isomorphism and polymorphism is related to mineralogy

1. ISOMORPHISM
The term isomorphism is applied to the
phenomenon of substances with analogous
formulas having closely related crystal
structures. The term was introduced by
Mitscherlich in 1819, who prepared crystals of
KH2PO4, KH2AsO4, (NH4)H2PO4 and
(NH4)H2AsO4 and found that they showed the
same forms and the interfacial angles between
corresponding faces were very similar.

2. ISOMORPHISM
Isomorphism is widespread among minerals
and is one of the bases of their classification.
Many isomorphous groups are recognized, for
example, the spinel group, the garnet group,
the amphibole group. The basis of the
phenomenon is that anions and cations of the
same numbers tend to crystallize in the same
structure type. This is well exemplified by some
of the carbonate minerals. The anhydrous
carbonates of the bivalent elements form two
isomorphous groups, one orthorhombic and
one trigonal.

3. ISOMORPHISM
Other substances with analogous formulas are
isomorphous with these carbonates. Thus soda niter
NaNO3, is isomorphous with calcite, whereas niter,
KNO3 is isomophous with aragonite, reflecting the
similar size of nitrate and carbonate groups and the
larger size of potassium ion as compared to sodium
ions. The borates of trivalent elements show similar
relationships. Until the development of X-ray
techniques for the determination of crystal structures,
it was somewhat of an enigma that substances as
different chemically as calcite and soda niter could
show complete similarity in crystal form.

4. ISOMORPHISM
Other isomorphous pairs at first sight do not even
have analogous formulas. Thus, the rare mineral
berlinite (AlPO4) is isomorphous with quartz; the true
analogy is seen when the formulas of quartz is written
SiSiO4. Both Al and P are similar in ionic size to Si
and can exist in a crystal structure in four-
coordination with oxygen; as a result AlPO4 can
crystallize with the same structure as quartz.
Similarly, tantalite, FeTa2O6, is isomorphous with
brookite, TiO2(TiTi2O6); the metallic ions are similar in
size and all show six fold coordination with oxygen.

5. ISOMORPHISM
Isomorphs
Minerals with the same structure and different
compositions are isomorphs.
– Forsterite (Mg2SiO4)- fayalite (Fe2SiO4)
– Halite (NaCl) - sylvite (KCl), periclase (MgO),
galena (PbS)
– Gold (Au) - silver (Ag)
– Quartz (SiO2) - berlinite (AlPO4)
– Muscovite-Biotite
Sometimes there’s crystalline
solution, sometimes not.

6. ISOMORPHISM
Polymorphs and Isomorphs
• Polymorph:
same composition, different structure
• Isomorph:
same structure, different composition

7. POLYMORPHISM
An element or compound that can exist in more than one
crystal form is said to be polymorphous. Each form has
different physical properties and a distinct crystal structure;
that is, the atoms or ions are arranged differently in different
polymorphs of the same substance. Polymorphism is an
expression of the fact that crystal structure is not exclusively
determined by chemical composition. Different polymorphs of
the same substances are found under different conditions of
pressure, temperature and chemical environment; hence the
presence of one polymorph in a rock will often tell something
about the conditions under which that rock was formed. For
example, marcasite can be formed only from acid solutions at
temperatures below 450°, and the presence of marcasite in
deposit thus puts some limits on the conditions of origin.

8. POLYMORPHISM
Two types of polymorphism are recognized,
according to whether the change from one
polymorph to another is reversible and takes
place at a definite temperature and pressure.
The first type is known as enantiotropy and is
exemplified by the relationship between quartz
and tridymite (quartz tridymite).

9. POLYMORPHISM
The second type is known as monotropy; an
example is the marcasite-pyrite relationship, in
which marcasite may invert to pyrite but pyrite
never changes to marcasite. With monotropic
polymorphs one form is always inherently
unstable and the other inherently stable; the
unstable form always tends to change into the
stable form, but the stable form cannot be
changed into the unstable form without first
completely destroying its structure by melting,
vaporization, or solution.

10. POLYMORPHISM
Polymorphs
Two minerals with the same composition, but
different structures are different minerals.
- Diamond and graphite are both pure carbon,
but are different minerals
- Quartz-tridymite-cristobalite-coesite
stishovite: all polymorphs of SiO2.
- Calcite and aragonite are CaCO3.
This is why we use mineral names rather than
chemical formulas.

11. POLYMORPHISM
Quartz SiO2 Hexagonal Common in igneous,
metamorphic, and sedimentary
rocks
Tridymite SiO2 Hexagonal A high-temperature, low pressure
SiO2 polymorph; certain volcanic
rocks.
Crystobalite SiO2 Isometric A high-temperature, low pressure
SiO2 polymorph; certain volcanic
rocks.
Coesite SiO2 Monoclinic A high pressure polymorph; deep
crustal and mantle rocks
Stishovite SiO2 Tetragonal A high pressure polymorph;
meteorites and impact
metamorphism
Opal SiO2-nH2O amorphous Forms by replacement or direct
spheres in precipitation in sedimentary and
hcp hydrothermal environments.

12. POLYMORPHISM
polymorphism, of minerals, property of
crystallizing in two or more distinct forms.
Calcium carbonate is dimorphous (two forms),
crystallizing as calcite or aragonite. Titanium
dioxide is trimorphous; its three forms are
brookite, anatase (or octahedrite), and rutile.
Polymorphism of an element is called allotropy.
The process was discovered (1821) by Eilhard
Mitscherlich.

13. ISOMORPHISM

14. ISOMORPHISM

15. ISOMORPHISM