THE MINERAL GMELINITE

  • Chemistry: (Na2, Ca)Al2Si4O12 - 6H2O, Hydrated sodium calcium aluminum silicate
  • Class: Silicates
  • Subclass: Tektosilicates
  • Group: Zeolites
  • Uses: Mineral specimen and chemical filter.
  • Specimens
Gmelinite is one of the lesser known and rarer zeolites. Zeolites 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 like gmelinite can act as a chemical sieve, allowing some ions to pass through while blocking others.

Crystals of gmelinite have a unique shape that could be described as being an angular version of a classic UFO flying saucer. It is hexagonal in outline and is dominated by a dipyramid (two identical pyramids at the top and bottom). However, the faces of the pyramids are relatively gentle in slope and are capped by a flat pedion face.

PHYSICAL CHARACTERISTICS:

  • Color is clear, white, pink, yellowish and reddish.
  • Luster is vitreous.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is hexagonal.
  • Crystal Habits include tabular to platy crystals with a clear hexagonal outline. The crystals have a gently sloping hexagonal dipyramid that terminates the top and bottom of the crystal that are often truncated by pinacoidal faces. The overall effect gives the look of an angular classic UFO flying saucer. Twinning is common with penetration twins.
  • Cleavage is absent.
  • Fracture is uneven.
  • Hardness is 4.5.
  • Specific Gravity is approximately 2.0 - 2.1 (very light)
  • Streak is white.
  • Other Characteristics: Vertical striations are seen on the dipyramidal faces.
  • Associated Minerals are quartz, calcite, chabazite, phillipsite, natrolite, heulandite, stilbite and other zeolites.
  • Notable Occurrences include Siegerland, Germany; Glenarm, Northern Ireland; New Jersey and Faroe Islands.
  • Best Field Indicators are crystal habit, density, hardness and associations.
GMELINITE specimens:
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GMELINITE specimen gme-1
$ 120.00
Dims: 1.3 x 1.1 x 0.7" (3.3 x 2.9 x 1.7 cm)
Wt: 12 g
Flinders, Victoria, Australia
At least 3 Gmelinite crystals are intergrown to form this small hand specimen. They are in very good condition, showing little damage, and reach diameters of nearly 0.8" (2.0 cm). Their hexagonal, platy form is excellent, showing well-defined edges and clean faces that have a vitreous luster. All have a pale pink color with a strong orange tinge and are transparent but intensely internally-fractured, completely spoiling their clarity. A small amount of a dull, dark gray material is present.
no photo
gme-1 ($120.00)
Flinders, Victoria, Australia
GMELINITE specimen gme-2
$ 48.00
Dims: 2.0 x 1.1 x 1.0" (5.0 x 2.9 x 2.5 cm)
Wt: 1.0 oz. (28 g)
Cairns Bay, Flinder's Shire, Victoria, Australia
Several thick, tabular Gmelinite crystals rest on the basalt base of this hand specimen. Most of these crystals are damaged and incomplete, but a few are intact and in excellnt condition. They reach diameters of about 0.4" (1.0 cm), and all have a very good hexagonal bipyramidal form with subtle but complex growth patterns on their faces. Their pink color and vitreous luster are standard for the specie, and all are dimly translucent.
no photo
gme-2 ($ 48.00)
Cairns Bay, Flinder's Shire, Victoria, Australia
 

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