Properties of silicate minerals

1
Properties of silicate minerals
Controls on density, hardness, cleavage and
resistance to weathering in nesosilicates
2
Specific gravity (G) tends to decrease from
nesosilicates to tectosilicates, as the degree of
tetrahedral polymerization increases. G mineral
formula (name) 3.27 Mg2SiO4 (olivine
forsterite) 3.58 Mg3Al2(SiO4)3 (garnet
pyrope) 3.2 CaMgSi2O6 (diopside
pyroxene) 3.0 Ca2Mg7Si8O22(OH)2 (amphibole
tremolite ) 2.86 KMg3(AlSi3)O10(OH)2 (mica
phlogopite) 2.54 KAlSi3O8 (feldspar) 2.32 SiO2
(cristobalite) 2.65 SiO2 (quartz)
3
Molar proportions of cations to oxygens First
number cations in C.N.gt 4 Second number cations
in C.N. 4 (network) Third number oxygen anions
Mg2SiO4 (olivine forsterite) 2 1
4 Mg3Al2(SiO4)3 (garnet pyrope)
5312 CaMgSi2O6 (pyroxene diopside ) 22
6 Ca2Mg5Si8O22(OH)2 (amphibole) 7822 KMg3(AlSi
3)O10(OH)2 (phlogopite) 4410 KAlSi3O8
(feldspar) 148 SiO2 (quartz) 012
4
Mg2SiO4 2 1 4 241248 Mg3Al2(SiO4)3
5 312 201248 CaMgSi2O6 2 2 6
161648 Ca2Mg5Si8O22(OH)2 7 822
151748 KMg3(AlSi3)O10(OH)2 4410
191948 KAlSi3O8 (feldspar) 148
62448 SiO2 (quartz) O12 02448
In these minerals, both Al and Si are found in
tetrahedra that are sharing corners, so they are
counted as network cations.
5
Mg2SiO4 2 1 4 241248 Mg3Al2(SiO4)3 5
312 201248 CaMgSi2O6 2 2
6 161648 Ca2Mg5Si8O22(OH)2 2 7
822 151748 KMg3(AlSi3)O10(OH)2 4410 1919
48 KAlSi3O8 (feldspar) 148 62448 SiO2
(quartz) O12 02448
From neso- to tectosilicates, the direct sharing
of oxygen anions among tetrahedra increases. The
valence need of oxygen anions are met
increasingly by Si4 or Al3 ions as you go down
the list.
6
Mg2SiO4 2 1 4 241248 Mg3Al2(SiO4)3 5
312 201248 CaMgSi2O6 2 2
6 161648 Ca2Mg5Si8O22(OH)2 9
822 191748 KMg3(AlSi3)O10(OH)2 4410 1919
48 KAlSi3O8 (feldspar) 148 62448 SiO2
(quartz) O12 02448
Reciprocally, the proportion of other cations
drops from nesosilicates to tectosilicates. Note
their fast drop, much faster than the increasing
proportion of cations (Al, Si) in tetrahedral
coordination!
7
viMg2SiO4 2 1 4 241248 viiiMg3viAl2(SiO4)
3 5 312 201248 viiiCaviMgSi2O6 2 2
6 161648 viiiCa2viMg5Si8O22(OH)2 7 822
191748 x-xiiKviMg3(AlSi3)O10(OH)2 4410 1919
48 KAlSi3O8 (feldspar) 148 62448 SiO2
(quartz) O12 02448
Other cations have lower charges (and higher
C.N.) than Si4 or Al3 ions in the structure.
More moles of these other cations are needed to
meet the valence requirement of oxygen anions
that are no longer shared by two Si4 or Al3
ions.
8
Mg2SiO4 2 1 4 241248 Mg3Al2(SiO4)3 5
312 201248 CaMgSi2O6 2 2
6 161648 Ca2Mg5Si8O22(OH)2 7 822
191748 KMg3(AlSi3)O10(OH)2 4410 191948 K
AlSi3O8 (feldspar) 148 62448 SiO2
(quartz) O12 02448
This affects G, because cations with lower charge
and higher C.N. sit in polyhedra that tend to
share more edges and faces than cations with
higher charge and lower C.N. These ions pack
efficiently.
9
In quartz (left) and cristobalite (right), SiO4
4-tetrahedra share all corners. There is wasted
space between these polyhedra, where small
cations could fit. Cristobalite (G 2.2-2.32)
has fewer but larger cages than quartz (G 2.65).
10
Zeolites (G 2.0-2.4) have particularly large
cages within their tectosilicate network, where
large cations or H2O fit..
Stilbite NaCa2Al5Si13O3614H2O
The same is true of feldspars KAlSi3O8.
11
Contrast the tectosilicate structures with the
structure of the garnet group. Most of the space
between the tetrahedra is packed with cations in
6- and 8-fold coordination. viiiA3viB2(ivSiO4) 3
These ions viiiA2 , viB3 with lower charges and
higher C.N. than ivSi4 are in larger polyhedra
that can share edges or faces. This leads to more
efficient packing and higher G values.
12
Specific gravity (G) tends to decrease from
nesosilicates to tectosilicates, as cations with
high C.N. are replaced by cations with lower
C.N. G mineral formula (name) 3.58
Mg3Al2(SiO4)3 (garnet pyrope) 3.27 Mg2SiO4
(olivine forsterite) 3.2 CaMgSi2O6 (diopside
pyroxene) 3.0 Ca2Mg5Si8O22(OH)2 (amphibole
tremolite ) 2.86 KMg3(AlSi3)O10(OH)2 (mica
phlogopite) 2.54 KAlSi3O8 (feldspar) 2.65 SiO2
(quartz) 2.32 SiO2 (cristobalite) 4.35 SiO2
(stishovite but Si4 has C.N. 6)
13
HARDNESS and CLEAVAGE cannot be evaluated solely
from a mineral formula. One must consider the
strength of bonds and the directions along which
strong bonds are dominantly oriented. Sometimes,
hardness reflects the presence of the weakest
bonds, but only these weaker bonds are
concentrated along a given plane throughout the
structure.
14

• CLEAVAGE reflects either
• Planes across which the weakest bonds are found
  (in micas, K-O bonds between the sheets)
• Planes held by a lower density of bonds (strong
  or weak doesnt matter as in diamond, where
  octahedral cleavage planes are held by a lower
  number of strong C-C bonds per area)
• AND
• bonding character that is dominantly ionic or
  covalent (rather than metallic)

15
Some bonds that might be expected to be fairly
ionic or covalent turn out to show a strong
polarization of the electron cloud. This is
often seen in bonding between a relatively small,
highly charged ion next to a large, weakly
charged ion. NaCl ionic, good cleavage NaI,
AgCl sectile solids with poor cleavage. (Easier
to cut it with a knife than to cleave a bit like
a metallic solid).
16

• Most nesosilicates are rather hard (H 6 or
  more) and show poor cleavage because
• their structures distributes the weaker bonds
  (e.g., Mg-O) fairly evenly among stronger bonds
  (e.g., Si-O, Al-O)
• 2) frequent edge sharing among the larger
  polyhedra gives high density of bonds across most
  planes.

17
Zircon formula Zr4Si4O4 Zr4 (high charge)
(rather large cation, C.N.8) forms a moderately
strong Zr-O bond (e.v. valence/C.N. 4/8
0.5 ) Si-O bonds are strong (e.v. 4/4 1
) ZrO8 polyhedra share many edges. Every
direction meets a high density of bonds. How
many cations does each oxygen bond to? Answer
one Si-O bond and two Zr-O bonds 2 e.v.
18
Zircon, ZrSiO4 tetragonal ZrO8 polyhedra share
edges with each other and with the SiO4
tetrahedra.
H 7.5 G 4.68 poor 010 cleavage
Zr electronegativity 1.33 between those of Mg
(1.2), Al (1.5) somewhat ionic.
19
Minerals which, like zircon, are heavy and
resistant to weathering, are found in the sandy
sediment of streams. This includes several
nesosilicates. Minerals which form at relatively
high pressures, in the upper mantle (Mg-rich
garnet), are used as indicators for the
prospection of diamonds. This is how some of the
diamond deposits that are now exploited in
Nunavut and northern Quebec were discovered
There were grains of high-pressure minerals in
sands that came from the kimberlite pipes.
20
These characteristics (H?6, poor cleavage) make
many nesosilicates desirable gemstones, at least
in specimens of remarkable colour and
transparency.
Peridot (gem-quality olivine)
Garnet
Topaz (careful, that one cleaves easily)
Zircon
21
Olivine structure viMg2SiO4 M1 and M2 octahedra
share edges G3.27, H6.5, conchoidal fracture
chrysotile Mg3Si2O5(OH)4
22
Olivine in basalt is usually quite close to
forsterite, Mg2SiO4 This makes it quite
susceptible to weathering, or hydrolysis when it
comes in contact with H2O.
One common result is chrysotile, Mg3Si2O5(OH)4
This is the origin of the chrysotile asbestos of
Thetford Mines.
23
The garnets viiiA3viB2(ivSiO4) 3 have no specific
plane of weakness Conchoidal Fracture H
6.5-7.5 The weaker bonds are equally distributed
among stronger bonds.
In addition, the high density of packing (a lot
of edge sharing) offsets the longer weaker bonds
that one might expect from having viiiA2 ions
(small charge, high C.N.).
24
Topaz Al2SiO4(F,OH)2 H 8, G 3.4
Axes a vertical, c horizontal (E-W)
Chains of AlO4F2 octahedra along the c axis
(horizontal). The 001 cleavage (vertical)
breaks only Al-O and Al-F bonds, no Si-O bonds.
25

• Maany minerals including nesosilicates ( in list
  below) are used as abrasive materials... to clean
  graffitis off walls, strip old paint from
  surfaces prior to repainting, or polish wooden
  furniture.
• silica sand (silica is cheap, but its dust is
  carcinogenic)
• hematite
• aluminum oxide
• (synthetic, slag)
• staurolite
• olivine
• garnet
• diamond

26
Weak bonds affect the hardness of a mineral if
they are predominantly holding planes in one
dominant direction. K-O bonds, for example, are
present in feldspars and in micas, two mineral
groups with very different hardnesses. In each
structure, K-O bonds are the longest, weakest
bonds (K cation is quite large and has a low
charge).
27
In feldspars, the weak K-O bonds are in cages
surrounded by a strong tectosilicate network.
K ions have little effect on feldspar
hardness, but result in good cleavage where
planes cut across a lesser density of Al-O and
Si-O bonds.
28
In micas, the K-O bonds are all at the edge of
sheets of SiO4 tetrahedra, and hold adjacent
sheets together. There are (001) planes parallel
to the sheets which cut only these weak bonds.