Compositional Groups of Minerals

 

Use your identifications of the minerals and your homework assignment where you listed composition of each mineral to do the following:

 

A.  Create several groups of minerals on the table top according to composition.  These are the groups of minerals you should have after completion of this activity.

 

                  Silicates (positively charged ions + silicate groups [Si,O])

 

                  Carbonates (positively charged ions + carbonate ion [CO3-3])

 

                  Oxides (positively charged ions + oxygen ion [O-2])

        

                  Sulfides (positively charged ions + sulfide ion [S-1])

 

                  Sulfates (positively charged ions + sulfate ion [SO4-2])

 

                  Halides (positively charged ions + chloride ion [Cl-1])

 

                  Native elements (e.g., C)

 

         B.  Which group(s) of minerals has (have) prominently colored streaks?

 

         C.  Which group(s) of minerals commonly has (have) metallic lusters?

 

D.  Take the pile of silicate minerals and separate it into two parts.  One part should contain the darker-colored silicate minerals and the other should contain the lighter-colored silicate minerals.

 

1.  What name is often given to the darker-colored silicate minerals?

 

2.  What name is often given to the lighter-colored silicate minerals?

 

 

E.  The accompanying table lists the chemical formulas of several framework and single and double chain minerals  The atomic weights, ionic radii, and charges of each element are also given.  Calculate the atomic weight percent of Si in each of the minerals.  Enter your results in the space labeled %Si. Do not worry about %Si+X at this time! The calculations for K feldspar (orthoclase - KAlSi3O8) are shown below the table as an example.

 

Element       Atomic Weight/atom        Ionic Radius (Ang)  Ionic Charge

                                                      (Angstroms=10-10m)

 

H               1                                   0.46                     +1

O              16                                 1.40                      -2

Na             23                                 0.97                     +1

Mg             24                                 0.66                     +2

Al              27                                 0.51                      +3

Si              28                                 0.39                     +4

K               39                                 1.33                      +1

Ca              40                                 0.99                     +2

Fe              56                                 0.64-0.74              +2, +3

 

 


Example

 

Element      Atomic Weight       x       Number of Atoms   =        Atomic Wt

                 

               K                   39                                 1                          39

      Al                  27                                 1                          27

Si                  28                                 3                         84     

      O                   16                                 8                         128

 

                                                                                          Total=278

 

Now, divide the total atomic weight times the number of atoms by the atomic weight of silicon times the number of atoms

 

                           84/278 x 100% = 30% Si

                

 

 

Individual tetrahedra -

 

Olivine MgFeSiO4                                      %Si                      %Si+X

 

Single chain silicate -

 

Hypersthene (pyroxene) MgFeSi2O6              %Si                      %Si+X

Diopside (pyroxene) CaMgSi2O6                    %Si                      %Si+X

 

Double chain silicate -

 

Tremolite Ca2Mg5(Si8O22)(OH)2                  %Si                      %Si+X

 

Sheet Silicates

 

Talc Mg3(Si4O10)(OH)2                                %Si                      %Si+X

 

Framework Silicates -

 

Orthoclase K(AlSi3O8)                          30 %Si                      %Si+X

 

Sodic Plagioclase Na(AlSi3O8)                    %Si                      %Si+X

 

Calcic Plagioclase Ca(AL2Si2O8)                  %Si                      %Si+X

 

Quartz SiO2                                            %Si                      %Si+X        

 

                

 

Examine your data for %Si for the minerals above, and answer these questions:

 

a.  List the silicate structural groups in order of generally increasing Si

 

 

 

b.  Consider the group with the overall highest Si%.  Which mineral(s) is (are), if any, exception(s) in this group?

 

 

 

c.  This (these) exception(s) is (are) because another element (let's call it "X") has substituted for Si in the tetrahedral part of the crystal structure.  Why do you think such substitution is possible?

 

 

 

 

 

d.  Examine the composition of the mineral(s) which you identified in "b" and use the table to examine the radii of ions in the mineral(s).  What do you think the most likely element for "X" is? 

 

 

Now, calculate the atomic weight percent of (Si+X) for this (these) mineral (s) and record it in the table.

 

Does this atomic weight percent now fit the pattern of Si content for this silicate group?

 

 

e.  What is the charge on a Si ion?                Charge on "X"?

 

f.  What is the charge on a molecule with the formula Si4O8?

 

g.  What is its charge after one atom of element X is substituted for one Si atom?  (i.e., charge on X1Si3O8)

 

h.  Consider the charges of other elements in the table:  Which elements (or elements) could be added to the structure of the altered molecule in "g" to neutralize charge on the ion group?

 

 

 

i.  What is the charge on a molecule with the formula Si4O8  after two atoms of element X are substituted for two Si atoms? 

 

j.  Which element (or elements) could be added to the structure of the altered molecule in "i" to neutralize the charge on the ion group?

 

 

k.  Finally, consider the double chain mineral

 

Hornblende NaCa2Mg4Al(AL2Si6O22)(OH)2               %Si             %Si+X

 

Calculate %Si and state whether this mineral follows the normal pattern for Si content of double chain silicates, or whether it is different.   Calculate the atomic weight percent of (Si+X) for this mineral and enter it in the table.  Does this atomic weight percent now fit the pattern of Si content for this silicate group?

        

 

 

l.  Is the substitution discovered in "d" restricted to minerals in the framework group of silicates?

 

 

m.  Do you think it would be better to determine the relationship between the silicate structural group and the ratio of the tetrahedral ion to oxygen by using Si alone, or by combining Si and X?