GEOL 210: Earth Materials Norms and igneous crystallization Due: 11/15 Your task

Question

GEOL 210: Earth Materials

Norms and igneous crystallization

Due: 11/15

Your task is to calculate the normative mineral composition for the two igneous rocks listed on the next page. Do not download a CIPW calculator from the internet to do this for you: the point is to figure out the sequence of minerals and necessary formulas yourself! For full credit, you should digitally submit one spreadsheet file that can be opened in MS Excel software (you may choose to use another spreadsheet software, but please export an .xlsx file). It is typically easiest to include two worksheets in the spreadsheet file, one for each rock. You may submit your file using the appropriate Blackboard online assignment dropbox. (Be sure the filename includes your name, so I can tell the files apart after downloading them!)

While you must build your own calculation template, you may consult additional references for general guidelines as you set up your spreadsheet (such as the very detailed appendix in your Winter textbook). Please use “diopside” (Ca-rich cpx) and “hypersthene” (Ca-free opx) instead of the Wo, En, and Fs used in some calculators, however. Please also be sure to assign Mg and Fe in consistent proportions to the appropriate minerals (olivine and pyroxenes); do not put all the Mg in one mineral and all the Fe in another, for example.

Please provide your results in terms of both mole percentages and oxygen percentages of normative minerals (oxygen percent more closely approximates volume, and, therefore, visible mineral mode). It is not required, but you may wish to also calculate the weight percentages of normative minerals, as a check on your calculations (reasoning: the sum of the weights of the mineral constituents should add up to the total rock weights, because mass is conserved!). An initial calculation example is provided in the class notes, which we will discuss in lecture.

This problem set aims to give you greater familiarity with the relationship between the chemistry and mineralogy of igneous rocks; to encourage you to think critically about the chemical compositions of common igneous minerals; to help you understand igneous rock classification better; and to lay additional groundwork for useful graphical tools for looking at rock compositions. Your results here will also resemble the rocks in your Granites Lab: one of these compositions is peralkaline, and one is peraluminous! (HINT: NOTE that this means you may have to modify the mineral order in your algorithm to avoid negative values…)

Explanation / Answer

For Quincy

Oxide GFW Wt% Mol proportion

SiO2 60.085 73.93 1.2

TiO2 79.899 0.18 0.002

Al2O3 101.961 12.29 0.12

Fe2O3 159.692 2.91 0.018

FeO 71.846 1.55 0.021

MnO 70.937 0.00 0

MgO 40.311 0.04 0.0009

CaO 56.079 0.31 0.005

Na2O 61.979 4.66 0.075

K2O 94.203 4.63 0.049

P2O5 141.945 0.00 0

H2O+ 18.015 0.41 0.022

H2O- 18.015 0.00 0

100.91

Now amount of MnO added to that of FeO

FeO=0.021+0=0.021

Now for plotting apatite

CaO=CaO-3.3P2O5=0.005

If FeO exceeds titanium then illemenite is plotted=CaO-TiO2

=0.005-0.002=0.003

For alloting alkali and aluminium

For plotting orthoclase:Al203=Al203-K20=0.12-0.049=0.071

Now SiO2=SiO2-6K20=1.2-6*0.049=0.906

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