STILBITE

Stilbite is a relatively common tectosilicate, a mineral of the Zeolite Group that typically occurs as a secondary mineral. It has no economic importance other than collecting mineral.

“Stilbite” is not actually a mineral, but just a generic term that refers to two minerals: stilbite-Ca  ( NaCa4(Si27Al9)O72.28H2O ) and stilbite-Na ( Na9(Si27Al9)O72.28H2O ).

The two stilbite species cannot be distinguished either macroscopically or under the petrographic microscope. Usually the “stilbites” are stilbite-Ca, as stilbite-Na is extremely rare.

Due to its idiomorphic development in cavities of volcanic rocks, the mineral is very well known and has been given a number of different names in various languages, like “desmin”, sometimes still used.

1. Characteristics

The data below refer to stilbite-Ca:

Crystal system: Monoclinic prismatic.          

Color: White, colorless, yellowish, cream, orange, light to dark brown, orange, pink.     

Habit: Thin tabular, in bundles, radial, even in spheres. Bow-tie-shaped aggregates are typical.

Cleavage: {010} perfect.       

Tenacity: Brittle.        

Twinning: Twins are common in stilbite, on {001}, interpenetrating or cruciform. All the most common zeolites show twinning.

Fracture: Irregular.       

Mohs Hardness: 3.5 – 4

Parting: No.         

Streak: White.         

Lustre: Vitreous to pearly.          

Diaphaneity:  Transparent.          

Density (g/cm³): 2.19

 

2. Geology and Deposits

Stilbite is a low temperature hydrothermal mineral (~100°C) that typically crystallizes in cavities and fractures of basic and acidic volcanic rocks such as basalts, andesites and rhyolites.

It can also be found associated with metamorphic rocks.

It forms in hot springs and can cement sedimentary rocks such as sandstones and conglomerates.

Cryptocrystalline stilbite crystals form in lake and marine sediments.

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

 

3. Mineral Associations

Stilbite associates with secondary minerals common in volcanic rocks: quartz (rock crystal, amethyst), chalcedony, opal, calcite, apophyllite, prehnite, datolite, clay minerals and, more rarely, cavansite and pentagonite.

Also with a high number of other zeolites, such as heulandite, chabazite, thomsonite, laumontite, stellerite, mordenite, analcime and others.

In metamorphic rocks it occurs with chlorite.

 

4. Transmitted Light Microscopy

The data below refer to stilbite-Ca:

Refraction indices:  nα: 1.484 – 1.500     nβ: 1.492 – 1.507    nγ: 1.494 – 1.513

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.    

Relief: Low to moderate low.           

Cleavage:  {010} perfect.          

Habits:  The crystals show typically thin tabular shapes, elongated along the “z” axis. These crystals constitute radiated clusters that can evolve into bow-tie-shaped aggregates. More rarely it can form spherical aggregates.           

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: 0.010 to 0.013, corresponding to medium 1st order colors: dark gray, light gray, white and straw yellow.           

Extinction: Tends to oblique, with angles of 3 – 12º. It can simulate being parallel!

Extinction is usually not complete in the crystal, but occurs through irregular zones, which can be bands or larger and smaller areas of the grain (sectorizing).           

Elongation sign: ES(+) or ES(-), it is not diagnostic.            

Twins: It has interpenetration and cross-shaped twins.         

Zoning: No, but sectorizing is common.             

CONVERGENT LIGHT

Character: B(-)          

2V angle: 30 – 49º         

Alterations: may alter to clay minerals or other zeolites.          

May be confused with: other zeolites, as there are several species very similar to each other. It is necessary to check the table below, including even gypsum and barite in the options, which are also similar with zeolites and can occur in the same paragenesis.

In XPL, stilbite with the typical tabular shapes, which can form a weave like here or clump together in a bow-tie shape. When anhedral, it is difficult to recognize stilbite among other zeolites like heulandite, scolecite, etc. In PPL all minerals in the image are colorless.
Stilbite in XPL, horizontally in the center of the image, showing a shape that tends to bow-tie, which is a very common habit. They are beams of crystals, radiated to both sides, and can form perfect ties. In PPL all crystals are completely colorless.
In XPL, in the extinction position, a stilbite crystal showing that i actually formed by a set of crystals.
In XPL, stilbites with a well-developed tabular habit, almost white in this position, with cleavage parallel to the elongation. Around her, other stilbites. The stilbite on the left, in shades of brown, shows very well the band-and-band extinction that the stilbite can show. The grain is not all black in a certain position.
Stilbites in XPL showing cleavage parallel to the length of the crystal. Generally, the cleavage is well developed, it may resemble the cleavage of micas.
Stilbite in vesicle filling in basaltic rock in XPL. The vesicle wall, formed by basaltic rock, is at the bottom of the image, with colored augites. opaque minerals (magnetite) and white to gray lamellae of poorly developed plagioclases. The filling of the vesicle starts with green clay minerals containing an idiomorphic quartz crystal (white). The second band is formed by the stilbite, tabular, almost parallel extinction, B(-) and, in this case, practically without twins. There is a fracture accompanying the stilbites, a defect in the making of the thin section. Towards the top, small idiomorphic quartz crystals begin to appear, macroscopically similar to white rice, which form a compact aggregate that constitutes the third band.
In XPL, a detail view of the same stilbite band of the previous image.
In XPL, a more idiomorphic stilbite in a similar situation. Basaltic rock (the vesicle wall) is at the bottom (dark).
Stilbite 01: In PPL, a stilbite crystal showing its cleavage (horizontally).
Silbite 01: The same section as of the previous image, now in XPL. It is clear that the crystal is twinned.
Stilbite 02: In PPL, a fracture in a basaltic rock filled with minerals, among them a stilbite (with vertical cleavage).
Stilbite 02: The same section as of the previous image, now in XPL. The stilbite shows a complicated twin pattern.
Stilbite 03: In PPL, an altered volcanic rock crossed by a vein filled with an unidentified dark zeolite, followed by stilbites-stellerites in radial aggregates and calcite. In PPL the stilbite stands out due to higher relief.
Stilbite 03: The same section as of the previous image, now in XPL. Calcite is colorful, stilbite in grays. The stilbite crystals are pinacoid terminated, forming radial aggregates, a habit typical for stellerite. Conclusive identification is not possible by optical means.
Stilbite 04: From the same sample of the two previous images, now a detail. In PPL, the radial aggregates of stilbite-stellerite crystals are well-defined due to their higher relief.
Stilbite 04: The same section as of the previous image, now in XPL. Calcite is colorful, stilbite-stellerite in grays. The radial aggregates are very clear. In hand specimen, sometimes spherical aggregates can be found.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of the stilbite is simple and is of excellent quality. Cleavage may be visible and some imperfections (holes) result from the orientation of the cleavage with respect to the plane of the polished section in certain crystals.

PLANE POLARIZED LIGHT – PPL

Reflection color:  Very light gray.      

Pleochroism: No.      

Reflectivity: Low (<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Widespread in the same color of the mineral in hand sample, always lighter than in the hand sample. Multicolored internal reflections may occur.      

May be confused with: many other light colored transparent minerals. The Reflected Light technique is not the correct technique for identifying stilbite. 

Stilbite 01: In PPL, aggregate of stilbite crystals that macroscopically have a yellowish color. In PPL, the color is quite light and some cleavages are visible. In black, cavities between the stilbite crystals.
Stilbite 01: The same section as of the previous image, now in XPL. Clear internal reflections are widespread.