Feldspar

Most abundant of all the minerals..

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.

Classification of Minerals

Common Silicate mineralsFeldspar 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

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.

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.

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 O8 Monoclinic

Orthoclase KAl Si3 O8 Monoclinic

Microcline KAl Si3 O8 Triclinic

Soda- Lime Feldspar

Plagioclase Series (Na,Ca) (Al, Si) Al Si2 O8 Triclinic

Potassium feldsparNote Pearly Luster!

KAlSi3O8

Plagioclase feldsparNote the Twinning, seems to have ‘stripes’

(Ca,Na)AlSi3O8

Look for some Labradorite over the break

Minerals in Granite Rock: What is the pink stuff?

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.

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.

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.

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

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.

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.

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