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THE MINERAL MESOLITE


Mesolite is a popular zeolite mineral for mineral collectors and zeolite collectors in particular. Its radiating sprays of ice-clear acicular crystals are a hallmark of this mineral. Often associated with minerals such as the green or clear apophyllite , the pink and pearly heulandite and the sparkling tiny crystals of quartz, mesolite is a wonderful addition in both style and rarity to these great mineral assemblages. Mesolite is often found in geodes containing other zeolites.

Mesolite's structure has a typical zeolite openness about it that allows large ions and molecules to reside and actually move around inside the overall framework. The structure 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 mesolite can act as a chemical sieve. Mesolite's structure contains chains of silicate tetrahedrons aligned in one direction and this produces the needle-like crystals and the cleavage results from the weaker bonds between the chains.

Mesolite, a sodium calcium zeolite, is intermediate between natrolite, a sodium zeolite, and scolecite, a calcium zeolite. They are closely related and sometimes found together. The presence of calcium in two of the minerals slightly alters the structure from that of natrolite from an orthorhombic symmetry to a monoclinic symmetry. Although twinning of scolecite and mesolite often make them look orthorhombic The three minerals are referred to as "chain" or "needle" zeolites. They are similar and hard to distinguish when in clusters with radiating, acicular habits. Natrolite forms thinner crystals with pyramidal terminations, and mesolite's fiber-like crystals are usually the thinnest crystals of the three minerals. Scolecite's larger crystals tend to be more robust and durable. These characteristics are only generalities and can not be used as dependable identifying traits. Absolute identification can not be made by ordinary means.

PHYSICAL CHARACTERISTICS:

  • Color is clear or white.
  • Luster is vitreous.
  • Transparency: crystals are transparent to translucent.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include sprays of needle thin acicular crystals with a slanted domal termination. Also nodules, fibrous and earthy masses.
  • Cleavage is perfect in two directions, prismatic. Cleavage is rarely seen due to small crystal size.
  • Fracture is conchoidal to uneven.
  • Hardness is 5 - 5.5.
  • Specific Gravity is approximately 2.2 - 2.4 (very light)
  • Streak is white.
  • Other Characteristics: Masses can have a chatoyant (cat's eye) effect.
  • Associated Minerals are quartz, apophyllite, datolite, heulandite, stilbite and other zeolites.
  • Notable Occurrences include Poona, India; Giants Causeway, Ireland; Skye, Scotland; Colorado, New Jersey and Oregon, USA and Berufjord, Iceland.
  • Best Field Indicators are crystal habit, density and associations.

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