ANALCIME

Analcime – Na2(Al2Si4O12).2H2O – is a tectosilicate from the Zeolite Group. It is a relatively rare mineral, commonly found in association with other zeolites. It has no economic importance others than the market for mineral collectors.

The chemistry and structure of analcime are more similar to those of feldspathoids. It is cubic/pseudocubic, but can be tetragonal, orthorhombic, triclinic or monoclinic, depending on the ordering. Analcime may show some substitution of Na by Ca and K. It forms a series with pollucite ((Cs,Na)2Al2Si4O12•2H2O) and another series with wairakite (Ca816H2O.)

The crystals, almost always as trapezoidal icositetrahedrons, reach up to 25 cm in diameter and can be confused macroscopically with leucite. There are 4 varieties (with Cs, with Fe and two synthetic varieties, one with Ge and one with Ag). It is weakly piezoelectric. When rubbed or heated, it is weakly electrostatic.

A detailed account of the occurrence, forms and other information of analcime can be found in the book “Zeolites of the World” by Rudy Tschernich, available for download on the internet. Very detailed information on the various ways in which analcime occurs can be found on the website of the Commission on Natural Zeolites: http://www.iza-online.org/ .

1. Characteristics

Crystal system: See above.

Color: White, colorless, grey, rarely pink, red, greenish or yellowish.

Habit: It usually forms trapezoids. Granular, compact, massive.

Cleavage: {100} poor.

Tenacity: Brittle.

Twinning: Polysynthetic in {001} and {110}.

Fracture: Sub-conchoidal.

Mohs Hardness: 5 – 5.5

Parting: No.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2.24 – 2.29

 

2. Geology and Deposits

Analcime occurs in the matrix or in vesicles of low silica, intermediate and mafic igneous rocks, typically basalts and phonolites, formed from late hydrothermal solutions or disseminated due to deuteric alteration.

It also occurs in lake sediments, by alteration of pyroclastics or clays, or as a primary, autigenic precipitate, in sandstones and siltstones.

 

3. Mineral Associations

In volcanic rocks it is associated with quartz, prehnite, calcite, aegirine, serandite, native copper, apophyllite and zeolites (natrolite, thomsonite-Ca, gmelinite).

In other paragenesis it is associated with rhodochrosite, microcline and quartz.

In sedimentary rocks occurs with glauconite.

 

4. Transmitted Light Microscopy

Refraction indices: nα: 1.479   nβ: 1.493    nγ: 1.494

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.

Relief: Moderate.

Cleavage: {001} poor, usually not visible under a microscope.

Habits: Anhedral, rounded grains, radiated groups. Rarely has inclusions, never shows oriented inclusions. May show concentric structure.

Analcime usually occurs as trapezoidal icositetrahedrons of the Cubic System, which under the microscope appear as rounded grains or, when idiomorphic, as sections with 6 or 8 sides.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: When it occurs interstitial it is isotropic.

In large grains, it has anomalous birefringence with colors up to 1st order: black to very light gray to white. The crystals are sectored, with portions with different colors (black/gray/white), somewhat reminiscent of leucite, but the pattern of these sectors is different.

Extinction: Isotropic or does not apply.

Elongation sign: Isotropic or does not apply.

Twins: Interpenetrating (polysynthetic) lamellae, but without the generalized complex twins of leucite.

Zoning: No.

CONVERGENT LIGHT

Character: isotropic, but may be anomalous B(-). Usually subdivided into irregular sectors.

2V angle: 0o a 85º

Alterations: analcime alters to natrolite.

May be confused with: very low birefringence and rounded contours are typical.

Leucite also shows round grains, but has twins that are different, reminiscent of microcline.

Chabazite also has very low interference colors, but the relief is lower and usually shows pseudo-cubic contours.

Fluorite has higher relief, usually shows colored spots and bands (blue, violet), may show cubic contours, does not show twins and occurs in other paragenesis, associated with granitic rocks.

 

Analcime 01: in PPL, large analcime grain, colorless and with a moderate relief. Cleavage never is visible. In the center, tufts of needle-shaped crystals may occur.
Analcime 01: the same grain as in the previous image, now in XPL. Gray colors, sectoring, irregular twins are typical. Around it, calcite.
Analcime 01: the same grain as in the two previous images, now in XPL and with the compensator. Blue and red colors emphasize the irregular twinning typical of analcime.
Two grains of analcime, in XPL, side by side, showing twins and sectorizing. Each grain has a distinct pattern, uniformity is not the rule.
Idiomorphic (trapezohedron!) analcime grain in XPL, showing a more regular pattern of twinning. Around it, calcite.
Analcime 02: in XPL, mosaic of small analcime grains above a sand infilling of a vesicle in basaltic rock. Very irregular twins are diagnostic, but recognition of analcime may be difficult.
Analcime 02: in XPL, detail of the mosaic of small analcime grains from the previous image.
Analcime 02: in XPL, the same mosaic as in the two previous images, now in more detail. Inserting the compensator helps to see the twinning pattern.
Analcime 03: in PPL, horizontal vein in a volcanic rock filled with small analcime grains. Relief is moderate and helps to individualize the grains.
Analcime 03: the same horizontal vein with analcime as in the previous image, now in XPL. Dark colors, twinning and sectorizing are diagnostic.
In XPL, vesicle in basaltic rock with quartz cristals on the edge (colorful, thin section is slightly thicker than 30 micra), calcite (brownish colors) and analcime in the center.

5. Reflected Light Microscopy

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

Sample preparation: analcime is easily polished, like calcite and fluorite.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, darker than quartz and much darker than calcite.

Pleochroism: No.

Reflectivity: Very low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.

Internal reflections: Widespread in the same color as the mineral in hand specimen.

May be confused with: many other light colored transparent minerals. If idiomorphic, it is simpler to recognize. Its characteristics in PPL are more diagnostic than those in XPL.

Analcime 01: in PPL, calcite (left, lighter, better polish), analcime (right, darker, worse polish) and quartz (lighter than analcime, idiomorphic basal section inside the analcime).
Analcime 01: in XPL the 3 minerals show almost the same internal reflections, identification of analcime in this case is not possible.
If the analcime shows strong colors in hand specimen, its recognition in the polished section is easy. XPL.
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