Tectosilicates: the feldspars
Nesse
EPSC210 Introductory Mineralogy
Gem & Mineral Show
Montreal Gem & Mineral Club
Club de Gemmologie et Minéralogie de Montreal
Hippodrome de Montréal / Blue Bonnets
(Namur station)
(87 dealers, minerals, gemstones, fossils)
Friday 4 p.m. to 10 p.m.
Saturday 10 a.m. to 7 p.m.
Sunday 10 a.m. to 5 p.m.
Admission: $4 (students with ID) ; $6 (adults)
November,19, 20, 21 2004
Feldspars make up about 50% of the Earth’s
crust.
Magmatic feldspars undergo important
changes during cooling.
They are involved in important metamorphic
reactions.
Their weathering produces much of the clay
minerals and aluminum oxi-hydroxides found in
sedimentary rocks.
Albite (variety cleavelandite is platy)
orthoclase KAlSi3O8 ...
... weathers to
kaolinite:
Al2Si2O5(OH)4
... which can weather
further to bauxite:
AlO(OH)
When a substitution takes place, in a mineral
structure, electrical neutrality must be
maintained.
In feldspars, this is illustrated by members
of the plagioclase series:
from Na Al Si3O8 to CaAl2Si2O8
Na + is replaced by Ca 2+
Si 4+ is replaced by Al 3+
When three or more ions are involved in the
substitution to preserve electrical neutrality,
we call it a coupled substitution.
The variety amazonite is an
example of colour
associated with a vacant
site created by the cation
substitution:
Pb2+ +  = 2 K+ in K-felspar
Colour center: A
vacant site may trap
an electron from a
nearby ion and it will
absorb specific
wavelengths, leading
to colour.
Feldspars have
names for very
specific ranges of
composition. Most
cannot be identified
in hand specimens.
This type of diagram illustrates the chemical
changes that take place during the crystallization
of liquids of intermediate composition between
albite and anorthite.
Bowen’s reaction series
The earliest feldspar to precipitate from a
rapidly cooling igneous rock is usually a Carich plagioclase. It is also the feldspar most
susceptible to weathering.
This type of diagram illustrates the chemical
changes that take place during the crystallization
of liquids of intermediate composition between
albite and anorthite.
Pure anorthite
crystallizes
at this
temperature
Pure albite
crystallizes
at this
temperature
Liquid of
T1
intermediate
composition A
starts to
crystallize at
T1, to give a
bit of
plagioclase,
composition
B.
As liquid
T2
cools, the
composition
of the
plagioclase
changes. At
T2, the
plagioclase of
composition
D. The liquid
has lost more
Ca than Na,
and is now of
composition C.
The last drop
of liquid,
composition E
crystallizes
at T3. If
cooling was
slow, all the
plagioclase is
now at
T3
composition F.
It has the
same
composition
as the original
liquid (A).
The slow cooling we just described ends up with a
plagioclase similar to the original liquid because ions
are exchanged back and forth between the growing
crystals and the molten liquid.
If cooling is fast, ions cannot leave or enter the
crystal in time to keep adjusting the plagioclase
composition.
The result is concentric zoning.
(This also happens in rapidly cooled olivines and
pyroxenes, by the way... The Mg-rich variety
crystallizes at a higher temperature than the Fe-rich
variety because Mg-O bond is shorter, stronger).
The core of the crystal is more
calcium-rich than the edges.
Exsolution is a chemical unmixing of the solid
crystal during cooling. It occurs by diffusion of
ions within the solid (but still hot) crystal,
because the mineral structure readjusts and
contracts during cooling. It becomes less tolerant
of impurities that have the wrong size.
Note where
complete solid
solution occurs
(shaded area)...
There is a big gap
between anorthite
and K-feldspar.
There is a greater
size difference
between Ca2+ and
K+, and between
Al3+ and Si4+, than
between Na+ and
Ca2+ (their C.N.=9).
T1, T2 (tetrahedral) sites become different during
cooling because Al3+ and Si4+ start to occupy different
tetrahedra within each unit cell as the structure shrinks.
The 2/m symmetry decreases to bar 1
In alkali feldspars, the
movement of Na and K
within the hot mineral
gives rise to perthite
during cooling. The result
of unmixing is often
visible to the naked eye.
In plagioclases,
the movement of
Na and Ca is
slowed down by
the Al-Si exchange
(Al-O and Si-O
bonds are harder Submicroscopic unmixing gives
to break).
rise to iridescence.
sanidine: the monoclinic K-feldspar which may
crystallizes from magma, inverts to orthoclase (and
eventually microcline) if cooling is slow. Monoclinic
sanidine is only preserved in some lavas or at the
rapidly cooled margin of an intrusion. Its Si and Al
are evenly distributed among all tetrahedra.
This diagram
spans a wider
temperature
range than those
in your book (Fig.
5.11, p. 96).
NOTE: If there is
no loop between
the liquid and
crystals, the
mineral will not
develop concentric
zoning even at fast
cooling rates!
High albite and
low albite are
the Naequivalent of
orthoclase and
microcline. They
show the same
tendency for Al
and Si to
become better
ordered among
the tetrahedra
during cooling,
but high and low
albite are
triclinic.
monoclinic
triclinic
triclinic
triclinic
As a result, albite (and
most plagioclase) grow with
polysynthetic twinning
even at high temperature
because their structure is
triclinic. (010) is the
pseudo-monoclinic twin law.
Microcline grows
directly as triclinic
from hydrothermal
waters (< 300
degrees), and shows
twinning by symmetry
elements of the
monoclinic system.
Ease of weathering often correlates with the ionic
character of the bonds. In the feldspars, this is not
so obvious.
In theory, the Ca-rich plagioclase has stronger Ca-O
bonds than the Na-O or K-O bonds of alkali feldspars
(Ca-O bonds are shorter, charge on Ca+2 is twice that
of K+. The weaker link of Ca-rich feldspars may be
the Al-O bonds, which are twice as abundant in Cafeldspars as in alkali feldspars. Al-O is longer than
Si-O, and Al3+ has a lesser charge than Si4+ .
Surprisingly, the most stable feldspar in a weathering
environment is K-feldspar (yet K-O bonds are longer
than Na-O bonds.)