CHEMICAL MINERALOGY REACTIONS

CERTAIN PHENOMENON OF CHEMICAL MINERALOGY
GUIDED BY
PRESENTED BY
DR. NARENDRA JOSHI
PRIYESH K VISHWAKARMA 1
GOVT. HOLKAR SCIENCE
COLLEGE

content
Solid solutions
 Substitutional solid solution
 Omission solid solution
 Interstitial solid solution
 Applied utility of solid solution
Exsolution
 Applied utility of solid solution
Isomorphism
2

3
Polymorohism
 Reconstructive polymorphism
 Displacive polymorphism
 Order-Disorder polymorphism
Pseudomorphism
 Substitution
 Encrustation
 Alteration
References

SOLID SOLUTION
Definition- A solid solution is a mineral structure in which
specific atomic sites are occupied in variable proportions
by two or more different chemical elements(or groups).
4

TYPES OF SOLID SOLUTION
1.Substitutional solid solution
2.Omission solid solution
3.Interstitial solid solutions
5

Substitutional solid solution
 SIMPLE SUBSTITUTION SOLID SOLUTION
When ions of equal charge and nearly equal size
one another, the solid solution is said to be simple.
Ex.-The mineral olivine forms a continuous solid solution
whose end members are forsterite (Mg2SiO4) and
(Fe2SiO4)
6

Fig no. 1-Simple substitution in olivine whose structure is viewed down
the a axis.The octahedral M sites between tetrahedra may be
occupied by either Mg2+ or Fe2+,The shaded wedge shown on each M
cation graphically indicates the percentage of the M sites that are
occupied by Fe2+ (22% in this case). 7

 COUPLED SUBSTITUTION SOLID SOLUTION
 Coupled substitution occurs if an ion of different charge
substituted. This results in having to make another
in order to maintain charge balance.
 Such coupled substitution is common in the silicate
where Al+3 substitutes for Si+4 in tetrahedral (C.N. = 4)
 Ex.-The mineral plagioclase is a good example. The end
members are albite (NaAlSi3O8) and anorthite (CaAl2Si2O8)
8

Fig no. 2-Coupled substitution in plagioclase shown
schematically.To maintain charge balance, substitution of Ca2+
for Na+ is balanced by substitution of Al3+ for Si4+.
9

Omission solid solution
 Omission solid solution occurs when an ion of higher
charge substitutes for an ion of lower charge. In order to
maintain charge balance, two of the lower charged ions
will be replaced, but the higher charged ion will occupy
only one site, thus the other site will become vacant, or
omitted.
 An example of this type of solid solution is found in the
blue-green variety of microcline, in which a Pb2+ ion
replaces 2K+ ions. One of the K+ sites is replaced by the
2+
10

Fig no. 3-Omission substitution of 3Fe2+ 2Fe3+ + in pyrrhotite shown
schematically.
11

Interstitial solid solution
 In some crystal structures there are sites that are not
normally occupied by ions. These are considered voids.
However, when an ion does occupy one of these voids it is
called interstitial solid solution.
 Ex.-The mineral Beryl (Be3Al2Si6O18) is constructed of rings
of silicon tetrahedra that are stacked atop each other to
form channel like cavities in the centre of the rings.
12

Fig no. 4-Interstitial ionic substitution in beryl. Insertion of large
monovalent cations (K+, Na+, Rb+, etc.) in the open channel was defined by
the ring of silicon tetrahedra is balanced by substitution of Al3+ for Si4+ in the
13

FACTORS DETERMINE AMOUNT OF SOLID SOLUTION
1. The size of the ions and the size of the crystallographic
sites into which they substitute.
2. The charges on the ions that are substituting for one
another.
3. The temperature and pressure at which the substitution
takes place.
14

Applied utility of solid solution
 Solid solution alloys show many desirable properties such as
increase in strength and modulus, change in density
depending on the density of the solute element ,and
elimination of undesirable second phases.
 Uniform precipitation of fine second phase particle in a
metallic matrix has long been known to increase the strength
and hardness of precipitation hardening alloy.

16
Fig no 5-The building blocks of 3D Zeolite
structure

17
Fig no 6-Zeolites open frame structure is used as cage for nuclear
waste

exsolution
The separating of an initially homogeneous mineral solid solution
into two (or more) zones of distinct mineral phases; for example,
the formation of perthite from a homogeneous feldspar during
cooling.
18
Fig no. 7- photograph (1 mm wide) of a thin section showing
exsolution in alkali feldspar

19
Some minerals show complete solid solution under one set of
temperature/pressure conditions, and only limited solid solution
under different temperature/pressure conditions.
When the conditions change to those where limited solid
solution is favored, the mineral exsolves or unmixes.
But, because the process is taking place in the solid state,
exsolution or unmixing cannot easily form two separate phases,
because the ions must diffuse through the solid.
In fact, what happens is that two separate phases form in
discrete domains within a single mineral grain. These domains
are crystallographically oriented, so they appear as lamellae or
lines across the mineral grain.

21
 At high temperature Na+ and K+ are readily substitute for each
other and solid solution extends across the complete
compositional range from albite to k feldspar.
 At low temperature ,only limited amounts of K+ may substitute
for Na+ in the albite structure , and vice versa in the K feldspar
structure.
 The range of permissible compositions as a function of
temperature is shown in figure 6.
 The curve on the diagram is called the solvus.
 Compositions of alkali feldspar to the right , left, and above the
solvus are stable, those within are not.

22
Fig no 8-The solvus for alkali feldspar

23
examples of exsolution
Fig no9- Perthite unmixing in
Microcline

24
Fig no 10– perthite intergrowth in
Orthoclase. XPL image. 10X (field of
view=2mm)

25
Fig no 11-Rapakivi texture in which rims of sodic
plagioclase mantle a phenocryst of K Feldspar in
Granite.

26
Applied utilities of Exsolution
 Exsolution is importance in Transition Metal (Co, Rh, and
Ir)-doped LaCrO3 Perovskite Catalysts for Boosting Dry
Reforming of CH4 Using CO2 for Hydrogen Production.

polymorphism
 The ability of a specific chemical substance to crystallize
with more than one type of structure(as a function of
change in temperature , pressure or both) is known as
polymorphism(from the Greek meaning ‘many forms’ ).
 The change that takes place between crystal structures of
the same chemical compound are called polymorphic
transformations.
27

Types of polymorphism
1. Reconstructive polymorphism
2. Displacive polymorphism
3. Order-Disorder polymorphism
28

Reconstructive polymorphism
 This involves extensive rearrangement of the crystal
structure and requires breaking of chemical bonds and
reassembling the atoms into a different crystal structure.
 This usually involves a large change in energy of the
structure which must occur at the transformation
temperature or pressure.
 Because of the extensive rearrangement involved, the rate
at which this type of transformation occurs may be very
slow.
 If the rate of the transformation is very slow, unstable
polymorphs (metastable) may exist for long periods of time.
29

For example, diamond is a metastable polymorph of Carbon at
the pressures and temperatures present at the Earth's surface.
31
Fig no 13-Diamond-graphite stability fields .
Diamond is stable at pressure normally found
only in the Earth’s mantle.

These involve only small adjustments to the crystal
structure.
 Generally no bonds are broken, but the angles between
the atoms may change slightly.
 Because there is little rearrangement, displacive
transformations involve no change in energy at the
transformation temperature or pressure, and the
transformations are instantaneous and reversible.
 Thus, no unstable polymorphs will occur.
Displacive polymorphism
32

33
 For example, at 1 atmosphere pressure high quartz (ß
quartz) is the stable form of quartz above 580˚C. When
high quartz is brought to a temperature below 580˚C , it
immediately is transformed into low quartz (â quartz).
Thus, high quartz is never seen in rocks at the surface of
the Earth. Fig no 14 -Structure of â quartz and ß
quartz. The top view looks down the c-axis
,the bottom view is from the side. the unit
cell is outlined.(a) ß quartz is the high
temperature polymorph (b) â quartz is the
low temperature polymorph. The inversion
from ß quartz to â quartz is accomplished by
a distortion that reduces the symmetry.

Order – Disorder polymorphism
 These involve the state of order or disorder in a
crystal structure. Perfect order can only occur at a
temperature of absolute zero
(-273˚C).
 As temperature increases, the degree of order or
randomness of a crystal structure decreases, so that
the higher temperature forms of minerals are more
disordered than the lower temperature forms.
 Because the state of order-disorder changes
gradually with increasing temperature, there is no
definite temperature at which a transformation 34

 An example of polymorphic transformations that involve
order-disorder is the compound KAlSi3O8.
 At high temperature the stable form is Sanidine
(Monoclinic). At lower temperature the structure changes
to one of orthoclase (also Monoclinic), and at even lower
temperature the structure becomes that of the more
ordered structure of microcline (Triclinic).
 There is no definite temperature at which Sanidine
changes to orthoclase or orthoclase changes to
Microcline, since the structure changes gradually as
temperature decreases.
 If the temperature change is rapid, then unstable35

36
(a) (b) (c)
25% Al,75% Si
100% Al
100% Si
Fig no 15-Order-disorder in k-feldspar (KAlSi3O8) polymorphs. (a)
Schematic view of two up-pointing tetrahedrons that contain the three
Si and one Al per formula unit. Two tetrahedrons are T1 sites, two are T2
sites. (b)High Sanidine Al is equally likely to be in any of the four sites, so
on average each contains 25% Al. (c)Maximum Microcline, Al is
preferentially placed in one T1 site, site occupies the other three sites
producing a distortion to the structure.

37
isomorphism
 It is found that certain minerals of analogous
composition crystalline in forms showing close
relationships with one another. Such minerals have
their atoms arranged on similar planes . This
phenomenon is called isomorphism.
 Two or more minerals whose atoms are arranged in
the same type of crystal structure are also said to be
isostuctural.
 Halite (NaCl) and Galena(PbS) are isostructural
because the arrangement of Pb and S in galena is
identical to the arrangement of Na and Cl in halite.

38
Galena Halite
Fig no 16 -Galena and Halite crystalize
in same structure and shows
Isomorphism

pseudomorphism
 Pseudomorphism is the existence of a mineral that has the
appearance of another mineral.
 Pseudomorph means false form.
 Pseudomorphism occurs when a mineral is altered in such a
way that its internal structure and chemical composition is
changed but its external form is preserved.
39

 Substitution - In this mechanism chemical constituents are
simultaneously removed and replaced by other chemical
constituents during alteration. An example is the replacement
of wood fibers by quartz to form petrified wood that has the
outward appearance of the original wood, but is composed of
quartz.
 Another example is the alteration of fluorite which forms
isometric crystals and is sometimes replaced by quartz during
alteration. The resulting quartz crystals look isometric, and are
Types of pseudomorphism
40

41
Fig no 17 –Petrified wood ,plant cell
substituted by Quartz

 Encrustation - If during the alteration process a thin crust of a new
mineral forms on the surface of a preexisting mineral, then the
preexisting mineral is removed, leaving the crust behind, we
say that pseudomorphism has resulted from encrustation.
 In this case the thin crust of the new mineral will have casts of
the form of the original mineral.
42

43
Fig no 18-Quartz pseudomorph after fluorite, crystal
encrustation

 Alteration - If only partial removal of the original mineral and
only partial replacement by the new mineral has taken place,
then it is possible to have a the space once occupied entirely
by the original mineral be partially composed of the new
mineral.
 This results for example in serpentine pseudomorphed after
olivine or pyroxene, anhydrite (CaSO4) pseudomorphed after
gypsum (CaSO4.2H2O), limonite [FeO.(OH).nH2O] after pyrite
(FeS2), and anglesite (PbSO4) after galena (PbS).
44

45
Fig no 19 -Alteration of pyrite into limonite

46
References
Solid solution
Introduction to mineralogy by Nesse William D. (pg no. 69-71)
Mineral sciences by Klein Cornelis (22nd edition 2002) (pg no. 90-94)
https://www.tulane.edu/~sanelson/eens211/mineral_chemistry.htm
 https://www.slideshare.net/ErPrabhakar1/solid-solutions-72263452
https://chembam.com/experiments/cages-for-nuclear-waste/
Exsolution
Mineral sciences by Klein Cornelis (22nd edition 2002) (pg no.143-148)
Mineralogy Perkins Dexter (3rd edition) (pg no. 151)
http://www.alexstrekeisen.it/english/pluto/perthite.php
http://www.alexstrekeisen.it/immagini/pluto/pertiti0185(2).jpg
http://myrmekite.blogspot.com/2006/02/introduction.html?m=1
http://www.alexstrekeisen.it/english/pluto/granophyre.php
https://petromramor.ru/index.php?lan=eng&nav=288

47
Polymorphism
Mineral sciences by Klein Cornelis (22nd edition 2002) (pg no. 134-141)
https://www.tulane.edu/~sanelson/eens211/twinning.htm
Isomorphism
 Introduction to mineralogy by Nesse William D.(pg no 65)
Pseudomorphism
Introduction to mineralogy by Nesse William D. (pg no. 92)
Mineral sciences by Klein Cornelis (22nd edition 2002) (pg no.151)
https://www.hemswell-antiques.com/antiques/antiquities/decorative-petrified-
wood-cut-cross-section-62019.html
https://therussianstone.com/products/rare-prophecy-stone-limonite-after-pyrite-
from-egypt-1-7
http://www.johnbetts-
fineminerals.com/jhbnyc/mineralmuseum/picshow.php?id=16025

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