THOMSONITE

Thomsonite – NaCa2[Al5Si5O20].6H2O – is a tectosilicate from the Zeolite Group. It typically occurs as a secondary mineral in vesicles and fractures of volcanic rocks and is not common; it is one of the rarest zeolites. It has no economic importance.

“Thomsonite” is currently no longer considered a mineral, but a generic term that refers to two minerals: Ca-thomsonite (which is much more common) and Sr-thomsonite. The distinction was introduced in 1997. Specimens that have not been analyzed in detail are simply called “thomsonite.”

Macroscopically, it can be confused with stellerite. It has a Sr variety and is pyroelectric. Ca-thomsonite has 20 synonyms, considering the names attributed to the mineral in various languages. Therefore, care must be taken with older literature.

1. Characteristics

Crystal system: Orthorhombic bipyramidal, pseudotetragonal.        

Color: Colorless, white, beige, more rarely yellowish, pale green, pink, brown. 

Habit: Columnar, prismatic,
fibrous, radial, spherical to globular, may be botryoidal, massive.

Cleavage: {010} perfect, {100} good. Striations // to [001]       

Tenacity: Brittle.        

Twinning: By {110}, cyclical, occasional.       

Fracture: Irregular, subconchoidal.       

Mohs Hardness: 5 – 5.5

Parting: No.         

Streak: White.         

Lustre: Vitreous, pearly.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.23 – 2.29

 

2. Geology and Deposits

Thomsonite, like many other zeolites, is common in vesicles (amygdales) of basic to ultramafic volcanic rocks, such as alkaline basalts, phonolites, theralites, and others. It can also occur in hypabyssal rocks.

In rocks that have undergone zeolite facies metamorphism, thomsonite can replace laumontite.

Ca-thomsonite can occur as cement in some sandstones.

Rarer occurrences are in contact metamorphic zones and granitic pegmatites.

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

 

3. Mineral Associations

It is associated with non-zeolites such as calcite, prehnite, datolite, apophyllite, clay minerals (smectites), chlorite, chalcedony, macrocrystalline quartz, and albite.

It occurs with zeolites such as natrolite, mesolite, scolecite, stilbite, phillipsite, chabazite, heulandite, and analcime.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.497 – 1.530      nβ: 1.513 – 1.532     nγ: 1.518 – 1.545

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless.

Relief: Low.           

Cleavage: May be visible.           

Habits: Columnar along the c-axis, it can be fibrous, forming radial or irregular aggregates. It rarely forms idiomorphic crystals.   

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Maximum birefringence of 0.015: colors ranging from first-order white to second-order blue, which is very high for zeolites and therefore quite diagnostic.

Common zeolites generally exhibit colors between gray and white, at most first-order straw yellow.

Extinction: Tends to be paralell.           

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

Twins: Sometimes.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 44-75º         

Alterations: No information available.          

May be confused with: A few other minerals, considering paragenesis.

Prehnite also has strong colors, but different habits and anomalous interference colors.

Datolite shows strong interference colors, but is generally granular, never fibrous.

Natrolite has lower interference colors, between gray and white.

The other zeolites that may exhibit a fibrous habit show oblique extinction.

Chabazite and analcime have much lower birefringence, different habits, and are uniaxial.         

Thomsonite 01: In PPL, basaltic rock with a vesicle filled with thomsonite. In PPL there is no diagnostic feature.
Thomsonite 01: The same section as of the previous image, now in XPL. Strong interference colors and straight extinction are very diagnostic.
Thomsonite 02: Vesicle in basaltic rock filled in the lower portion with a radial aggregate of prismatic thomsonite crystals and, in the upper portion, by fibrous mesolite. In PPL, the thomsonite is colorless and of low relief.
Thomsonite 02: The same section as of the previous image, now in XPL. Thomsonite is colorful, mesolite is gray.
Thomsonite 03: In PPL, a vesicle in basaltic rock with some thomsonite crystals at the base and the rest filled with fibrous mesolite.
Thomsonite 03: The same section as of the previous image, now in XPL. Thomsonite is colorful, mesolite is gray. The image does not accurately convey the vibrant colors of thomsonite due to excessive brightness, but the bright colors are clearly visible under a microscope.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying thomsonite. However, it is important to prepare a polished slide or section to identify opaque minerals that may occur associated with thomsonite, such as native copper and secondary copper minerals.

Sample preparation: Thomsonite, like other zeolites, acquires a great polish without difficulty.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, more or less like quartz and feldspars.

Pleochroism: No.      

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was be observed.        

Internal reflections: Clear, colorless, milky, and generally generalized.      

May be confused with: Many other minerals, including many from the same paragenesis, especially other zeolites. Thomsonite cannot be identified by reflected light.

Caption