Orthoclase
Member of Feldspar Group
Feldspars form complex isomorphous series between the endmembers albite and anorthite on the one hand and the endmembers albite and orthoclase on the other hand. Members of the albite-anorthite series are summed up under the term plagioclase. Members of the albite-orthoclase series are termed alkali feldspars.
Classical moonstones are orthoclase feldspars.
In the gem trade one occasionally encounters yellow, transparent orthoclase, mostly from Madagascar
Origin of name: originally named orthose by French mineralogist Rene Just Haüy in 1801, after Greek ὀρθός orthos, meaning "straight" or "right" in allusion to the fact that orthoclase has two cleavage directions at right angles to each other. Haüy identified the mineral as feldspar but did neither state the type locality nor its composition.
In 1823 it was renamed orthoclase (from ὀρθός orthos see above and κλάσις klasis meaning cleavage or fracture) by German mineralogist Johann Friedrich August Breithaupt.
Synonyms and trade names: none
Can be confused with: other transparent yellow gems like citrine, scapolite, beryl, chrysoberyl and yellow sapphire
Localities: orthoclase is petrogenetic (rock forming) and one of the most common minerals. However, gemmy orthoclase is very rare and almost exclusively found in Madagascar.
Rare green orthoclase from Madagascar
Handling: orthoclase cleaves perfectly in two directions and is quite brittle. Thus it must be handled and worn with care. Sensitive to heat, acids and brines. Do not clean ultrasonically, no galvanic bath!
Worth knowing: orthoclase is the reference stone for hardness 6 on Mohs scale of hardness
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