THE MINERAL THOMSONITE

  • Chemistry: NaCa2Al5Si5O20 - 6H2O, Hydrated sodium calcium aluminum silicate.
  • Class: Silicates
  • Subclass: Tektosilicates
  • Group: Zeolites
  • Uses: Mineral specimen and chemical filter.
  • Specimens
Thomsonite is one of the rarer zeolites. It forms tight acicular radiating clusters and sphericules as well as some blockier crystals that are found in the vesicles or bubbles of volcanic rock as are most other zeolites. Natrolite, another zeolite, is usually square in cross section but is otherwise difficult to distinguish from thomsonite. The color is usually colorless or white, but a few specimens have shown a lovely yellow color. Thomsonite is a rare mineral and is sought after by collectors of rare zeolite minerals.

Zeolites are known to have an openness about their structure that allows large ions and molecules to reside and actually move around inside the overall framework. The structure actually contains open channels that allow water and large ions to travel into and out of the crystal structure. The size of these channels controls the size of the molecules or ions and therefore a zeolite can act as a chemical sieve, allowing some ions to pass through while blocking others.

PHYSICAL CHARACTERISTICS:

  • Color is clear, white and yellow .
  • Luster is vitreous.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is orthorhombic.
  • Crystal Habits include tight acicular radiating clusters and sphericules. Blocky, prismatic, fibrous and tabular crystals are also known.
  • Cleavage is perfect.
  • Fracture is uneven.
  • Hardness is 5 - 5.5
  • Specific Gravity is approximately 2.2 - 2.4 (light).
  • Streak is white.
  • Associated Minerals are quartz, calcite, chabazite, natrolite, heulandite, stilbite and other zeolites.
  • Notable Occurrences include Saxony, Germany; Italy; Faroe Islands; Kilpatrick Hills, Scotland and Kern Co., California and Cape Lookout, Oregon, USA.
  • Best Field Indicators are crystal habit, density, hardness and associations.
THOMSONITE specimens:
(hover for more info)
THOMSONITE specimen thm-1
$ 60.00
Dims: 1.3 x 1.1 x 0.4" (3.2 x 2.7 x 1.1 cm)
Wt: 11 g
Minnesota, U.S.A.
This polished thumbnail specimen consists of a polished piece of a black (probably basalt) matrix that contains several Thomsonite "eyes". Each "eye" consists of an acicular spray of Thomsonite needles that extends from a more massive, concentrically-layered base. None of the Thomsonite crystals are isolated and free of the black matrix, and all that are visible are incomplete due to the grinding and polishing process. They have a milky-white coloration, but some have a pale pink tinge. Their round, layered bases have a color that ranges from pale pink to a brownish-red, and polish to a high gloss. A small chunk has been broken out of one of the rounded edges.
no photo
thm-1 ($ 60.00)
Minnesota, U.S.A.
THOMSONITE specimen thm-2
$ 25.00
Dims: 1.6 x 1.6 x 1.2" (4.2 x 4.1 x 3.0 cm)
Wt: 1.4 oz. (41 g)
Minnesota, U.S.A.
This small hand specimen consists of a vein of massive, acicular and fibrous, radiating Thomsonite that extends between two thin basalt slabs. The crystals do not exceed 0.2" (5 mm) in length, and though many of the exposed needles are damaged and all appear to be somewhat weathered, their orthorhombic form is likely definable. The crystals have a dull white coloration and only a dull pearly or greasy luster at best. The compact, more massive Thomsonite beneath them has a color that ranges from white to pale brown and a dull, almost silky luster.
no photo
thm-2 ($ 25.00)
Minnesota, U.S.A.
THOMSONITE specimen thm-3
$ 84.00
Dims: 1.97x1.46x0.79" (5.0x3.7x2.0cm)
Wt: 1.07oz. (30.3g)
Globe, Oregon, USA
A half-dozen globes of white thomsonite lie on a crust of heulandite in this specimen. The thomsonite balls have a distinct texture, looking very much like tiny snowballs (although having a slightly off-white color). The heulandite crystals are transparent and colorless. Tiny amounts of another mineral disturb the perfection of the thomsonite balls by giving an appearance of dirt on the surface, in several small areas. Still, this is an unusually nice specimen of this rare mineral.
no photo
thm-3 ($ 84.00)
Globe, Oregon, USA
THOMSONITE specimen thm-4
$ 32.00
Dims: 1.38x0.98x0.51" (3.5x2.5x1.3cm)
Wt: 0.24oz (6.8g)
Cape Grim, Tasmania, Australia
This specimen is all thomsonite, as one (and a fraction) globes of acicular crystals. Close examination reveals that the individual crystals are colorless, transparent, square in cross section, and some have basal terminations (unlike the similar mineral natrolite).
no photo
thm-4 ($ 32.00)
Cape Grim, Tasmania, Australia
 

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