Plagioclase feldspars form a complete solid solution series from pure albite (NaAlSi, Os) to pure anorthite (CaAl2Si2Og). The composition of different members is given in.

Where: Ab = albite (NaAlSi3Og) and an = anorthite (CaAl, Si, 08). Figures as subscript indicate the composition in percent. For example Ab70 An30 is equal to albite 70% and anorthite 30%. From the above table it is evident that the composition of each member of the plagioclase feldspar is variable within certain limits.

Physical properties:

(i) Form: Cleavable masses or irregular grains, lamellar and granular forms.

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(ii) Colour: Colourless, white and gray; less commonly greenish, flesh-red.

(iii) Lustre: Vitreous to pearly.

(iv) Streak: White.

(v) Hardness: 6.

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(vi) Cleavage: Perfect 2 sets.

(vii) Fracture: Conchoidal to uneven.

(viii) Specific gravity: 2.605 – 2.765.

(ix) Diaphaneity: Transparent to translucent.

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Use:

Plagioclase feldspars are used in the manufacture of porcelain and pottery, for the production of glazes on earthenware, sanitary ware enameled brick etc. It is also used in manufacture of opalescent glass, as a binder for abrasives and in facing of artificial building material.