EPISTILBITE

Epistilbite – CaAl2Si6O16.5H2O – is a tectosilicate belonging to the Zeolite Group. It is a relatively rare mineral. It has no economic significance other than as a collector’s specimen.

It is dimorphous with goosecreekite and may contain Na, Mg, Fe, and K. It is piezoelectric.

1. Characteristics

Crystal system: Monoclinic, can occur in various classes.     

Color: Colorless to white, more rarely pinkish or yellowish.

Habit: Prismatic, typically in radial aggregates (“open accordion” pattern). Spherulitic, acicular, tabular.

Cleavage: {010} perfect.

Tenacity: Brittle.     

Twinning: It always forms simple twins on {110} and {102}, resulting in pseudo-orthorhombic shapes. It also forms cross-shaped interpenetration twins on {100} and {110}.

Fracture: Irregular.

Mohs Hardness: 4

Parting: No.   

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2.22 – 2.28

 

2. Geology and Deposits

Epistilbite typically occurs in cavities (vesicles – former gas bubbles) within basaltic and andesitic volcanic rocks. It may also be found in cavities within gneisses.

It is not a common zeolite; its abundance is significantly lower than that of other zeolites, such as stilbite, heulandite, and laumontite. However, in some regions it can be found easily.

 

3. Mineral Associations

It occurs with secondary minerals common in cavities within basaltic rocks, such as quartz (including the amethyst variety), chalcedony, calcite, fluorapophyllite, and others.

Various different zeolites may occur in these cavities: laumontite, chabazite-(Ca), mordenite, okenite, stilbite, scolecite, heulandite-Ca, and gyrolite.

 

4. Transmitted Light Microscopy

Refraction indices: nα: 1.502-1.505     nβ: 1.510-1.515      nγ: 1.512-1.519

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.   

Relief: Low.     

Cleavage: Usually not visible. 

Habits: It occurs as pointed crystals, intergrown with various others to form radial clusters.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.010, corresponding to interference colors ranging from dark gray to light gray, white, and up to straw yellow. These colors are slightly higher than those of other zeolites, which typically exhibit colors between gray and white.  

Extinction: Oblique in 10º.  

Elongation sign: ES(+).     

Twins: Abundant, always present.      

Zoning: No.     

CONVERGENT LIGHT

Character: B(-).     

2V angle: 44º

Alterations: No information available.      

May be confused with: other zeolites, which generally have lower interference colors.

Phillipsite is B(+) and exhibits parallel extinction.

Thomsonite shows parallel extinction.

Stilbite has lower refractive indices and two cleavage directions.

In XPL, epistilbite aggregate, illustrating the complex forms and twinning that epistilbite exhibits. In PPL, epistilbite is entirely colorless and displays low relief.
In XPL, large twins in epistilbite crystals. The thin section is slightly thicker than the standard. On the right, small quartz crystals.
In XPL, small twins in epistilbite crystals.
In XPL, typical terminations of epistilbite crystals within an aggregate that tends toward a radial habit. The thin section is a bit over 30 microns thick.
In XPL, a (horizontal) twin in epistilbite. The upper side of the twinned crystal exhibits anomalous interference colors.
In XPL, a cluster of epistilbite crystals is seen, which grew freely within a vesicle. This habit is common for epistilbite: the crystals form a radial aggregate, which, macroscopically, resembles an open accordion (or concertina). The longest crystals reach 1 cm in length. The specimens were impregnated and prepared to a thickness slightly greater than 30 microns; this reveals—through variations in color—the immense complexity of the forms and twins they exhibit.
In XPL, a cluster of epistilbite crystals is seen, which grew freely within a vesicle. This habit is common for epistilbite: the crystals form a radial aggregate, which, macroscopically, resembles an open accordion (or concertina). The longest crystals reach 1 cm in length. The specimens were impregnated and prepared to a thickness slightly greater than 30 microns; this reveals—through variations in color—the immense complexity of the forms and twins they exhibit.
In XPL, a cluster of epistilbite crystals is seen, which grew freely within a vesicle. This habit is common for epistilbite: the crystals form a radial aggregate, which, macroscopically, resembles an open accordion (or concertina). The longest crystals reach 1 cm in length. The specimens were impregnated and prepared to a thickness slightly greater than 30 microns; this reveals - through variations in color - the immense complexity of the forms and twins they exhibit.
In XPL, a cluster of epistilbite crystals is seen, which grew freely within a vesicle. This habit is common for epistilbite: the crystals form a radial aggregate, which, macroscopically, resembles an open accordion (or concertina). The longest crystals reach 1 cm in length. The specimens were impregnated and prepared to a thickness slightly greater than 30 microns; this reveals - through variations in color - the immense complexity of the forms and twins they exhibit.

5. Reflected Light Microscopy

Reflected Light Microscopy is evidently not the recommended analytical method for identifying epistilbite. However, it is important to create a slide or polished section to identify the opaque minerals that may occur associated with epistilbite, such as magnetite and native copper.

Sample preparation: Polishing epistilbite is straightforward and presents no difficulties. However, the quality of the polish is not excellent, due to the mineral’s cleavage and numerous twinning planes.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspars.

Pleochroism: No.

Reflectivity: Low (<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: No anisotropy is observed as a result of internal reflections.

Internal reflections: Generalized, ranging from clear to milky; may be multicolored in some areas.

May be confused with: It is not possible to identify epistilbite under reflected light.

Epistilbite 01: In PPL, epistilbite (below, idiomorphic) and colorless resin (at the top) are visible. There are no diagnostic features in the epistilbite; even its shape can be similar to that of heulandite.
Epistilbite 01: The same section as of the previous image, now in XPL. Clear internal reflections are widespread.