ROCK-FORMING MINERALS
You must know the name and the exact mineral formula of each end-member in each mineral group.





Know the types of polyhedrons and their coordination number in each structure.
You should also be able to identify each mineral group from drawing of its crystal structure.
You should know the type of polyhedron that each cation goes to.
Know the valances of all cations.
Know the meaning of displacive and reconstructive polymorphic transitions.
SiO2 polymorphs





transition between high and low quartz is displacive
transitions between other polymorphs are reconstructive
tridymite can be found in volcanic rocks
coesite can be found in rapidly exhumed high-pressure rocks
coesite can also be found in surficial rocks that were hit by a meteorite
Olivine - (Fe,Mg)2SiO4




olivine has complete solid solution between: Mg end-member, forsterite, and Fe end-member,
fayalite
Fe and Mg are in nearly identical M1 and M2 octahedral sites; Si is in tetrahedral sites
olivine has the lowest Si/(Mg+Fe) ratio of the most common ferromagnesian silicates
olivine occurs mostly in ultramafic and mafic rocks (peridotite, basalt, gabbro)
Pyroxene (XYSi2O6)



pyroxenes are the most common ferromagnesian minerals
pyroxenes can found in all igneous rock types, although in most silica rich varieties only rarely
pyroxenes can be described by the pyroxene quadrilateral in terms of three dominant
components: MgSiO3 (enstatite), FeSiO3 (ferrosilite), and CaSiO3 (wollastonite):
CaSiO3
(wollastonite)
diopside
CaMgSi2O6
augite (C2/c)
hedenbergite
CaFeSi2O6
pigeonite (P21/c)
enstatite
Mg2Si2O6
hypersthene (Pbca)
ferrosilite
Fe2Si 2O6
The pyroxene structure has three principal sites, M1 (Y in the general formula), M2 (X in the general
formula), and T. Substitutions that occur in these sites are:
M1 (octahedral) - Fe2+, Mg2+, Mn2+, Fe3+, Al3+, Cr3+, Ti4+
M2 (octahedral - Fe2+, Mg2+, Ca2+, Mn2+, Na+, Li+
T (tetrahedral) - Si4+
The substitution of the relatively large Ca into the pyroxene structure results in distortion, which is
accommodated by going from orthorhombic to monoclinic symmetry in Ca-rich pyroxenes. Because the
symmetries of Ca-rich pyroxenes are different from Ca-poor pyroxenes, there is only limited solid
solution between high and low-Ca pyroxenes.
Aside from the quadrilateral pyroxenes, one may run into
jaedite (NaAlSi2O6), which occurs in high-pressure rocks that may have contained feldspar at low
temperatures
spodumene (LiAlSi2O6), which occurs in Li-rich granitic pegmatites
Wollastonite (CaSiO3)
Wollastonite makes-up the apex of the pyroxene ternary diagram, but is not a pyroxene; it is a
pyroxenoid. The octahedral sites, which contain Ca, are too large for other cations. It occurs only in
calc-silicate metamorphic rocks.
Amphibole [W0-1X2Y5Z8O22(OH)2]
Amphiboles can occur in the same rock types as pyroxenes, but the rocks must contain water.
Amphiboles can be considered as hydrated pyroxenes. They are less common in the mantle, which
contains only very tiny amounts of water, but are more common that pyroxenes in granite magmas,
which often contain substantial amounts of water. Quadrilateral amphiboles are also common in mafic
metamorphic rocks.
Like pyroxenes, amphiboles can be described by Mg, Fe, and Ca end-members. These end-members
define the amphibole quadrilateral:
tremolite
Ca2Mg 5Si 8O 22(OH)2
actinolite (C2/m)
ferroactinolite
Ca2Fe5 Si8 O22(OH)2
cummingtonite (C2/m)
anthophyllite (Pnma)
Mg7Si 8O 22(OH)2
grunerite
Fe7 Si8 O22(OH)2
Elemental substitutions in pyroxenes are:
A(W) - Na+,K+, or vacancy
M4 (X) - Mg, Fe2+, Ca, Na, Mn
M1, M2, M3 (Y) - Mg, Fe2+, Mn2+,Fe3+, Al3+, Ti4+
T (Z) - Si4+, Al
The most common amphibole in igneous rocks is hornblende [ , Na)Ca2(Mg,Fe,Al)5(Si,Al)8O22(OH)2].
It is analogous to augite in the pyroxene group.
Micas
Micas are layered (sheet) silicates. They contain (OH) groups, and therefore water must be around
during their crystallization. There are dioctahedral and trioctahedral micas, the former having two
octahedral sites and the latter three octahedral sites. Two micas are dominant in igneous and
metamorphic rocks:
muscovite - KAAl2VIAlIVSi3O10(OH)2; occurs in aluminous (pelitic) metamorphic rocks and granites
biotite -
KA(Mg,Fe)3VIAlVISi3O10(OH)2; The Fe end-member is annite; the Mg end-member is
phlogopite occurs in pelitic metamorphic rocks and intermediate to high-silica igneous
rocks.
Note that these two micas can be built from simple hydroxides structures by progressive substitutions:
Dioctahedral
Trioctahedral
Gibbsite - Al2(OH)6
Brucite - Mg3(OH)6
O
O
+ Si2O5 - (OH)2 =
Kaolinite (Clay) - Al2Si2O5(OH)4
+ Si2O5 - (OH)2 =
Antigorite (Serpentine) - Mg3Si2O5(OH)4
T
O
T
O
T
O
T
O
+ Si2O5 - (OH)2 =
Pyrophyllite - Al2Si4O10(OH)2
+ Si2O5 - (OH)2 =
Talc - Mg3Si4O10(OH)2
T
O
T
T
O
T
T
O
T
T
O
T
+ KAl - Si =
Muscovite - KAl2AlSi3O10(OH2)
+ KAl - Si =
Phlogopite - KMg3AlSi3O10(OH)2
+CaAl - KSi =
Margarite - CaAl2Al2Si2O10(OH)2
+CaAl - KSi =
Xantophyllite - CaMg3Al2Si2O10(OH)2
Feldspars
Along with pyroxenes, feldspars are the most abundant mineral group. They are found in all crustal
igneous rocks and most metamorphic rocks. There are three feldspar end-members:
anorthite - CaAl2Si2O8 albite - NaAlSi3O8



K-feldspar - KAlSi3O8
Going from anorthite to albite involves the coupled substitution Na+Si4+ for Ca2+Al3+. Going
from albite to K-feldspar involves the simple substitution K+ for Na+.
Minor substituting cations include Fe2+, Sr2+, Ba2+.
There is very limited solid solution between anorthite and K-feldspar and limited solid solution
between albite and K-feldspar at low temperatures:
The unmixing (exsolution) of albite and K-feldspar can be shown well on a temperature-composition
phase diagram. Note that there are several polymorphs of K-feldspar and albite.
Feldspathoids
Leucite (KAlSi2O6) and nepheline (Na,K)AlSiO4 principally occur in alkalic basalts and syenites. These
rock types have high (K+Na)/Si ratios. Feldspathoids cannot coexist with quartz because of the reaction
relationship, such as
KAlSi2O6 + SiO2  KAlSi3O8 .
Garnet (X3Y2Si3O12)
the X-sites are distorted cubes with coordination number of 8; contain 2+ cations Ca, Mg, Fe2+, Mn
the Y-sites are octahedrons with coordination number of 6; contain 3+ cations Al, Cr, Fe3+, (V3+)
the T-sites are tetrahedrons with Si.
There are two garnet series:
pyralspites (X ≠ Ca; Y = Al)
pyrope Mg3Al2Si3O12
almandine - Fe3Al2Si3O12
spessartine - Mn3Al2Si3O12





ugrandites (X = Ca)
uvarovite - Ca3Cr2Si3O12
grossular - Ca3Al2Si3O12
andradite - Ca3Fe23+Si3O12
pyrope-rich garnet occurs in the mantle
pyrope-almandine solid solutions occur in high-pressure mafic rocks (subducted ocean crust)
almandine-spessartine solid solutions occur in metapelites and peraluminous granites
grossular occurs in calc-silicate metamorphic rocks
andradite and uvarovite occur as solid solutions within grossular; end-members are rare
Al2SiO5 polymorphs
The minerals andalusite, kyanite, and sillimanite occur dominantly in aluminous pelitic rocks. They
are diagnostic of the pressure-temperature relationships that the rocks experienced.
Minor, but ubiquitous minerals in igneous and metamorphic rocks
zircon - ZrSiO4
apatite - Ca5(PO4)3(OH,Cl,F)
sphene - CaTiSiO5
Fe-Ti oxides
The iron-titanium oxides are minor but ubiquitous in both igneous and metamorphic rocks.
magnetite - Fe3O4
hematite - Fe2O3
ulvospinel - Fe2TiO4
ilmenite - FeTiO3
rutile - TiO2
At very high temperatures, a phase from the ilmenite-hematite series will coexist with a phase from the
ulvospinel-magnetite solid solution series, as shown by the tie-lines.
TiO2
rut
ilm
ulv
Fe
wusite
FeO
mag
hem
1/2 Fe2O3
O
At submagmatic condition in both igneous and metamorphic rocks, the two solid solution series become
unstable and end-member minerals will form. The following Fe-Ti oxide assemblages can occur,
depending on the oxidation state of the rock:
rut
ilm
rut + mag + ilm
ulv
rut + mag
+ hem
Fe
wusite
FeO
mag
hem
1/2 Fe2O3
O
Fe-Ti oxides are important indicators of the oxidation state of magmas and metamorphic rocks. For
example, we can have the following oxidation reactions:
2Fe3O4 + 0.5O2 ` 3Fe2O3