Orthoclase Feldspar Gemstone & Information | Gemopedia by JTV | Gemopedia™

Orthoclase Feldspar

While feldspar is one of the most abundant mineral families in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, as they often possess rare optical effects. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire. Some varieties are commonly encountered and many are seen in granites, for example, "Black Pearl" granite countertops are actually composed mostly of feldspar. While orthoclase, a potassium-rich feldspar, is better known to collectors, there is one variety that many are familiar with--moonstone, prized for its shimmering adularescence.

Colors

Colorless, White, Green, Orange, Yellow To Brown, Gray

  • Orthoclase-feldspar classification

    CommonName
    Orthoclase Feldspar
    Species
    Orthoclase
    Variety
    Colors
    Colorless, White, Green, Orange, Yellow To Brown, Gray
    AlternateNames
    Adularia, Moonstone, Sunstone
    GemstoneGroups
    Feldspar
    KeySeparations
    refractive index, birefringence, optic character and possibly spectrum and fluorescence
    ClassificationComments
    Feldspar is a group of minerals of which orthoclase is one species. This information is illustrative of the yellow or orange-brown transparent faceted material depicted above, not necessarily other species and varieties within the group. Orthoclase feldspar can display a 4 rayed star.Adularia: colorless variety sometimes with adularescence.Moonstone: the semitransparent to opaque variety with white or blue adularescence. Sunstone: the semitransparent to opaque orange to brown variety that displays aventuresence.
  • Orthoclase-feldspar chemistry & crystallography

    ChemicalName
    potassium aluminum silicate
    ChemicalFormula
    KAlSi3O8
    Synthesis
    CrystalSystem
    Monoclinic
    ChemistryClassification
    Silicate
    Nature
    Natural
    Crystallinity
    Crystalline
    ChemistryComments
  • Orthoclase-feldspar optical properties

    Transparency
    Transparent - Opaque
    Dispersion
    Strength: weak fire Value: 0.12
    OpticalComments
    RefractiveIndex
    1.518-1.526
    Tolerance:(+0.010/-)0.010)
    Birefringence
    0.005-0.008
    OpticCharacter
    Biaxial
    OpticSign
    Negative
    PolariscopeReaction
    Doubly Refractive (DR)
    Fluorescence
    SWUV: transparent yellow: inert to weak reddish orange
    LWUV: transparent yellow: inert to weak reddish orangesemitranslucent white: red
    CCFReaction
    Pleochroism
    Unobservable
  • Orthoclase-feldspar characteristic physical properties

    Hardness
    6-6.5
    CharacteristicComments
    Streak
    White
    SpecificGravity
    2.55-2.61 Range:+/-0.03 Typical:2.58
    Toughness
    Poor
    Inclusions
    Orthoclase feldspar might display centipede like inclusions. Material with aventuresence or asterism might contain reflective inclusions such as fine needles or elongated hematite platelets.
    Luster
    Vitreous
    Stability
    Fracture
    Uneven, Splintery
    Cleavage
    Perfect, in two directions