silikat mineral

1. SILICATE MINERALS
Prepared by Dr. F. Clark,
Department of Earth and Atmospheric
Sciences, University of Alberta
Sept. 05

2. ISOLATED SILICATES [Nesosilicates]
In this group, silicon tetrahedra share no oxygen anions
with other tetrahedra, and so have an excess negative
charge of 4-. In the mineral olivine, this is balanced by
the insertion of a pair of divalent cations in the crystal
structure, either or both of Mg2+ and Fe2+. The chemical
formula for olivine is written (Mg,Fe)2SiO4, which tells us
that for every silicon, there are four oxygen and two
cations, either or both of Mg and Fe. This option, which
illustrates ionic substitution, is indicated by separating
these elements by a comma within the parentheses. We
have variation within fixed limits, those limits being 100%
Mg and 100% Fe, or any proportion in between.

3. OLIVINE
These three
specimens are
of an igneous
rock consisting
almost
exclusively of
crystals of
olivine that are
approximately
1mm across.
Olivine exhibits its classic glassy, olive green appearance in these
specimens, as well as its common granular [somewhat like sugar]
habit. There is no cleavage, only conchoidal fracture, so that there are
no plane surfaces reflecting light.

4. SINGLE CHAIN SILICATES
[Inosilicates]
In the single chain silicates, each silicon tetrahedron shares
two oxygen anions, one with one neighbouring
tetrahedron, and one with another, to produce long,
strongly bonded chains. Each shared oxygen accounts
for only 1- rather than the usual 2-, so that for each
silicon tetrahedron, the excess negative charge is now
only 2-, which still requires insertion of cations in the
crystal structure. These cations are bonded to, and serve
to link, the chains, but these bonds are weaker than
those within the chains. The single chain silicates thus
cleave parallel to the chains, along two planes that meet
at approximately 90 degrees.

5. Pyroxene [e.g. augite]
In these three views of two specimens, the upper face and left side
vertical face meet at right angles, a common characteristic of the single
chain silicates. Note how irregular the faces are on the two images on
the right, yet how these small steps are parallel to each other. The
hardness, around 5 ½ to 6, white streak, and typical dark colour make
this otherwise very similar to amphibole, a double chain silicate. The
square cross-sections of pyroxene crystals distinguishes them from
amphiboles.

6. DOUBLE CHAIN SILICATES
[Inosilicates]
As with single chain silicates, chains are constructed by
sharing of two oxygen for each silicon tetrahedron. The
double chains are constructed by having every second
silicon along the chain share a third oxygen with a silicon
from the facing chain. The net result is that on average,
each silicon shares 2 ½ oxygen, so the excess negative
charge per silicon is reduced to 1 ½. Cations serve to
balance charge and link the strongly constructed double
chains, whose extra width causes the cleavage planes to
change orientation and meet at approximately 60 and
120 degrees, producing hexagonal cross sections.

7. Amphibole [e.g. hornblende]
In the double chain silicates, the extra width of the double chain skews
the intersection angle between cleavage faces, so that they meet to
form hexagonal cross sections to the crystals, as highlighted by the
yellow lines in the right-hand image. In this case, we are sighting along
the length of the chains. Other major properties are as for pyroxene.

8. SHEET SILICATES [Phyllosilicates]
In this group, each silicon tetrahedron shares three oxygen
anions with neighbouring tetrahedra, so that the net
negative charge per silicon is now 1-. This produces a
kind of hexagonal honeycomb sheet, in which all
tetrahedra point in the same direction. This enables
these layers to bond with layers of cations at the centres
of octahedra with oxygen as the apices. There is great
variety in the combinations that are possible. However,
there is an asymmetry to the charge distribution which
leads to net surface charges on the sheets, which are
then weakly bonded to each other by cations. This leads
to the characteristic property of this group – one perfect
cleavage, parallel to the sheets.

9. Biotite
As with all mica group minerals within the sheet silicates, biotite cleaves
readily to produce flexible cleavage flakes whose surface has significant
reflectance, such that small flakes or crystals within a rock typically glint.
It is soft as well, and not readily confused with anything else. You may
rely on the colour to be this consistent, almost black or very dark brown
shade, to distinguish from other micas, such as muscovite.

10. Muscovite
Soft, and with flexible, highly reflective cleavage flakes, the mica group
mineral muscovite is distinguished consistently and reliably from the
darker cousin biotite by its clear to silvery colour. Muscovite contains
aluminum, whereas biotite has iron and/or magnesium in the same site
in the crystal struture, which accounts for the consistent colour
difference.

11. Chlorite
Belonging to a
different group
of sheet
silicates than
the micas,
chlorite has
brittle rather
than flexible
cleavage
flakes.
Chlorite is also soft and readily flakes along its perfect cleavage, and is
highly reflective as well. Its characteristic dark green colour imparts a
green tone to the rocks which most typically contain it – low grade
metamorphic rocks that are called greenschists.

12. FRAMEWORK SILICATES
[Tectosilicates]
Finally, all four oxygen are shared, each one with a
different silicon tetrahedron, which eliminates the excess
negative charge, given the basic formula SiO2 (the two
oxygen are in effect four ½ oxygen, each being shared).
One might therefore expect the framework silicates to be
the simplest group to deal with, but complexity is
introduced in the feldspar group, as we shall soon see.

13. Quartz
Among the most common rock-forming minerals, quartz is also among
the easiest to identify. With a hardness of 7, it is not scratched by a
knife blade, but ends up with a thin streak of metal on its surface. Most
commonly it has a somewhat dull, grey glassy appearance. It has no
cleavages to produce plane reflecting surfaces when incorporated in
rocks (see right image), but rather exhibits conchoidal fracture. Its
characteristic habit is as hexagonal prismatic crystals (see left view)
with pyramid terminations, seen in the specimen under the scale bar in
the left image, and in the middle image.

14. Feldspar Group – Potassium Feldspar
In the feldspars, we see coupled ionic substitution, rather
than the simple substitution exhibited by olivine. By
virtue of its size, Al3+ fits between the oxygen anions of
the tetrahedra in place of Si4+. Of course, this introduces
a positive charge deficiency. Statistically, either one out
of every four tetrahedra, or two out of every four
tetrahedra, may have a silicon cation replaced by
aluminum (any more than that cannot be accommodated
by the crystal structure). In the case of potassium
feldspar, one out of every four tetrahedra has aluminum,
and the charge deficiency is balanced by insertion of a
potassium (K+) cation.

15. Potassium Feldspar
The most common variety of potassium feldspar is orthoclase, number
6 on Mohs hardness scale. Although it is commonly a salmon pink
colour, this is not a diagnostic feature (see plagioclase feldspar images
to confirm this point). It has two cleavages that meet at right angles, to
produce square edges as seen in these specimens. Streak is white.
This mineral may have simple twinning, but never exhibits the multiple
twinning that plagioclase feldspar may show.

16. Potassium Feldspar
This specimen is included to emphasize the fact that one can not say
with confidence that potassium feldspar is pink, and plagioclase white,
although this is often the case. In the left-hand image, the upper face
and lower left faces are cleavages, and in the right-hand image, the
upper face and shaded lower right face are cleavages. Note again that
cleavages tend to be expressed in a somewhat discontinuous fashion.

17. Feldspar Group – Plagioclase Feldspar
Explanation of the plagioclase feldspars carries on from
potassium feldspar. The substitution of one Al3+ for Si4+
could also be balanced by Na+. This is albite, the sodium
plagioclase feldspar. If we substitute two Al for Si out of
every four Si, the charge deficiency of 2+ is balanced by
Ca2+, and we have anorthite. The ionic radii of Na+ and
Ca2+ are almost identical, so the two freely substitute,
along with Al3+ for Si4+, to produce the plagioclase feldspar
solid solution series. One might expect there to be free
substitution between albite and potassium feldspar, but
because the ionic radius of potassium is approximately
40% larger than that of sodium, such substitution is
limited to elevated temperatures.

18. Plagioclase
Feldspar
This mineral
has many
properties in
common with
potassium
feldspar, which
we emphasize
on this slide.
The hardness is 6, colour is variable, including salmon pink as seen
here, and the streak is white. Two excellent cleavages meet at right
angles (upper and left-facing surfaces in the image above). However,
plagioclase feldspar may have multiple (or polysynthetic) twinning
striations, as seen on the upper face parallel to the red arrow.

19. Plagioclase Feldspar
This reoriented specimen exhibits the twinning striations more clearly,
parallel to the blue arrows. Resembling very fine scratches, they
represent the intersection between twin planes and the upper surface of
the crystal, and are flush with that surface. They are not seen on the
faces marked with blue stars, because they are parallel to those faces,
but could be seen on the faces highlighted by green arrows.

20. Plagioclase Feldspar
The same crystal yet again conveys the idea that the twinned crystal
consists of slices with alternating orientation of the crystal structure. As
a result, some slices catch the light in such a way as to reflect it, and
others do not, showing up salmon pink rather than being washed out.
We stress that the striations are an optical effect produced by the fact
that crystal structure controls the interaction of light with a specimen.

21. Plagioclase Feldspar
This specimen illustrates the variability in colour exhibited by feldspars.
Twinning striations are visible on the upper surface, parallel to the red
arrows. Because the twin planes are parallel to the right side face in the
right side image, they could not be seen there, but hypothetically could
be seen on the lower, shaded face in that image. The irregularity of this
fracture surface makes this most unlikely in practice.

22. Plagioclase Feldspar
This white specimen, with twinning striations running parallel to the red
arrows, illustrates the fact that portions of the twinned crystal are not
necessarily all of the same thickness, although they tended to be nearly
so in the specimens in the earlier slides. Note the broad uniform band,
almost 1 cm wide, sandwiched between twins whose planes are less
than 1 mm apart.