LEUCITE

Leucite – K(AlSi2O6) – is a tectosilicate of the Feldspathoid Group, constituting an important alkaline rock forming mineral. Leucite itself is not mined, but alkaline rocks as a whole can be used in the glass and ceramics industries as fluxes, for example.

Leucite crystallizes in the cubic system in volcanic rocks at high temperatures (~900°C) and, with lowering the temperature to approximately 700-600°C, acquires a stable tetragonal modification at room temperature and typically develops pseudo-cubic crystals in the form of a trapezoidal icositetrahedron (”leucitohedron”), which exhibit under the microscope very characteristic polysynthetic twins, whose arrangement changes from grain to grain.

As impurities in leucite, there may be Ca and Na, Fe and Mg, Ti, Ba, Rb, Cs and H2O.

1. Characteristics

Crystal system: Tetragonal, bipiramidal, pseudo-cubic.

Color: White to gray, never shows colors.

Habit: Idiomorphic pseudo-cubic crystals. Usually rounded.

Cleavage: {110} very poor.

Tenacity: Brittle.

Twinning: Polysynthetic, common.

Fracture: Conchoidal.

Mohs Hardness: 5.5 – 6

Parting: No.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2.45 – 2.5

 

2. Geology and Deposits

Leucite is the most common and most characteristic feldspathoid of K-rich and silica-deficient volcanic rocks. It occurs almost exclusively as a phenocryst in undersaturated volcanic and hypabyssal rocks (mafic and alkaline), such as monzosyenites, trachytes, trachyandesites, phonolites, basanites, leucittites, melilitites, lamproites and lamprophyres.           

It can occur in plutonic alkaline rocks.

 

3. Mineral Associations

Leucite occurs with other feldspathoids (nepheline, nosean) sanidine, natrolite, analcime, olivine, fluorapatite, melilite, phlogopite, carbonates (calcite, aragonite), clinopyroxenes (augite), sodic and calc-sodic amphiboles (arfvedsonite), magnetite and perovskite.

 

4. Transmitted Light Microscopy

Refraction indices:  nε: 1.509     no: 1.508  

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, never has pleochroism.

Relief: Low negative (very low), no crystal limits are seen.

Cleavage: {110} poor, cannot be seen under a microscope.

Habits: form rounded grains. Leucite almost always forms trapezoidal icositetrahedrons of the holohedral class of the Cubic System. These trapezoidal icositetrahedrons, under the microscope, are visualized as rounded shapes and may show 8-sided sections. They usually do not form polygons, they are just rounded. They are known as “leucitohedrons”.

Inclusions in radial alignments, usually glass, are very common.

Colorless idiomorphic inclusions of hauyna and nosean with rhombododecahedral forms, hexagonal forms etc., with a few microns in diameter, may occur.

A pseudo-leucite crust can form around the leucite grains, consisting of nepheline and K-feldspar.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic. Its maximum birefringence is 0.001: dark to medium gray colors. It is not completely black, it doesn’t look like an isotropic mineral, there’s always a definite luminosity.

Extinction: Oblique (incomplete) to undulating extinction. Due to the rounded shape, the extinction cannot be classified as parallel or oblique, only as undulating. 

Elongation sign: Does not apply because the grains are rounded (pseudo-cubic).

Twins: Usually presents complex polysynthetic twins in many directions, very typical, often with a concentric arrangement or at angles of approximately 60º. The twin pattern changes from grain to grain. Due to leucite’s low birefringence, twins are difficult to recognize.

The recognition of the twins is easier introducing the compensator, so that the lamellae of the twins turn pink and blue (see on one of the images below).

Zoning: Frequently zoned.

CONVERGENT LIGHT

Character: U(+), may be anomalous biaxial (2V low). Twins make it difficult or impossible to obtain good figures.

2V angle: No, but it may have an anomalous low 2V angle.

Alterations: easily alters to pseudo-leucite, a mixture of nepheline and K-feldspar. It can undergo the process of analcimization, when it changes initially to other zeolites and finally to analcime.

May be confused with: few other minerals due to paragenesis. Typical are generally rounded shapes and the polysynthetic twins.

Nosean occurs in the same paragenesis and can also be rounded, but is isotropic.

Analcime has a higher refractive index, does not have the characteristic inclusions of leucite, is generally anhedral and has no twins, only subgrains, which have a different appearance.

Microcline never occurs in volcanic or hypabyssal rocks.

Chabazite presents anomalous subgrains, better cleavage and rhombohedral, pseudo-cubic habit.

Colorless volcanic glass also has a very low relief, but it never presents polysynthetic twins and is completely isotropic, it has no luminosity in CPL.

Leucite 01: in PPL, a leucite crystal with typical characteristics. Low relief and colorless, inclusions may be possible.
Leucite 01: the same crystal of the previous image, now in XPL, showing very low anomalous birefringence and the typical polysynthetic twins, always in a disordered pattern, different from grain to grain.
In some much rarer situations leucite twins cross exactly at an angle of 60º, as in this crystal. XPL.
Leucite in PPL with its typical, but not always present, pattern of inclusions arranged in an oriented manner, in concentric bands.
Leucite 02: Volcanic rock with large amounts of leucite grains in a fine-grained matrix.PPL.
Leucite 02: The same section as in the previous image, now in XPL. Typical polysynthetic twins can be seen better in large grains, as they are often absent in small ones or the very low birefringence does not allow them to be observed. Inclusions arranged in concentric patterns also do not occur in all grains.
Leucite 03: Complex polysynthetic twins in an exceptionally large crystal of leucite. XPL.
Leucite 03: the same section as in the previous image, now in XPL + compensator. Twins turn pink and bluish, an easy way to recognize the twins when the leucite crystals are very small and with low birefringence (too dark). It is a trick that can be very useful in differentiating small leucites from other minerals.
In PPL, leucite in phonolite with many colorless idiomorphic hauyna and/or nosean inclusions. In the forms, the rhombododecahedron of the Cubic System predominates.
In PPL, detail images, obtained with the 63x objective + camera zoom + photoshop, of colorless idiomorphic inclusions (rhombododecahedrons of the Hexaoctahedral Class of the Cubic System) of hauyna and/or nosean in leucite. In XPL, the crystals are isotropic. In many cases, only a hexagonal contour is observed. In other cases, apparently the forms are combined.
In PPL, detail images, obtained with the 63x objective + camera zoom + photoshop, of colorless idiomorphic inclusions (rhombododecahedrons of the Hexaoctahedral Class of the Cubic System) of hauyna and/or nosean in leucite. In XPL, the crystals are isotropic. In many cases, only a hexagonal contour is observed. In other cases, apparently the forms are combined.
Leucite 04: Leucite crystal with a crust, a corona, of pseudo-leucite, consisting of nepheline and K-feldspar whose crystals are orientated approximately perpendicularly to the surface of the leucite grain. In PPL, a slightly cloudy but still colorless band is observed at the grain boundaries.
Leucite 04: The same crystal of the previous image, now in XPL. Leucite is dark and there is a gray band of different texture between it and the rock.
Leucite 04: from the same crystal of the previous two images, now in detail, in PPL. Leucite at the bottom, the rock at the top and, in the middle, horizontally, the pseudo-leucite crust. In PPL, there is a slight haze (turbidity) in the pseudo-leucite.
Leucite 04: The same section as of the previous image, now in XPL. Nepheline and K-feldspar crystals oriented perpendicularly to the leucite can be seen.
It is possible to visualize the contour of the leucite grain, in a sectional view that corresponds to the trapezoidal icositetrahedron of the Hexaoctahedral Class of the Cubic System, a shape known as a leucitohedron. There was probably a staged growth of leucite crystals here and the deposition of inclusions in the first grain has just marked its contour. PPL.
It is possible to visualize the contour of the leucite grain, in a sectional view that corresponds to the trapezoidal icositetrahedron of the Hexaoctahedral Class of the Cubic System, a shape known as a leucitohedron. There was probably a staged growth of leucite crystals here and the deposition of inclusions in the first grain has just marked its contour. PPL.
It is possible to visualize the contour of the leucite grain, in a sectional view that corresponds to the trapezoidal icositetrahedron of the Hexaoctahedral Class of the Cubic System, a shape known as a leucitohedron. There was probably a staged growth of leucite crystals here and the deposition of inclusions in the first grain has just marked its contour. PPL.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of leucite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with leucite.

Sample preparation: the polishing of leucite is simple and of excellent quality, as well as the polishing of the other minerals characteristic of this paragenesis.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, the same color as other feldspars such as nepheline, sanidine, orthoclase and microcline.

Pleochroism: No.

Reflectivity: Low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Generalized clear to colorless, may be white, milky, in spots. As the mineral is completely colorless and transparent, dark or reddish minerals around it and, especially, below it, influence the color of the reflections, which can change to reddish or darker.

May be confused with: feldspars and feldspathoids that occur in the same paragenesis, such as sanidine and nepheline, if they are rounded.

General Characteristics: 

Grain shape is typically rounded and easy to recognize.           

Cleavage is not observed.

Polishing scratches do not occur.

Twinning is not visible, the same situation as with the plagioclase and microcline twins in Reflected Light. However, in many situations, there are prismatic mafics (pyroxenes) below the observed grain and the dark reflections of these minerals simulate twins.

Leucite 01: in PPL, leucite crystal in a phonolite.
Leucite 01: the same crystal of the previous image, now in XPL.
In PPL, in phonolite, two grains of leucite side by side. The grain on the left has clearer internal reflections and pseudo-twins due to mafics located below it. The grain on the right has darker, reddish reflections.
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