Transcript Unit 3 - Igneous Lab Self

IGNEOUS ROCKS
SELF-DIRECTED
NAME: _______________________________
(PRINT CLEARLY)
DIRECTIONS
There are several pages to read and some diagrams to study. Once you have done
this, you will be ready to work the problems and identify the different rocks (identified
by their mineral composition). You will be doing similar work in your Plutonic and
Volcanic Rock Labs.
2.
1
EXPLANATION OF THE ROCK NOMENCLATURE TRIANGLES
The included triangles show three end-members that exit at each end of the triangle. On
the Felsic diagram, the maximum amount of quartz is 100% at the top of the triangle. As
you move down either side of the triangle, the amount of quartz decreases. These end
members are generally referred to as Q, A and P. This is quartz or (SiO 2), alkali feldspar
(containing potassium (K) and plagioclase feldspar containing calcium and sodium.
The bottom of the triangle represents the alkali feldspar to plagioclase feldspar
percentages. At the far left is 100% alkali feldspar, to the far right is 100% plagioclase
feldspar. The rocks that are defined by this triangle has X% of quartz and Y% of feldspar
which equals 100% of the entire rock. The amount of feldspar, Y%, can be broken down
into A% for alkali feldspars and B% for plagioclase feldspars. This means that the entire
rock is considered to be X% quartz and Y% feldspar (A% + B%). The combined X% and
Y% = 100%.
If we have a rock with 45% quartz, then the remaining amount is 65% feldspar (ignoring
trace amounts of other minerals). If there is 92% alkali feldspar and 8% plagioclase
feldspar, then the amounts are 45, 92, and 8. Plotting this on the triangle places this rock
into area 2 on the Felsic triangle. Therefore the rock is either plutonic, and is an alkali
feldspar granite, or is volcanic and is an alkali feldspar rhyolite.
These categories for Felsic rocks look at the amounts of feldspar and silica (therefore the
name). But other constituents may exist in a rock, you may have muscovite in the sample.
If there are appreciable amounts of other minerals that are not part of the end-members, not
quartz or feldspar, this mineral can become part of the name.
2
Normalization
If we have a rock with 6% biotite, 44% quartz and 50% feldspar (25% alkali feldspar
and 25% plagioclase feldspar), where would the rock fall on the Felsic triangle?
We ignore the biotite and normalize for the end-members. That is 100% - 6% gives
94% of the constituents that are the end-members of the triangle. Then divide the 44%
quartz by 94% and the 25% alkali by 94% and 25% plagioclase by 94%. This gives
normalized values of 46.8% quartz, 26.5% alkali and 26.5% plagioclase. Which is 47
(rounded up) plus 53 that equals 100% of the normalized amount.
This would fall in area 3 on the triangle. The rock would be a granite if it is plutonic and
rhyolite if it is volcanic. Based on the nature of the crystals and whether or not there were
vesicles (holes) in the rock, the we would determine if the rock is plutonic or volcanic. If
the biotite is visible with other mineral crystals visible, the rock would be called a biotite
granite.
As the rock sample’s constituents move further and further away from the quartz end,
the rocks typically become darker in color, less milky or glassy quartz. The alkali-rich
rocks usually become red to pink in color. The plagioclase rocks tend to be white in color.
Overall the rocks on this triangle are light in color. They vary from milky or glassy white to
red to pink, pink and white, and white.
We look at color to determine if it is Felsic. If it is dark in color it falls on the Mafic
triangle. Then we look to see what amounts are alkali or plagioclase. If it is red, it is very
alkali. If it is very white, it is plagioclase rich.
Texture is when the rock is smooth or coarse. Very coarse crystals show there was
slow cooling of an intrusive magma. If the texture is very smooth the rock was extrusive
or volcanic. We also look for vesicles (holes) to indicate that the rock may have been
volcanic. Often volcanic rocks have a smooth texture with occasional large crystals or
phenocrysts (crystals visible with the naked eye).
The Mafic triangle shows that alkali feldspars and plagioclase end-members are
present. In lieu of quartz, we see feldspathoids as the third end-member. All of the mafic
rocks are poor in quartz (little to none).
3
The following table is an example of a simplified method of igneous rock identification. In
lieu of considering the mineral content of quartz, feldspars or feldspathoids (on the second
triangle included), we will look at color and texture to determine a general composition,
make an educated guess as to the mineral composition, and ascertain if they were formed
from a magma or a lava.
Felsic and Mafic Rock Identification Table
FELSIC OR
MAFIC
FELSIC
MORE WHITE
THAN DARK
MINERALS
FELSIC
MORE WHTE
THAN DARK
MINERALS
MAFIC
MORE DARK
THAN WHITE
MINERALS
MAFIC
MORE DARK
THAN WHITE
MINERALS
TEXTURES
COLORS
MINERALS
COARSE WITH
CRYSTALS
VISIBLE
IGNEOUS
WHITE AND/OR
PINK WITH
LITTLE BLACK
QUARTZ
WHITE FELDSPAR
OR PINK
FELDSPAR
WHITE AND/OR
PINK WITH
LITTLE BLACK
A FEW FLEDSPAR
CYRSTALS BUT
MOST MINERALS
CANNOT BE
DETERMINED BY
HAND SAMPLE
GLASSY OR
FEW VISIBLE
CRYSTALS
OCCASIONAL
LARGE
CRYSTALS
SOME HOLES
VOLCANIC
COARSE WITH
CRYSTALS
VISIBLE
IGNEOUS
GLASSY OR
FEW VISIBLE
CRYSTALS
OCCASIONAL
LARGE
CRYSTALS
SOME HOLES
VOLCANIC
DARK GRAY TO
BLACK COLOR
DARK GREEN
TO BLACK
COLOR
SOME CRYSTALS
SHOWING 120
DEGREE ANGLES
AND STUBBY
ALMOST SQUARE
CRYSTALS WITH
STRIATIONS
(AMPHIBOLE AND
PYROXENE)
INTRUSIVE
OR
EXTRUSIVE
INTRUSIVE
EXTRUSIVE
INTRUSIVE
UNDETERMINABLE
DARK GRAY TO
BLACK COLOR
DARK GREEN
TO BLACK
COLOR
A FEW CRYSTALS
GREEN TO
BLACK IN COLOR
ARE VISIBLE
EXTRUSIVE
4
Using the Felsic and Mafic Rock Identification Table on the
previous page, and these descriptive narratives, identify
the following rocks. Circle the appropriate terms for each
sample.
1.
2.
3.
The sample is light pink with a fairly smooth texture. Some small crystals
smaller than one millimeter in length are visible with the unaided eye.
felsic
or
mafic
coarse texture
or
fine texture
intrusive
or
extrusive
light color
or
dark color
This is a solid rock with a blackish to dark red color. There are no visible
crystals. It is filled with vesicles (holes) that range in diameter from 1 to 3
millimeters.
felsic
or
mafic
coarse texture
or
fine texture
intrusive
or
extrusive
light color
or
dark color
Black, massive, heavy and with a fine texture. There are no vesicles.
There are no crystals visible with the unaided eye.
felsic
or
mafic
coarse texture
or
fine texture
intrusive
or
extrusive
light color
or
dark color
5
MODAL CLASSIFICATION AND NOIMENCLATURE OF THE PLUTONIC
IGNEOUS ROCKS AND VOLCANIC ROCKS THAT ARE FELSIC
Quartz
1
60
60
2
3
4
5
20
20
6
7
10
8
9
35
65
10
90
Alkali Feldspar
Plagioclase
ORTHOCLASE, MICROCLINE, SANIDINE
ALBITE, OLIGOCLASE, LABRADORITE, ANORTHITE
PLUTONIC FELSIC ROCKS
VOLCANIC FELSIC ROCKS
1
QUARTZOLITE
2
ALKALI FELDSPAR GRANITE
2 ALKALINE FELDSPAR RHYOLITE
3
GRANITE
3 RHYOLITE
4
GRANODIORITE
4 DACITE
5
TONALITE
5 PLAGIODACITE
6
ALKALINE FELDSPAR SYENITE
6 ALKALINE FELDSAPR TRACHYTE
7
SYENITE
7 TRACHYTE
8
MONZONITE
8 LATITE
9
MONZODIORITE,
9 LATITE-ANDESITE, MUGEARITE
MONZOGABBRO
10
DIORITE, GABBRO
& ANORTHOSITE
10
ANDESITE, BASALT
6
MODAL CLASSIFICATION AND NOIMENCLATURE OF THE PLUTONIC
IGNEOUS ROCKS AND VOLCANIC ROCKS THAT ARE MAFIC
ALKALI FELDSPAR
PLAGIOCLASE
ORHTOCLASE, MICROCLINE,
SANADINE & ALBITE
OLIGOCLASE - ANORTHITE
10
35
6
65
7
90
8
10
9
10
10
50
11
12
13
14
For rocks falling in
zones 6 through 10
please refer to the
previous triangle.
60
60
16
15
ULTRAMAFICS
> 90% MAFIC – DARK
COLORED MINERALS
FLEDSPATHOID
NEPHELINE, LEUCITE, ANALCITE, SODALITE AND MORE
PLUTONIC MAFIC ROCKS
VOLCANIC MAFIC ROCKS
11
FELSDPATHOID SYENITE
11
PHONOLITE
12
FELDSPATHIOD MONZONITE
12 TEPHERITIC PHONOLITE
13
ESSEXITE
13
PHONOLITIC TEHPRITE
14
THERALITE
14
TEPHERITE, BASANITE
15
FOIDITE
15
FOIDITE, NEPHELINITE, LEUCITITE
16
ULTRAMAFIC ROCKS WITH OVER 90 PERCENT COLORED MINERALS
(SUCH ROCKS ARE CALLED: HORNBLENDITE, PYROXENITE, DUNITE, CARBONATITE,
LHERZOLITE, PERIDOTITE, GARNET PERIDOTITE, KIMBERLITE, GRIQUAITE, MELILITITE
AND OTHERS)
7
USING THE FELSIC PLUTONIC AND VOLCANIC ROCK CHART
IDENTIFY THE FOLLOWING
Obtain the names from the chart and place it beside the number. First give the
Plutonic (intrusive) name, then give the volcanic (extrusive) name.
1.
________________
________________
12% QUARTZ [30% PLAGIOCLASE, 70% ALKALI] = 88% FELDSPAR
2.
________________
________________
10% QUARTZ [05% PLAGIOCLASE, 95% ALKALI] = 90% FELDSPAR
3.
________________
________________
67% QUARTZ [65% PLAGIOCLASE, 35% ALKALI] = 33% FELDSPAR
4.
________________
________________
30% QUARTZ [70% PLAGIOCLASE, 30% ALKALI] = 70% FELDSPAR
5.
________________
________________
17% QUARTZ [96% PLAGIOCLASE, 04% ALKALI] = 70% FELDSPAR
6.
________________
________________
25% QUARTZ [70% PLAGIOCLASE, 30% ALKALI] = 70% FELDSPAR
7.
________________
________________
10% QUARTZ [50% PLAGIOCLASE, 50% ALKALI] = 90% FELDSPAR
8.
________________
________________
35% QUARTZ [40% PLAGIOCLASE, 60% ALKALI] = 65% FELDSPAR
9.
________________
________________
50% QUARTZ [07% PLAGIOCLASE, 93% ALKALI] = 50% FELDSPAR
10. ________________
________________
97% QUARTZ [01% PLAGIOCLASE, 02% ALKALI] = 03% FELDSPAR
8
USING THE MAFIC PLUTONIC AND VOLCANIC ROCK CHART
IDENTIFY THE FOLLOWING
Obtain the names from the chart and place it beside the number.
First give the plutonic name, intrusive, then give the volcanic name, extrusive.
1.
________________
________________
30% FLEDSPATHOID [95% PLAGIOCLASE, 05% ALKALI] = 70% FELDSPAR
2.
________________
________________
20% FELDSPATHOID [95% PLAGIOCLASE, 05% ALKALI] = 80% FELDSPAR
3.
________________
________________
67% FELDSPATHOID [65% PLAGIOCLASE, 35% ALKALI] = 33% FELDSPAR
4.
________________
________________
30% FELDSPATHOID [70% PLAGIOCLASE, 30% ALKALI] = 70% FELDSPAR
5.
________________
________________
17% FELDSPATHOID [96% PLAGIOCLASE, 04% ALKALI] = 70% FELDSPAR
6.
________________
________________
25% FELDSPATHOID [70% PLAGIOCLASE, 30% ALKALI] = 70% FELDSPAR
7.
________________
________________
25% FELDSPATHOID [40% PLAGIOCLASE, 60% ALKALI] = 60% FELDSPAR
8.
________________
________________
80% FELDSPATHOID [70% PLAGIOCLASE, 30% ALKALI] = 20% FELDSPAR
9.
________________
________________
50% FELDSPATHOID
[07% PLAGIOCLASE, 93% ALKALI] = 50% FELDSPAR
10. ________________
97% FELDSPATHOID
________________
[01% PLAGIOCLASE, 02% ALKALI] = 03% FELDSPAR
9