Feldspar

1. Feldspar
Most abundant of all
the minerals..

2. Introduction
Most abundant constituents of the igneous
rocks.
Major constituents of most gneisses and
schists, and occur also in many thermally as
well as regionally metamorphosed rocks.
Second in abundance to quartz in the
arenaceous sediments.

3. Classification of Minerals
Common Silicate minerals
Feldspar Group
–Most common mineral group
–3-dimensional framework of tetrahedra
exhibit two directions of perfect cleavage at
90 degrees
–Orthoclase (potassium feldspar) and
Plagioclase (sodium and calcium feldspar)
are the two most common members
–Pearly Luster

4. They are closely related in form and physical
properties , but they fall in to two subgroups.
Potassium and barium
feldspars
Monoclinic, or very
nearly monoclinic in
symmetry.
When the cations are
very large (K, Ba) the
symmetry is monoclinic
or pseudo- monoclinic
Sodium and calcium
feldspars ( the plagioclases)
Triclinic
Smaller Cations (Na, Ca) the
structure is slightly distorted,
and the symmetry becomes
triclinic.

5. General formula may be
W Al (Al, Si) Si2 O8.
In which ‘W’ may be Na, K , Ca, Ba.
The variable (Al, Si) being balanced by variation in
the proportions of univalent and bivalent cations.
The structure of feldspars is a continuous three-
dimensional network of SiO4 and Al O4
tetrahedra, with the positively charged sodium,
potassium , calcium and barium situated in the
interstices of the negatively charged network.

6. Barium feldspars are rare and of no importance as rock-
forming minerals.
Major three components are Orthoclase, Albite and Anorthite,
and the composition of feldspar is often expressed in
percentages, An10 Ab85 Or5
Potash Feldspar
Sanidine KAl Si3 O8Monoclinic
Orthoclase KAl Si3 O8 Monoclinic
Microcline KAl Si3 O8 Triclinic
Soda- Lime Feldspar
Plagioclase Series (Na,Ca) (Al, Si) Al Si2 O8 Triclinic

7. Potassium feldspar
Note Pearly Luster!
KAlSi
3
O
8

8. Plagioclase feldspar
Note the Twinning, seems to have ‘stripes’
(Ca,Na)AlSi
3
O
8
Look for some Labradorite over the break

9. Minerals in Granite Rock: What is the pink stuff?

10. At high temperatures (above 700°C) there is a
continuous solid solution between alkali
feldspars of different compositions and the
structure of sanidine.
The more potassic members sanidine and
orthoclase, are mooclinic, sanidine being
characteristic of volcanic rocks, whereas
orthoclase is characteristic of plutonic rocks.
The more sodic members of the series are
triclinic and are called anorthoclase.

11. At lower temperatures solid solutions intermediate
between potash feldspar and albite are unstable
and under conditions of slow cooling break down
into an oriented intergrowth of subparallel
lamellae, alternately sodic and potassic in
compostion. Such and intergrowth is called
perthite or antiperthite.

12. In perthites the plagioclase occurs as uniformly oriented
films, veins, and patches witin the orthoclase (or microcline);
in antiperthites this relation is reversed.
The system Ab-An shows a complete solid solution at high
temperatures, but complications enter at low temperatures.
There is essentially no solid solution between orthoclase
and anorthite, and hence varieties intermediate between
these two components do not occur.

13. occurrence
Sanidine occurs in potash rich volcanic rocks
such as rhyolite and trachyte.
Orthoclase is the characteristic potash feldspar
of igneous rocks, occurring both alone and in
pethitic intergrowth with albite
It also occurs in metamorphic rocks.
Adularia is a variety of potash feldspar.
The name given to a form with a distinctive
crystal habit found in low temperature
hydrothermal veins

14. Microcline is the common potash feldspar of pegmatites and
hydrothermal veins.
It also occurs in metamorphic rocks.
Anorthite is usually found in contact metamorphosed
limestone
Bytwonite and labradorite are characteristic of igneous rocks
of gabbroic compositions and anorthosites.
Andesine of andesites and diorites
Oligoclase of mozonites and granodiorites.
Albite occurs in some igneous rocks but more common in
pegmatites.

15. The plagioclases are common in
metamorphic rocks; in low-grade schists and
gneisses the plagioclase is usually albite
In medium grade rocks usually oligoclase
and andesine.

16. THANK YOU……………..