358 University of California Publications.
ordinary
natrolite. Instead of the usual prisms with square pyramidal
terminations there are formed small roof-shaped ridges as shown in
plate 34, commonly with curved or more strictly broken roof lines and
coxcomb-like groups. These are made up of elements that are bounded by
a basal plane and unit prism. The smaller angle of the prism generally
projects out into the open cavity. In other words, the tendency is to
place the b axis at right angles to the wall. The apex of the
roof-shaped form is then a prism edge. The radiate groups are produced
by laying the elements together along the diverging basal planes whose
intersections are roughly parallel.
More
complicated aggregates are common. In certain druses lined with
greenish amphibole needles the natrolite occurs as small equant groups
looking in the distance as if individual crystals perched on the ends
of the amphibole fibres, as shown in figure 2, plate 35. A close
examination shows them to be complex aggregates in which the tabular
elements described above can frequently be observed. This mode of
occurrence is undoubtedly closely related to the botryoidal groups
which are occasionally found and which are illustrated in plate 36.
Under the microscope the structure of the natrolite gangue is rather
complex. As viewed on a fracture surface or in section, the radiate
groups described show divergent strips that are elongated at right
angles to the vertical axis. As the axis is perpendicular to the basal
plane the elongation in thin section will always be optically negative.
This is just the reverse of that usually seen in divergent «natrolite
groups and a positive elongation is generally looked upon as a
character of diagnostic importance.14
In sections parallel to the base is often seen the twinning described by Lacroix,15 with (110) as twinning plane forming, as it were, a sort of irregular mosaic when viewed with the aid of a gypsum plate.
A group of natrolite crystals, rare for this occurrence, was found showing the normal development of habit and planes. The
