SCOLECITE

Scolecite – CaAl2Si3O10·3H2O – is a relatively common tectosilicate, a zeolite that typically occurs as a secondary mineral associated with basic igneous rocks. It has no economic importance other than a collecting mineral.

It is classified in the Zeolite Group, in the Natrolite Subgroup, where there are three very common and similar zeolites: scolecite (of Ca), natrolite (of Na) and mesolite (intermediate term). Macroscopically they are very similar; all form radiated aggregates of long prismatic crystals, usually colorless, often white, always with a vitreous luster.

Scolecite can contain Na and K and is usually twinned according to {100}, with the twinning axis on [001], forming contact or interpenetration twinnings. When free crystals form, it is relatively simple to visualize the reentrant angle of the twin at the end of the prisms.

It may fluoresce between yellow and brown under short-wave and long-wave Ultraviolet Light, but this is not a diagnostic property as it occurs only sporadically. It is pyroelectric and piezoelectric.

1. Characteristics

Crystal system: Monoclinic domatic.          

Color: Colorless, white, more rarely pink, salmon, green or red.     

Habit: Long prismatic, in radial aggregates may be pseudo-orthorhombic.       

Cleavage: {110} perfect, {1-10} perfect. Striations // (001).       

Tenacity: Brittle.       

Twinning: Usually twinned according to {100}, with the twinning axis on [001], forming contact or interpenetration twinnings.       

Fracture: Irregular.       

Mohs Hardness: 5 – 5.5

Parting: No.         

Streak: White.         

Lustre: Vitreous, silky when fibrous.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.25 – 2.29

          

2. Geology and Deposits

Scolecite is a mineral typically found in cavities and fractures in basaltic rocks. It also occurs associated with gneisses and amphibolites. It can occur in syenitic and gabbroic bodies.

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

 

3. Mineral Associations

Scolecite occurs associated with the minerals that form the host rocks, generally mafic and ultramafic igneous rocks: olivine, clinopyroxene (augite, pigeonite), plagioclase (labradorite), magnetite, ilmenite, chalcopyrite, pyrite, native copper and others.

It composes the assembly of secondary minerals, with celadonite, calcite, macrocrystalline quartz, chalcedony, fluor-apophyllite, powellite, prehnite, datolite and other zeolites (heulandite, laumontite, stilbite, epistilbite, chabazite, etc.).

In tectonic cracks occurs with quartz, chlorite and adular.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.507 – 1.513    nβ: 1.516 – 1.520    nγ: 1.517 – 1.521

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless.   

Relief: Low.          

Cleavage: {110} perfect, usually not visible under a microscope. 

Habits: Long prismatic, very typical, forming radiated aggregates. More rarely fibrous, nodular or massive.

It can have V-shaped endings when it is twinned, which is very common.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: 0.011 to 0.012: 1st order medium colors – light gray to white, not reaching 1st order straw yellow (like all common zeolites).           

Extinction: Oblique, of 15 – 18º.           

Elongation sign: ES(-) (diagnostic!)            

Twins: Very common on {100}, visible in both longitudinal and basal sections. They are interpenetration and contact twins.         

Zoning: No.           

CONVERGENT LIGHT

Character:  B(-)         

2V angle: 36 – 56º         

Alterations: No information available.          

May be confused with: very similar to other zeolites with a long prismatic habit.

Natrolite and mesolite, the other two members of the series are extremely similar, but natrolite shows ES(+).

Thomsonite shows much higher interference colors and is ES(+). 

Scolecite 01: In PPL, scolecite prims with typical low relief and no cleavage.
Scolecite 01: The same section as of the previous image, now in XPL. As all the crystals are twinned, there will always be one individual in the twin in the extinction position (black) and the other individual in the twin in the lighting position, showing the interference colors between gray and white.
In XPL, twinned scolecites sectioned perpendicularly to the elongation, showing the twin planes (straight lines separating the white from the black portions.
In XPL, a large aggregate of twinned scolecites fills a cavity (vesicle) in a volcanic rock (basalt).
As the sections of the twinned scolecite prims move away from the perpendicular to the elongation, they become longer and longer and the twin plane assumes an increasingly diagonal position. Sections exactly parallel to the elongation are shown to be long prismatic, divided in half (parallel to the elongation) by the twin plane, always with a dark side (crystal in extinction position) and with the other side light gray (crystal in illumination position, maximum or not). XPL
In XPL, twinned scolecite prisms in sections oblique to the elongation of the prims.
In XPL, a twinned scolecite prism exactly paralell to the elongation. At top right, the typical reentrant angle at the top (end) of the twinned crystal is perfectly visible.

5. Reflected Light Microscopy

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

Sample preparation: scolecite accepts polishing very easily, despite the cleavage and its habit of long prisms. Even with a quick polish it ends up with a polishing quality much superior to that of quartz and feldspars, for example.

PLANE POLARIZED LIGHT – PPL

Reflection color:  Dark gray, similar to quartz and feldspar.      

Pleochroism: No.      

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Widespread colorless, pale to grayish. Depending on the orientation of the cleavages, multicolored reflections.      

May be confused with: many other minerals and especially many other zeolites. The Reflected Light technique does not allow the identification of the scolecite.       

General Characteristics: 

Grain shape: the typical long prisms of scolecite, forming radiated beams, can usually be recognized if the orientation of the section is parallel or approximately parallel to them.

Cleavage may occasionally be visible, as well as contacts between prisms.  

Twins are not possible to identify.   

Scolecite 01: In PPL, diagonally on the image, long paralell prisms of scolecite.
Scolecite 01: The same section as of the previous image, now in XPL. Widespread colorless, milky and multicolored internal reflections asre similar to reflections of other zeolites and even quartz.