Smith College - Geology 222b - Petrology

Petrographic Data File

Leucite
Property
Value
Comments
Formula KAlSi2O Feldspathoid
Crystal System Tetragonal (Psuedoisometric) Above 625 degrees Celcius, the structure is isometric; below that temperature it is tetragonal.
Physical Properties White streak; vitreous luster; H=6; G= 2.47  
Occurrance Uncommon. Found in potassium-rich rocks low in silica content.
Crystal Habit Equant trapezohedral.
Commonly occurs as phenocrysts.
Twinning Characteristic lamellar twinning often forming concentrically or at 60 degree angles. Twinning is caused during cooling from the isometric to tetragonal structure.
Cleavage (110) very poor (pseudododecahedral) Not seen in thin section and does no determine shape.
Relief in Thin Section Low negative  
Color/Pleochroism White to gray in hand sample. Colorless in grain mount and in thin section under ppl, gray to black under crossed polars, no pleochroism.
Interference Figure Uniaxial (+) Sign is usually positive. Twinning hinders obtaining interference figures. Isogyres can show slight separation during rotation.
Optical Properties Often octagonal cross section; complex polysynthetic twinning in many directions; inclusions common.
Refractive Indices:
ordinary =
extraordinary =

1.508-1.511
1.509-1.511 

Birefringence 0.000 - 0.001 Very weak and resulting in first order gray interference colors.
Elongation Not prismatic; crystals display no elongation.  
Extinction Inclined (incomplete) to undulating.  
Distinguishing Features Leucite's low birefringence, crystal habit and twinning differentiate  it from analcime and microcline.  Analcine also has higher indices of refraction and microcline has greater birefringence and lower relief. Inclusions of other minerals are common.
Occurrence Occurs almost exclusively as phenocrysts in  silica deficient lavas, especially in the vicinity of Mt. Vesuvius and Leucite Hills, Wyoming.
Editors
Erica Bradstreet (02), Clea Paine (02), Jamie Patterson (01), Lara Kapelanczyk (05), Kelsey Winsor (06)

 
Leucite commonly exhibits octahedral cross sections.  There is complex "tartan" twinning in crossed polars, which is often confused with microcline, but microcline has greater birefringence and lower relief. 
Leucite often includes inclusions along with the complex twinning pattern. 
Leucite commonly exhibits octahedral cross sections.  There is complex "tartan" twinning in crossed polars, which is often confused with microcline, but microcline has greater birefringence and lower relief. 

Leucite in cross polarized light showing twinning.

The same leucite crystal as above in plane polarized light
Leucite crystal in plane polarized light under 500x magnification. Click image to view rollover between plane polars and crossed polars. Cross hatching is commonly visible in crossed polars.
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