In this section there is discussion of mineral classification and related topics 
                                       and concepts influencing these

     Mineral classification is based primarily on the chemical composition, atomic
     structure, degree of ionic substitution, and color and crystalline state of minerals 

I.  Mineral Classification

    A.  Mineral Classes
- minerals are classified primarily on the main anion ( O-2, S-2,  etc.), anionic complex (oxyacid
          anion) (OH-1, SO4-2, CO3-3, PO4-3, BxOy-Z, SixOy-Z, etc), or lack of an anion(native elements)
         - some of the classes are listed below with the chemical characterisic used to classify them--find
            more mineral classes with the corresponding anion or complex anion in the text

               Native elements ( comprised of atoms of only one element and no anion)----covalent by
                 nature--- atomic structure cannot be determined by Pauling’s Rule #1(radius ratio)
               Sulfides, including Sulfarsenides, Arsenides, Sulfosalts ( main anion is S-2)---covalent by
                 nature—atomic structure cannot be determined by Pauling’s Rule #1 (radius ratio)
               Oxides ( main anion is O-2)----almost all are comprised of isodesmic bonds--atomic
                  can be determined by Pauling’s Rule #1 (radius ratio)
               Hydroxides   (main anion complex is OH-1)
               Halides ( main anion is a halogen as Cl-1,  F-1, Br-1, I-1)
               Carbonates ( the oxyacid anion, CO3-3)
               Nitrates ( the oxyacid anion, NO3-1)
               Borates ( the oxyacid anion, BxOy-
               Phosphates ( the oxyacid anion, PO4-3)
               Sulfates ( the oxyacid anion, SO4-2)
               Tungstates ( the oxyacid anion, WO4-2)
               Silicates ( the oxyacid anion, SixOy-Z)

    B.  Mineral Subclasses
        -some classes
can be subdivided based on chemical or structural grounds--examples are
         the 1.
Native Element Class which is divided into minerals with metallic bonding (metals),
         those with mostly covalent bonding ( nonmetals), and those with a mixture (semimetals); 
         and the 2. the 
Silicate Class, with 6 subclasses (neso-, soro-, cyclo-, phylo-, tecto-silicates)
         based on the linkage of the silica tetrahedra--details concerning these subclasses will be
         treated later under a discussion of the non silicates and silicates

    C.  Mineral Groups
classes or subclasses can be further divided based on atomic structure and similar chem-
         istry--examples are isomorphic (isostructural) groups, polymorphic groups and groups based
         on a general empirical formula with consistent properties

          1.  isomorphic group is a group of minerals with the same atomic structure but different
                 chemical formulas--atoms of different elements representing equivalents in minerals
                 of this group have the same C.N.--FeCO3 (siderite) and CaCO3 (calcite) belong to
                 the same isomorphic group in the carbonate class because in both cases there are 6
                 O around each Fe and Ca respectively, 3 O around each C, and one C and 2 Fe or
                 Ca around each O--often the same atomic structure in different minerals reflects 
                 similar chemical and physical properties and similar crystallography
                - some examples of isomorphic (isostructural) groups are:

                    --in the oxide class-
                           hematite group, spinel group, rutile group
                    --in the carbonate class-
                           calcite group, aragonite group
                    --in the sulfate class-
                           barite group
                   --in the silicate class-
                           (and nesosilcate subclass)--garnet group
                           (and inosilicate-pyroxenes subclass)--sodium pyroxene group
                           (and inosilcate-amphibole subclass)--sodium amphibole group

              -isomorphism can exist with minerals which are not in the same mineral class--since they
               are not in the same mineral class they cannot be in the same isomorphic group--NaNO3
                    (nitratite) is isomorphic or isostructural with the minerals in the calcite group of the carbonate 
            class including siderite and calcite

          2.  polymorphic group is a mineral group belonging to the same mineral class, all having the
                same chemical formula but different atomic structures--these usually form or are stable
                under different temperatures or pressures whereby the same cation forms a different
                C.N. with the same anion--or the same CN exists but there is a different bond angle
                between polyhedra--the difference in atomic structures result in polymorphs 
                often forming in different crystal systems

              -some examples of polymorphs are:

                    a. calcite and aragonite--CaCO3---calcite is hexagonal and aragonite, orthorhombic
                    b. pyrite and marcasite--FeS2---pyrite forms at a high temperature and is isometric
                       while marcasite forms at a low temperature and is orthorhombic
                    c. quartz, tridymite, cristobalite, stishovite and coesite--SiO2---quartz forms at a
                        low temperature and forms in the hexagonal system, cristobalite forms at a high
                        temperature and forms in the tetragonal system, while tridymite is an intermediate
                        temperature form which is orthorhombic---coesite is stable at high pressures and
                        is associated with meteor impact and is a monoclinic mineral---stishovite is tet-
                        ragonal and is thought to be associated with rocks from Mars
                     d. kyanite and andalusite--Al2SiO5---kyanite is triclinic and is formed at a high
                         temperature and andalusite is orthorhombic and is the low temperature form
                     e. microcline, orthoclase, sanidine--KAlSi3O8---microcline, a triclinic mineral is
                         the low temperature variety, sanidine, a monoclinic mineral is the high temper-
                         ature variety and orthoclase is a monoclinic mineral which forms at an inter-

                  kinds of polymorphism:
                   -two types of polymorphism are recognized according 1. to whether a change from
                    one polymorph to another is reversible and takes place at a definite temperature and
                    pressure, or 2. is irreversible and can change in only one direction at a certain temperature

                        1. enantiotropy
                           -a reversible change as:
                               quartz >< tridymite 
                                graphite >< diamond 
                       2. monotropy
                          -a one way change between polymorphs as:
                               marcasite > pyrite   marcasite to pyrite but not vice versa (irreversible)

                  -also, polymorphs can also be categorized as to the nature of their change in respect to 
                   the degree of reconstitution of the atomic structure

                       1. reconstructive change
                           - is the breaking of atomic bonds and a reassembly of structural units--this type of
                             change involves alot of energy and the change is not readily reversed and is
                                   quartz > tridymite > cristobalite
                       2. displacive change
                            -atomic bonds are not broken and the original structure is maintained--there is
                             only a slight displacement of the atoms resulting in different bond angles--this
                             change is instantaneous and involves little energy
                                 high quartz > low quartz

                        3. ordered-disordered change
                             -microcline (KAlSi3O8) has an ordered arrangement of the Si and Al in its
                              structure while the same for orthoclase is disordered--the disordered form  
                             will have more symmetry since it forms at a higher temperature

          3.  Other Groupings
              -minerals grouped based on the same general or empirical formula such as the pyroxene,
               amphibole and mica groups

    D.  Mineral Series
-classes and groups can be subdivided into mineral series in which solid solution is most
            prominently displayed

           solid solution is a homogeneous crystalline mineral of variable composition comprised of
             a mixture of end members in which there is ionic substitution between some cations of the
             end members--the principles of ionic substitution was treated earlier in the semester
            -the type or quantity of cation(s) which can proxy for locations in the atomic structure
             of a mineral during mineral formation to a large degree is a function of temperature--in
             most cases examples of proxying cations in a mineral series are Ca+2 and Na+1              
             Al+3 and Si+4, and Fe+2 and Mg+2

            -some examples of solid solution series are:     
                a. Plagioclase series (coupled ionic substitution)
                     -end members are CaAl2Si2O8 (anorthite) (An) and NaAlSi3O8 (albite) (Ab) in 
                      which there is a proxying between  both Na and Ca, and Al and Si--a table below       
                      expresses the different plagioclase minerals based on the degree of ionic sub-
                      stitution of Na and Ca, and Al and Si in end members:





plagioclase name

  plagioclase type/temperature of formation




sodic plagioclase/low temp. in magma




"               "     /"    "        "     "




intermediate plagioclase/intermediate " " "




intermediate plagioclase/           "        " "  "




calcic plagioclase/high temp."        "




    "            "               "      "     "        "


                  -the determination of the specific plagioclase mineral in a rock by use of the petro-
                   scopic microscope is paramount in the naming of igneous rocks

               some examples of
simple ionic substitutions are: 
                b. Olivine series ((Fe,Mg)2SiO4)
                    -end members are fayalite (Fe2SiO4) and forsterite (Mg2SiO4) in which there is
                     proxying between Fe and Mg concentrations in end members

                 c. Enstatite-Ferosilite series ((Fe,Mg)SiO3)
                    -end members are enstatite (MgSiO3)-ferosilite (FeSiO3) in which there is proxying
                      between Fe and Mg concentrations in end members
                 d. Tremolite series (Ca2(Fe,Mg)5SiO22(OH)2)
                    -end members are tremolite-Ca2Mg5Si8O22(OH)2 and actinolite-Ca2Fe5Si8O22
in which there is proxying between Fe and Mg concentrations in end mem-

other kinds of solid solution will be discussed later   


    E.   Mineral Varieties

          1. based on chemical rich minerals
             -a mineral which can be appropriately expressed with an adjective denoting an
              unusually large amount of chemical constituent(s)--some examples of  modifers are:
                            aluminian = Al-rich      ferrian = Fe+3-rich
                            calcian = Ca-rich         magnesian = Mg-rich
                            chromian= Cr-rich       manganoan = Mn-rich
                            ferroan = Fe+2-rich
              -some specific mineral varieties would be manganoan aegerine, ferrian diopside or
               magnesian augite

          2. based on crystalline types
             -based on crystal size, color, or special appearance--many examples apply to quartz

               a. coarsely crystalline quartz
                     rock crystals--colorless;  amethyst--purple:  rose quartz--rose;  smoky quartz--
                       smoky yellow to brown to black;  citrine--light yellow;  milky quartz--milky
                       white; rutilated quartz--with inclusions of rutile

               b. microcrystalline quartz  
                    1. fibrous varieties:
                        chalcedony--waxy brown to gray often mammillarychrysoprase--apple green;
                        agate--parallel somewhat curved bands;  heliotrope (bloodstone)--green with
                        small red spots in it
                    2. granular varieties:
                        flint or chert--flint is black with some chalk while chert is light colored;  jasper--
red color caused by hematite

II.  Related Topics

     A.  Exsolution
          -is the association of similar composition minerals formed almost instantaneously from a
           cooling magma resulting in stringers of one mineral appearing in the other more abundant
           mineral--this can occur if the two minerals have cations close to ionic substitution at high
           temperature but not at the lower solidification point or temperature of formation of the
           minerals--examples are perthites and antiperthites (macro-, micro- and crypto-)
                        perthite is albite (Ab, NaAlSi3O8) stringers in orthoclase (Or, KAlSi3O8)
                        antiperthite is Or stringers in Ab

    B. Pseudomorphism
         -a form or shape displayed by "mineral A" in occurrence with "mineral B" in which
          the shape displayed by mineral A( not a normal form of A) is that of mineral B--ie. the existence  
         of a mineral with the outward form of another

                  kinds of pseudomorphism:
                        1. encrustation
                           -one mineral is deposited over crystals of another as quartz (hex.) encrusting
                             cubes of fluorite (iso.)--the more solublized fluorite can then be dissolved
                             leaving a cast of its form in the quartz
                        2. alteration
                           -a mineral can be altered to produce an external or outer layer of another
                            which takes on the form of the altered mineral--examples are: the change of
                            anhydrite, CaSO4 to gypsum, CaSO4.H2O;   the change of galena, PbS to
                            anglesite PbSO4;  the oxidation or change of pyrite, FeS2 or siderite, FeCO3
                                      to limonite, FeO(OH).nH2O

                        3. substitution
                           -a gradual removal of the original mineral and a molecule for molecule replace-
                            by another simultaneously--examples are: silica replacing fluorite; silica replac-
                            ing wood

     C.  Mineraloids
      -substances resembling minerals but are non crystalline since they have no ordered atomic
           arrangement--these mineral gels or glasses are not true minerals because they are amor-
          -these substances form under low temperature and low pressure conditions and commonly
           are products of chemical weathering processes--they often occur in mammillary, botry-
           oidal, or stalactitic masses as opal, and limonite

     D.  Metamict Structure
-minerals whose original atomic structure has been altered by radiation from the decay
           of radioactive elements--the original ordered atomic arrangement may be reconstituted
           often by the exposure of heat by the altered arrangement 
         -the reconstitution process may result in emission of heat which can cause the mineral to ap-
           pear incandescent
         -a solid with a metamict structure may be considered a type of amorphous (mineraloid)
         -examples of metamict substances can occur with allanite and zircon    

    E. Geological Thermometry (Geothermometry)
        -the amount of ionic substitution of one cation for another in a mineral can be a function of
         the temperature of formation of the mineral--the substitution of Ca+2 and Na+1 in
         plagioclase is an example--the higher the temperature of formation of plagioclase in a mag-
         ma the less Na+1 substitutes for Ca+2 and vice versa--this explains calcic rich plagioclase
         present in igneous rocks formed at higher temperatures as in gabbros and sodic rich
         plagioclase in rocks formed at lower temperatures as in granites
       -certain minerals can be used as an indicator of temperature of formation of that mineral or
         a related assembledge of minerals--sphalerite, (Fe,Zn)S is an example of this in which the
         concentration of Fe is directly proportional to the temperature of formation--the table
         below shows the concentration of Fe in sphalerite as a function of temperature of
         formation. Although (Fe,Zn)S is a solid solution, FeS and ZnS are not isomorphic.

geothermometer.jpg (12860 bytes)