite
'BASIC’ - low in silica 45-55%
Dark coloured MELANOCRATIC
High density
Rich in MAFIC/FERROMAGNESIAN minerals
40% PYROXENES (eg Augite)
60%PLAGIOCLASE FELDSPAR (LABRADORITE Ca rich)
eg Basalt, Dolerite, Gabbro
'ULTRABASIC' - under 45% silica
50% PYROXENE, 50%OLIVINE
eg Peridotite
SEE IGNEOUS ROCK CLASSIFICATION SHEET
IGNEOUS ROCK TEXTURES
V fast cooling of acidic magma: no crystals form: GLASSY texture eg
OBSIDIAN
Glass altered by DEVITRIFICATION has a SPHERULITIC texture eg
'SNOWFLAKE OBSIDIAN', often with curving cracks or PERLITIC TEXTURE
Fast cooling eg lava at surface, small/shallow intrusions
- finely crystalline, poorly formed = ANHEDRAL
Slow cooling eg large pluton/batholith intrusions
- coarsely crystalline, well formed = EUHEDRAL
Uninterrupted, steady cooling
- EQUIGRANULAR texture
Interrupted ie 2 stage cooling eg a) migration of magma from pluton to
dyke/sill
b) eruption from magma chamber to surface
PORPHYRITIC texture
- large, usually euhedral PHENOCRYSTS: slow cooling of large magma body
at depth
- fine GROUNDMASS: faster cooling in smaller/shallower magma body or at
surface
INTERGROWTH TEXTURES
GRAPHIC - intergrowth of quartz and orthoclase feldspar, probably the result
of crystallisation of acidic magma at or near to the eutectic point (looks like
angular writing)
GRANOPHYRIC - small scale graphic texture
POIKILITIC- small crystals enclosed in larger ones eg small crystals of olivine
enclosed in larger amphiboles
OPHITIC - small euhedral plagioclase feldspar crystals enclosed by large
pyroxenes; common in dolerites
REACTION TEXTURES
ZONED CRYSTALS - Common in plagioclase feldspars; Ca rich core, Na rich
rim due to d cooling preventing completion of reaction of early formed crystal
with melt
CORONA STRUCTURE - Olivine core surrounded by Pyroxene, surrounded
by Amphibole; formed by reaction of early formed crystals with melt
LAVA TEXTURES
VESICULAR - bubbles caused by gas coming out of solution due to release of
pressure when lava is erupted; may be elongated in direction of flow
AMYGDALOIDAL - vesicles infilled with minerals eg calcite, quartz, agate
DESCRIBING IGNEOUS ROCKS
THE OUTCROP
1
Location and general form of outcrop; sketch if appropriate
2
Orientation and spacing of joints
3
Relationship with veins, dykes, sills; apply law of cross cutting
relationships
4
Nature of contact with surrounding rocks
THE HAND SPECIMEN
1
Weathered surface; texture, colour eg rusty indicates oxidation of iron
rich pyroxenes and olivines in basic rocks
2
Collect representative samples ie average and variations
3
Colour of fresh surface; Leucocratic/ Mesocratic/ Melanocratic
4
Relative density
5
Use HANDLENS + GRAINSIZE CARD (or ruler!) to observe and record
TEXTURE (SIZE/ SHAPE/ARRANGEMENT of crystals)
a)
crystal size (in mm as well as glassy/ fine/ medium/
coarse)
b)
crystal shape (euhedral / anhedral)
c)
Crystal arrangement (equigranular/ porphyritic/ flow
banded)
d)
Presence of vesicles/ amygadales
6
Estimate number and proportions of different minerals present
Record mineral colour, cleavage, lustre, crystal shape,hardness where
possible
Suggest identification of minerals based on observations/tests
FELSIC MINERALS
QUARTZ; colourless/grey, no cleavage, vitreous lustre, usually anhedral,
harder than 5.5
ORTHOCLASE FELDSPAR; orange/pink, 2 poor cleavages at 90',
dull/vitreous lustre, rectangular 'lath' shaped crystals (may show simple
twinning) harder than 5.5
PLAGIOCLASE FELDSPAR; white/grey, 2 poor cleavages at 90', dull/vitreous
lustre, rectangular 'lath' shaped crystals (May show multiple twinning), harder
than 5.5
MUSCOVITE MICA; white/silver, 1 perfect cleavage (flakes), vitreous lustre,
tabular crystals, hardness >2.5 <3.5
MAFIC MINERALS
Very difficult to identify with certainty!
PYROXENE; black anhedral crystals
AMPHIBOLE; greenish black (not as dark as pyroxene) anhedral crystals
OLIVINE; olive green, poor cleavage, vitreous lustre, anhedral crystals, harder
than 5.5
BIOTITE MICA; black/brown, 1 perfect cleavage (flakes), vitreous lustre,
tabular crystals, hardness >2.5 <3.5
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Ben Church
Establishment: Monmouth Comprehensive School Date:24:05:05