SME
Physical Geology
Minerals and rocks
Use this for “A rock is 25% quartz, 30% K-feldspar, 20% plagioclase and 25% biotite. Name it.”
Diagram
Q′ = 33.3%, A′ = 40%, P′ = 26.7% of Q + A + P.
P
A + P
=
40%.
The point is in field 3b: monzogranite (a granite).
This is an example. Type your own values to start a new problem.
Q
Quartz
example · %
A
Alkali feldspar
example · %
P
Plagioclase
example · %
M
Mafic minerals
calculated · %
S
Quartz and feldspars
worked out (not typed) · %
Q′
Quartz, scaled
calculated · %
A′
Alkali feldspar, scaled
calculated · %
P′
Plagioclase, scaled
calculated · %
r
Plagioclase share
calculated · %
Formulas
S
=
100
−
M
75 = 100 − 25
S
=
Q
+
A
+
P
75 = 25 + 30 + 20
Q
′
=
100
×
Q
S
33.3333
=
100
×
25
75
A
′
=
100
×
A
S
40
=
100
×
30
75
P
′
=
100
×
P
S
26.6667
=
100
×
20
75
r
=
100
×
P
A
+
P
40
=
100
×
20
30 + 20
Assumptions
•
QAP names coarse-grained (plutonic) rocks with less than 90% mafic minerals.
•
The mafic minerals (biotite, hornblende, pyroxene) are left out; Q, A and P are scaled to 100%.
•
The field the point falls in names the rock (the IUGS fields).
Step-by-step
WE KNOW
Q = 25%, A = 30%, P = 20%
FIND
quartz and feldspars (S), quartz, scaled (Q′), alkali feldspar, scaled (A′), plagioclase, scaled (P′), plagioclase share (r), mafic minerals (M)
Step 1 · Find quartz and feldspars (S)
Use S = Q + A + P
Add the three minerals the triangle plots.
S
=
25
+
30
+
20
S
=
75%
Step 2 · Find quartz, scaled (Q′)
Use Q′ = 100 × Q ÷ S
Quartz as a percent of quartz and the two feldspars only: the mafic minerals are left out.
Q
′
=
100
×
25
75
Q
′
=
2,500
75
Q
′
=
33.3333%
Step 3 · Find alkali feldspar, scaled (A′)
Use A′ = 100 × A ÷ S
Alkali feldspar as a percent of quartz and the two feldspars only: the mafic minerals are left out.
A
′
=
100
×
30
75
A
′
=
3,000
75
A
′
=
40%
Step 4 · Find plagioclase, scaled (P′)
Use P′ = 100 × P ÷ S
Plagioclase as a percent of quartz and the two feldspars only: the mafic minerals are left out.
P
′
=
100
×
20
75
P
′
=
2,000
75
P
′
=
26.6667%
Step 5 · Find plagioclase share (r)
Use r = 100 × P ÷ (A + P)
The plagioclase share of the feldspar picks the column of fields.
r
=
100
×
20
30 + 20
r
=
100
×
20
50
r
=
2,000
50
r
=
40%
Step 6 · Find mafic minerals (M)
Use S = 100 − M
The rest of the rock is mafic minerals, which QAP leaves out.
M
=
100
−
S
M
=
100
−
75
M
=
25%
Check
Put the numbers back in. Both sides should match.
75 = 100 − 25 ✓
75 = 25 + 30 + 20 ✓
33.3333
=
100
×
25
75
✓
40
=
100
×
30
75
✓
26.6667
=
100
×
20
75
✓
40
=
100
×
20
30 + 20
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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