NATROLITE

This article appears in Volume V19, Page 273 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: NAN-NEW

NATROLITE , a

mineral

MINERAL

species belonging to the zeolite group.
It is a hydrated sodium and aluminium silicate with the

formula

Na2Al2Si3O1o2H2O, and containing sodium (Na2O, 16.3%),
was named natrolite by M. H. Klaproth in 1803. " Needle-
stone " or " needle-zeolite " are other names, alluding to the
common acicular habit of the crystals, which are often very
slender and are aggregated in divergent tufts. Larger crystals
have the form of a square prism terminated by a low pyramid:
the prism

angle

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

being nearly a right

angle

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

(88 452'), the crystals
are tetragonal in

appearance

APPEARANCE (from Lat. apparere, to appear)

, though actually orthorhombic.
There are perfect cleavages parallel to the faces of the prism.
1 E.

Thomas

, Histoire des ateliers nationaux, p. 29.
273
The

mineral

MINERAL

also often occurs in compact fibrous aggregates, the fibres having a divergent or radial arrangement (hence the name radiolite for one variety). From other fibrous zeolites natrolite is readily distinguished by its optical characters: between crossed nicols the fibres extinguish parallel to their length, and they do not show an optic figure in convergent polarized light. Natrolite is usually white or colourless, but some-times reddish or yellowish. The lustre is vitreous, or in finely fibrous specimens sometimes silky. The spec. gray. is 2.2, and the hardness 52. The mineral is readily fusible, melting in a candle-flame, to which it imparts a yellow colour owing to the presence of sodium. It is decomposed by hydrochloric acid with separation of gelatinous silica.
Natrolite occurs with other zeolites in the amygdaloidal cavities of basic igneous rocks. The best specimens are the diverging groups of white prismatic crystals found in compact basalt at the Puy-de-Marman, Puy-de

Dome

DOME (Lat domus,
house
- HOUSE
- HOUSE (O. Eng. hiss, a word common to Teutonic languages, cf. Dut. huis, Ger. Haus; in
  Gothic
  - GOTHIC
  it is only found in gudhiss, a temple; it may be ultimately connected with the root of " hide," conceal)
; Ital. duomo,
cathedral
- CATHEDRAL
)

, France. The largest crystals are those from Brevig in Norway. The walls of cavities in the basalt of the Giant's Causeway, in Co. Antrim, are frequently encrusted with slender needles of natrolite, and similar material is found abundantly in the volcanic rocks (basalt and phonolite) of Salesel, Aussig and several other places in the north of Bohemia.
Several varieties of natrolite have been distinguished by

special

SPECIAL

names. Fargite is a red natrolite from Glenfarg in Perthshire. Bergmannite or Spreustein is an impure variety which has resulted by the alteration of other minerals, chiefly sodalite, in the augitesyenite of southern Norway.

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