• Chemistry: BaAl2Si6O16 - 6H2O, Hydrated barium aluminum silicate.
  • Class: Silicates
  • Subclass: Tectosilicates
  • Group: Zeolites
  • Uses: Mineral specimen and chemical filter.
  • Specimens

Harmotome is one of the rarer zeolites, but is popular among zeolite and twin collectors. Its twins are considered classics among collectors who like twinned minerals. If perfectly formed, a twinned crystal of harmotome can appear to be composed of three prismatic crystals grown through the each others center at nearly 90 degree angles. Each termination can look like the end of a blunted phillips head screw driver. The result is a fairly complex looking crystal that is glassy, angular and very attractive.

Harmotome is occasionally found with other zeolites like heulandite and stilbite in the typical zeolite occurrence inside of vesicles or bubbles of volcanic rocks. However it is one of the only zeolites to be found in ore deposits.

Phillipsite is another zeolite that is difficult to distinguish from harmotome. It to commonly forms twins and in fact can form nearly the same twin as harmotome, although less likely to do so. The slightly less dense phillipsite is more likely to be found as aggregates and not as individual twinned crystals and is never found in ore deposits.

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 harmotome can act as a chemical sieve, allowing some ions to pass through while blocking others.


  • Color is colorless, white and yellowish.
  • Luster is vitreous.
  • Transparency: Crystals are transparent to translucent, often milky.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include a classic penetration twinned crystal that is seemingly composed of three prismatic crystals growing through each other at near 90 degree angles. Although examples showing a perfectly formed twin crystal are scarce, twinning is wide spread and almost all crystals of harmotome are twinned. Aggregates can be radiating and columnar.
  • Cleavage is poor in one direction.
  • Fracture is conchoidal to uneven.
  • Hardness is 4.5.
  • Specific Gravity is approximately 2.4 - 2.5 (light, but heavier than most zeolites).
  • Streak is white.
  • Associated Minerals are quartz, heulandite, stilbite, pyrite, brewsterite, edingtonite, barite and other barium and some manganese minerals.
  • Notable Occurrences include several localities in Germany also found at Kongsberg, Norway; Strontian, Highland, Scotland; Wales, Westchester Co., New York, USA and in other localities around the world.
  • Best Field Indicators are crystal habit, twinning, density, hardness and associations.
HARMOTOME specimens:
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HARMOTOME specimen har-1
$ 80.00
Dims: 2.9" x 2.1" x 1.1." (7.3 x 5.3 x 2.8 cm)
Wt: 4.16 oz. (118.0 g)
Strontian, Scotland
It would appear that many of the Harmotome crystals that erupt from the base rock of this specimen are not twinned! I see several monoclinic prismatic crystals that seem to be individual, without any of the symmetry that would signify twinning. Others, however, are obviously twinned, and some of these have a seemingly hexagonal prismatic form. The largest of these crystals has dimensions of 0.4 x 0.4 x 0.2" (1.0 x 1.0 x 0.5 cm), and shows only a small amount of damage along its edges. Other crystals are almost as large, but are heavily damaged and usually incomplete. Crystals range in size down to 1 mm or less in each dimension, and tend to be in generally better condition as their size decreases. They are milky-white and have a vitreous luster. Some crystals show small portions of transparence, but these are spoiled by intense internal fracturing, which I believe contributes to their color. They rest on a rather nondescript gray-green, igneous host rock that shows a few veins of either calcite or more, massive Harmotome.
no photo
har-1 ($ 80.00)
Strontian, Scotland


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