Estimate Mineral Composition of a Phaneritic Rock by Point Counting Activity 5.6

Question

Estimate Mineral Composition of a Phaneritic Rock by Point Counting Activity 5.6 Date Name: a technique called point-counting to estimate the relative abundance c igneous rock is shown in the photograph marked A. The four major of different minera minerals in the rock are identified in the map marked B. You will use the map and a adapted for this lab to classify the rock. s in a rock A biotite potassium plagioclase feldspar The basic idea is to identify the mineral found at each of several points on the rock. We draw a square grid with 5 vertical and 5 horizontal lines on some opaque paper. Each of the 25 spots where the grid lines cross will be called a node. At each node, we will create a small hole in the grid so that we can see the map of the rock through the hole. We count the number of node points that are filled with each of the four major minerals (that is, with each of the four colors on the map), and use that number to estimate the total volume of each major mineral in the rock. You can see more than one mineral through the hole at some node points, so either pick the mineral that fills most of the hole, or if two minerals each fill about half the hole, count each as 0 5 Figure A5.6.1 Fort Court, G:; Number of nodes filled with the mineral (__ x 4) =--% potassium feldspar (-_.x 4)·-% plagioclase feldspar (-_-.x 4) = % quartz (gray) ( x 4) % biotite (black) (pink) (white) Figure A5.62 144

Explanation / Answer

Point count Grid C:

8.5 X 4 = 34% potassium feldspar

7.5 X 4 = 30% Plagioclase feldspar

5 X4 = 20% Quart

3 X4= 12% Biotite

Point count Grid D

10.5 X 4 = 42% potassium feldspar

4-5 X 4 = 18% Plagioclase feldspar

7.5 X4 = 30% Quart

1.5 X4= 6% Biotite

Point count Grid E

5.5 X 4 = 22% potassium feldspar

6.4 X 4 = 25.6% Plagioclase feldspar

5.3 X4 = 21.2% Quart

4.9 X4= 8.2% Biotite

Point count Grid F

8 X 4 = 32% potassium feldspar

7.5 X 4 = 30% Plagioclase feldspar

8 X4 = 32% Quart

1.5X4= 6% Biotite

Plagioclase feldspar

0.11

The type of Phaneritic Igneous rock is Granite shown in photograph A

Name of mineral Average Standard deviation Potassium Feldspar 35.33 10

Plagioclase feldspar

24.53 6.07 Quartz 27.73 5.74 Biotite 6.73

0.11

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