I was studying about feldspar and got me confused one thing
In the picture down is saying that moonstones can be labradorite or orthoclase and also says that orthoclase has no twinning.
How come some moonstones can be orthoclase if moonstones are characterized by adalurescence effect which just happens because of the twinnings inside the stone?
We’re looking into the gem listings for feldspar in general as well as moonstone to try and clarify this. We’ll be back with a response hopefully soon.
Hi you are right to be confused about feldspar. Yes Moonstone can be lumped in with labradorite… I do not like to do that. Moonstone is a layer cake of orthoclase and albite . Labradorite is all so a layer cake but just made of alternating layers ( along twinning plains ) of labredorite. Ok the flour for this cake is Si4O8 but Al can slip in too so Al(Si3)O8 is the most often seen ( flour ) in a feldspar cake. To flavor this cake we can use Ka and you get K spar or orthoclase. Na we get the flavor albite. Labradorite has a mix of Ca and Na. Orthoclase or K spar and albite are the ends of a solid solution ( flavor mix ) called alkai feldspar . Labradorite with its mix of Ca and Na is near the center of the Plagioclase feldspar solid solution.
Orthoclase has twinning plains but they are not the reason for the play of light in Moonstone it is the layers of albite and orthoclase.
As the chemistry changes so does the crystallography. The Al(Si3)O8 wants to stay the same but is (warped in shape) so the crystal shape warps this is what makes feldspar so cool.
I hope this helps Hmm now I want cake .
Hello Fufumahfouz,
The statement in the feldspar article about no twinning in orthoclase was wrong. It’s been removed. Orthoclase can indeed have twinning.
Thank you for finding this inconsistency.
Pedro
IGS
Thank you guys very much
707chrisa you deserve a piece of cake 
That was a great question. There are so many details and different minerals associated with stones that sometimes another’s inquiry clears up confusion you didn’t know was there… I believe I have many moonstones.
