Introduction: orthoclase belongs to the alkali feldspar family and may contain up to 15% albite (NaAlSi₂0₆) component (expressed as Or₈₅). Although alkali feldspars form a complete solid solution at magmatic crystallization temperatures, on cooling the sodic and potassic components in potassium-rich feldspar ‘unmix’ to form orthoclase microperthite. At even lower temperatures orthoclase can convert to microcline, which is triclinic. Orthoclase is most commonly found as a rock-forming mineral in igneous rocks, particularly in granitoids and pegmatite and is also characteristic of high grade metamorphism of pelite.

Occurrence in Wales: orthoclase is probably widely developed in Wales, although specific references to it in the literature are relatively few; most commonly reference is made just to the presence of ‘alkali feldspar’ . In many instances this may be because there is uncertainty as to the structural state of the feldspar (microcline or orthoclase). Documented occurrences are listed below.

Last saved full search

Alphabetical search:

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z