Crystal habit, clay

Crystal habit modification. Several crystal habits have been reported in the open and patent literature for zeolite omega. Elongated hexagonal rods (15,16) or fibres (17) have been reported when the zeolite resulted from the recrystallization of another zeolite, Y (15) or S (17), or of clays (16). Moreover the natural counterpart of zeolite omega, mazzite, appears as bundles of needle-shaped particles (18). All these solids have been grown at low supersaturation levels, hence under conditions in which our results show that the growth in the direction <001> prevails on the growth normal to the c-axis (Fig.8). 

These minerals have different stacking of the silica and alumina layers, as well as, incorporating metal hydrates of Na, K, Mg, Al, or Fe between the silica and alumina layers. Clay minerals can also be characterized according to their morphological features including crystal habit (i.e., plates, rods, or rolled-up platelets) stacked in either a house of cards or blocklike aggregates giving a partide-size distribution. 

Clay minerals have been used for a long time in various fields of application (e.g., paper coating) because of their platelike crystal habit in colloidal dimensions and the ability to bond to one another. These phenomena of structure formation are predominantly controlled by Coulombic forces between the negative charges on the basal planes and the positive charges around the edges. 

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

Pyrite averages 0.2 vol%, and only in a few samples forms up to 1.3 vol%. It shows two occurrence habits (i) fine crystals (< 2. im) or framboids scattered in kaolinized or chloritized detrital clays and micas, or engulfed by coarse carbonate cements (Fig. 15E) and (ii) coarsely crystalline (up to 200 pm across), intergranular replacive cement. 

---