Description:
Natrolite is a common and popular zeolite mineral.
Its radiating sprays of ice clear acicular crystals
are not exclusive to natrolite but they are a hallmark
of this mineral. Natrolite can make a fine specimen
in itself but it often is an accessory to other minerals
and can enhance the beauty of associated minerals such
as apophyllite, heulandite, benitoite and others. Natrolite'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 zeolites like natrolite can act
as a chemical sieve. Natrolite's structure contains
chains of silicate tetrahedrons aligned in one direction;
this produces the needle-like crystals. Its cleavage
results from the weaker bonds between the chains. Natrolite,
a sodium zeolite, scolecite, a calcium zeolite, and
mesolite, a calcium and sodium zeolite, are closely
related and sometimes found together. The presence of
calcium in two of the minerals makes the structure slightly
different from that of natrolite; it is altered from
an orthorhombic symmetry to a monoclinic symmetry. However,
the twinning of scolecite and mesolite often make them
appear orthorhombic. All three minerals are referred
to as "chain" or "needle" zeolites.
They are similar and difficult to distinguish when in
clusters with radiating, acicular habits. Natrolite
tends to forms thin crystals with pyramidal terminations,
but mesolite's fiber-like crystals are usually the thinnest
of the three. Scolecite's larger crystals tend to be
more robust and durable. These characteristics are only
generalities, though, and can not be used as dependable
identifying traits; absolute identification cannot be
made by ordinary means. |