Geoscience Reference
In-Depth Information
6
Hydrothermal Synthesis and
Growth of Zeolites
6.1 Introduction
Zeolites form an important group of hydrated aluminosilicate minerals of the alka-
lies and alkaline earths, with an infinitely 3D anion network and with the atomic
ratio O:(Al
2. The history of zeolites began with the discovery of stilbite in
1756 by a Swedish mineralogist, Cr¨nstedt
[1]
, who heated this silicate mineral,
which fused readily with marked intumescence. The name zeolite signifies boiling
stone, the meaning derived from the two Greek terms, zein, to boil, and lithos,
stone, an allusion to the intumescence of most zeolites with a borax lead, and refers
to the frothy mass which will result when a zeolite is fused in the blowpipe.
Volatile zeolitic water forms bubbles within the sinter or melt
[2]
.
Zeolite minerals were first recognized as minor but were later regarded as wide-
spread components in amygdales and fissures, chiefly in basic volcanic rocks.
They are also found in veins and other later-stage hydrothermal environments. Big
crystals of zeolites can develop in these amygdales and fissures through the miner-
alizing action of trapped or circulating solutions on the alkaline matrix. It is these
crystals which, because of their beauty and diversity of form, usually comprise
zeolite collections
[3]
. Zeolites are also found in some igneous rocks and occur as
alteration products of aluminosilicates, such as the felspars or felspathoids. Their
occurrence is not just restricted to basalt matrices, although the scale and variety
of natural depositions have been appreciated only over the last three decades. They
occur as authigenic minerals in sandstones and other sedimentary rocks. Loew
[4]
reported chabazite in a sedimentary tuff bed near Bowie, Arizona. Similarly,
Several kinds of zeolites were identified by Coombs
[5]
.
Si)
1
5
6.2 Mineralogy of Zeolites
Zeolites belong to the tectosilicates group covering the felspars and felspathoid
family of minerals. However, the structure of felspars is a more compact frame-
work, with or without relatively small cavities. Thus, the cations and framework
are strongly interdependent so that the cations cannot easily move unless frame-
work bonds are broken, and replacement of Na or K by Ca necessarily involves
