# The Naked Scientists Forum

### Author Topic: How is a QAP Diagram used to identify rocks?  (Read 14381 times)

#### endgamex

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##### How is a QAP Diagram used to identify rocks?
« on: 21/10/2010 22:51:39 »
Hello there, I am studying for a Geoscience degree with the Open uni and at the moment I am learning how to plot a QAP diagram for plutonic igneous rocks but I am confused.  I understand how to get the three percentages but I don't understand how you can use them three percentages to identify the rock.  I've spent a while now trying to figure it out and it's probable very simple! If some kind soul can tell me how to do it but in very simple terms...perhaps step by step, I would be very grateful.

Many thanks for any help.
« Last Edit: 06/12/2015 15:59:31 by chris »

#### Bass

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##### QAP Diagram?
« Reply #1 on: 22/10/2010 02:14:24 »
Wikipedia has a decent explanation of the QAPF diagram

http://en.wikipedia.org/wiki/QAPF_diagram

Q=quartz
A=alkalai feldspar
P=plagioclase feldspar
F=feldspathoids

If you ignore the rare feldspathoid rocks, you get the QAP diagram for most common igneous rocks.  Plot the percentages of feldspar, then move the plot to the percentage quartz in the rock and that point should give you the type of rock.  For example- 15% Q (quartz) + a mixture of 50% alkali feldspar and 50% plagioclase feldspar = quartz monsonite (plutonic) or quartz latite (volcanic).

#### endgamex

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##### QAP Diagram?
« Reply #2 on: 22/10/2010 19:42:34 »
Hi, thanks for that but I still don't understand how plotting 15% Q, 50% A and 50% P can land on the part of the triangle that says quartz monzonite.  Using your example I would do the following,

To plot quartz I'll measure 15% up from the bottom right corner.  That takes me to the second row which has quartz monzodiorite in it.  Then for alkali I measure 50% upwards from the bottom left hand corner which takes me to the row with granite in it.  Measuring from the bottom line of the triangle (left to right) 50% plagioclase takes me to the middle area with monzonite in it.

The quartz and plagioclase percentage lines overlap in the quartz monzonite box but alkali's percentage doesn't match up.  I would have thought that if you drew a line from each percentage they are all meant to overlap at one point and give you your rock name.

Where am I going wrong? Again, thanks for any help that is given.

#### JimBob

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##### QAP Diagram?
« Reply #3 on: 23/10/2010 05:29:46 »
Delete the triangle with the feldspathoids for this discussion. Anything with a 'foid in it will be in the triangle deleted.

I am going to use the two triangle diagram Bass refereed to to explain this, just disregard the bottom triangle of the QAPF diagram.

Draw a perpendicular line from the apex of each of the three intersections of the triangle to the long edge of the triangle opposing it. The intersection at the apex represents 100% of a mineral type and the long base 0% of a mineral type.

Go to the 100% quartz apex. Then go down the line you drew to 15% quarts. It will be near the bottom, close to the  Anywhere along that line parallel to the base - the A-P line - is 15% Quartz.

To better understand lets modify the rocks composition somewhat. Use 38% anortosite and 12% plagioclase to make up the rest of the rock.  Move along this the 38% anorthosite line (to the left parallel to the right oblique line P-Q until you intersect the line of 12% plagioclase compositin, This line is parallel to line A-Q. At that one spot there is only one composition.

The extrusive rock is  a basalt andesite and the intrusive rock is a quartz-monzodiorit or quartz-monzogabbro, depending on the grain size. A gabbro has larger crystals visible than a diorite.

If you still have questions let us know.

#### Bass

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##### QAP Diagram?
« Reply #4 on: 24/10/2010 02:12:26 »
Welcome to the forum Endgamex! (I should have said that in the first post)

JimBob made it clear as mud- as usual ;-))

Keep it simple:

First determine the relative percentages of total feldspars (on the bottom line).  Are the feldspars alkali or plagioclase?  Back to the case of quartz monzonite- the alkali to plagioclase percentages will be 50%-50% of the total feldspars in the rock.

Second, from this point on the bottom line, move up to percentage of quartz in the rock (15% for the quartz monzonite).

Because feldspars are a solid solution series that changes as the magma differentiates (becomes more silicic), their composition is the main determinant of the classification.

Still clear as mud, eh?
« Last Edit: 26/10/2010 15:16:11 by Bass »

#### endgamex

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##### QAP Diagram?
« Reply #5 on: 24/10/2010 15:44:43 »
Brilliant, thank you so much. Completly understand it now.

#### JimBob

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##### QAP Diagram?
« Reply #6 on: 27/10/2010 03:55:52 »
Thanks Bass, I need the encouragement.

#### Don_1

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##### QAP Diagram?
« Reply #7 on: 28/10/2010 15:11:35 »
My pleasure,

EDITED BY MYSELF;

Opps! Beg pardon, I just read your question again, with my specs on this time and I see you wanted a QAP not an OAP.
« Last Edit: 28/10/2010 15:13:51 by Don_1 »

#### Ivana

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##### Re: QAP Diagram?
« Reply #8 on: 06/12/2015 15:09:51 »
So I'm ripping up an old topic here.
I'm not able to get the QAP diagram to work.
One examples that was made here, you have Q=15%, P=12%, A=38% in a rock, which makes up 65% total of the rock. But, I thought that to be able to plot in the QAP diagram, and get a classification where all the "Lines" meet, you have to have 100%? I thought that I understood the principle on how to draw a line out of how much I have of each Component, but it dosent make sense (ex. Q=15% I make a line parralell to the AP, and it only is up to 15% of the 0-100% quartz chart).

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##### Re: QAP Diagram?
« Reply #8 on: 06/12/2015 15:09:51 »