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Formula | KAlSi2O6 | 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 |
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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), Naomi Jahan (18) |