KALSILITE
Kalsilite – KAlSiO4 – is a rare tectosilicate, a feldspathoid typical of K-rich and silica-deficient igneous rocks. It has no importance as an ore.
It is classified in the Feldspathoid Group, being dimorphic from kaliophilite (trigonal). May contain impurities such as Ca, Na, Fe and Mg. Gelatinizes in concentrated HCl.
1. Characteristics
Crystal system: Hexagonal trapezohedral.
Color: Colorless, white to gray.
Habit: Tabular thin to thick, prismatic to short prismatic, may be acicular, almost fibrous. Granular, massive. Crystals up to 5 mm.
Cleavage: {10-10} poor, {0001} poor.
Tenacity: Brittle.
Twinning: Always present, several types.
Fracture: No information available.
Mohs Hardness: 6
Parting: No.
Streak: White.
Lustre: Vitreous, resinous.
Diaphaneity: Transparent.
Density (g/cm³): 2.59 – 2.62
2. Geology and Deposits
Kalsilite is rare, but it is a rock-forming mineral in some rare K-rich and silica-deficient mafic volcanic rocks (lava and tuffs) such as pyroxenites (kalsilite-pyroxenites), kamafugites, melilitites (kalsilite-phlogopite-melilitites, kalsilite-melilite-lavas) and olivine-melilitites (kalsilite-leucite-olivine-melilitites and kalsilite-phlogopite-olivine-leucite-melilitites). In the mellilites of melilitites, small regions of droplets oriented parallel to the “z” axis may occur, the so-called “peg structure”, with at least three phases: kalsilite, nepheline and a silica-rich mineral.
It is rare in plutonic rocks, but can occur in syenites as a primary magmatic mineral, associated with potassium feldspar and biotite. In rocks containing leucite, leucite decomposition forms intergrowths between kalsilite and potassium feldspar.
It was found in a granulite facies emery deposit.
In plutonic rocks, the kalsilite component of nepheline can form lamellae of kalsilite exsolution in nepheline, which then has a milky color. Nepheline with these exsolution lamellae is called “elaeolite”.
3. Mineral Associations
In the type locality (Kamengo, Chamengo Crater, Uganda) it ocurrs with phlogopite, olivine, calcite and diopside.
Occurs with minerals from the Melilite Group (akermanite, alumoakermanite), other feldspathoids (nepheline, leucite, kaliophilite) and micas (biotite, phlogopite, fluorphlogopite).
With clinopyroxenes (aegirine), zeolites (thomsonite-(Ca), willhendersonite), melilite, microcline, titanite, fluorapatite and eudyalite.
With rare minerals like vladykinite and noonkanbahita.
4. Transmitted Light Microscopy
Refraction indices: nω: 1.538 – 1.543 nε: 1.532 – 1.537
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Low (low negative to low positive)
Cleavage: {10-10} bad and {0001} bad are not visible on a thin section.
Habits: Tabular thin to thick, prismatic to short prismatic. May be acicular, almost fibrous. It may be intergrown with nepheline or microcline. Granular to massive. Crystals up to 5 mm.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.005 to 0.006, very low, corresponding to interference colors between dark gray and slightly lighter gray, does not reach the white.
Extinction: Paralell.
Elongation sign: No information available.
Twins: Common, many types.
Zoning: No.
CONVERGENT LIGHT
Character: U(-)
2V angle: No.
Alterations: No information available.
May be confused with: nepheline, which is very similar. To differentiate nepheline from kalsilite it is necessary to use coloring techniques or X-Ray Diffraction.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of kalsilite. It is not possible to recognize kalsilite by Reflected Light nor do associated opaque minerals occur.