HEULANDITE

Heulandite – (Ca,Na,K)5(Si27Al9)O72.26H2O – is a tectosilicate of the Zeolite Group. It is a very common mineral, abundant in certain situations. It has great importance as an ore when it occurs cryptocrystalline. When it occurs hydrothermal, in larger crystals, it has only some importance as a collection mineral.

“Heulandite” is actually not a mineral, but just an informal term for a group of minerals in the Heulandite Group, which includes heulandite-Ca, heulandite-Na, heulandite-Ba, heulandite-K and heulandite-Sr. Heulandite-Ca is very common; the others are very rare to extremely rare. It is not possible to differentiate the “heulandites” under the petrographic microscope.

It can be triclinic due to the Al-Si ordering. It forms a series with clinoptilolite-Ca. There is a variety, “beaumontite”, which has orthorhombic shapes and is usually golden in color.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Colorless, white, more rarely red, green, brown or yellow.     

Habit: Tabular, coffin-shaped (trapezoidal). Granular, massive. Idiomorphy is common.       

Cleavage: {010} perfect.       

Tenacity: Brittle.        

Twinning: On {100}.  

Fracture:  Irregular, conchoidal.      

Mohs Hardness: 3.5 – 4

Parting: No.         

Streak: White.         

Lustre: Vitreous, pearly on cleavage planes.          

Diaphaneity: Transparent.          

Density (g/cm³): 2.1 – 2.2

          

2. Geology and Deposits

Heulandite occurs as a filling of cavities in basaltic rocks (basalts, diabases, gabbros), acidic volcanic rocks (rhyolites, rhyodacites, dacites) and andesites. It can also occur associated with serpentinites, pegmatites and active hydrothermal systems.

It is formed as a product of devitrification of volcanic glasses and tuffs, when it constitutes ore. It integrates the zeolite facies of regional metamorphic rocks and may constitute the diagenetic matrix of sandstones.

A detailed account of the occurrence, forms and other details of heulandite 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 heulandite occurs can be found on the website of the Commission on Natural Zeolites: http://www.iza-online.org/ .

 

3. Mineral Associations

Heulandite occurs with the minerals that form the host rocks, generally mafic and intermediate igneous rocks: olivines, clinopyroxenes (augite, pigeonite), plagioclases (labradorite), magnetite, ilmenite, chalcopyrite, pyrite, native copper and alteration minerals such as goethite, covellite, cuprite, malachite and others.

It is part of the assembly of secondary minerals common in basaltic rock cavities, such as quartz (including amethyst variety), chalcedony, calcite, powellite, prehnite, datolite, babingtonite, fluorapophyllite, clay minerals (celadonite) and others. Other zeolites such as laumontite, chabazite-(Ca), mordenite, okenite, analcima, stilbite-Ca, scolecite and gyrolite can also be found in these cavities.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.496 – 1.499      nβ: 1.497 – 1.500     nγ: 1.501 – 1.505

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. It may appear reddish due to Fe oxide inclusions.

Relief: Low to moderate.           

Cleavage: {010} perfect.  

Habits: tabular to elongated crystals, wider at the center and narrower at both ends: “coffin-shaped” (trapezoidal). It forms subparallel aggregates. Perfect cleavage can result in holes in the thin sections where portions of heulandite from sections perpendicular to the cleavage were torn off during the making of the thin section.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: birefringence of 0.005 to 0.009, corresponding to 1st order medium colors: gray in various shades and white, not reaching straw yellow.           

Extinction: oblique, angle between 0 and 32º.

Sections with the {010} cleavage vertically show parallel extinction.

Sections parallel to {010} show no cleavage, have oblique extinction to form, and show very low birefringence (dark gray colors).     

Elongation sign: ES(-) 

Twins: Form twins along the {100} plane. 

Zoning: No, but sectorizing is very common. 

CONVERGENT LIGHT

Character: B(+). It can simulate being uniaxial.    

2V angle: 0 – 55º, usually between 10º – 48º, more commonly between 30º – 34º.

Alterations: heulandite alters to clay minerals and other zeolites. 

May be confused with:  several other zeolites.

Scolecite and laumontite are B(-).

Stilbite has other twins and other habits (bow-tie-shaped radiated aggregates).

Natrolite exhibit parallel extinction and square basal sections.        

Heulandite in XPL with typical interference color and shape (c axis vertical, section 010). In PPL it is colorless.
Close to the extinction position, heulandite shows twins and sub-grains (sectorizing). XPL.
Aggregate of heulandite crystals in XPL. Heulandite shows interference colors between dark gray and white. Cleavage is sometimes difficult to visualize and idiomorphic forms are common. In PPL all the crystals are colorless.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of heulandite is relatively simple and is of good quality. Despite the well-developed cleavage, no polishing pits appear.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like quartz and feldspar, slightly lighter than the lighter gray of calcite pleochroism colors.       

Pleochroism: No.      

Reflectivity: Low (<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Widespread, clear to milky. Depending on the orientation of the cleavage in relation to the plane of the polished section, multicolored internal reflections may appear in parallel bars, resembling twins. Random colored internal reflections are also visible at times.

May be confused with: many other transparent, colorless or light colored minerals. It is not possible to identify heulandite through Reflected Light.

Heulandite 01: in PPL, calcite (darkest gray, lots of polishing scratches) and heulandite (lighter gray, very good polish).
Heulandite 01: The same section as of the previous image, now in XPL. Calcite has slightly darker internal reflections. Heulandite shows clear internal reflections in various shades of light gray and white. Both minerals have polygonal contours; it is a cavity lining (vesicle), where the crystals were able to crystallize freely.