Page 12 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 6, 7, 8
Q u a r tz

(continued)

The quart7 grain in the centre of the field of view in 6
appears to be a single homogeneous crystal. In 7 however,
where the same field of view is seen under crossed polars,
the quartz grain is clearly made up of pans of two
crystals. One, cromprising the upper left portion of the
grain is showing a mid-grey interference colour, whereas
the rest of the grain comprises a crystal with areas
�ho" ing :.lightly different interference colours. The left­
and right-hand sides are in extinction and interference
colour!> become progressively paler towards the centre of
the gram. Such a grain would show sweeping extinction
"hen rotated. This phenomenon. known as undulose
ntinction. i!> a result of strain and is found in quartz
gram� from both igneous and metamorphic sources.
Quartt cry:;tals may sometimes incorporate mineral
mclusions and identification of the minerals may yield
information about the provenance of the sediment.
The quartz grain in the centre of 6 and 7 has a number of
needle-shaped inclusions, although they are too small for
the mineral to be identified at the magnification shown.
Inclusions of the fluid present at the time of crystallization
arc common in quartz crystals and are known as fluid
inclusions or vacuoles. 8 shows a quartz grain with
abundant vacuoles. These appear as dark specks, and in
the sample illustrated, many are concentrated in lines
running at a low angle to the length of the picture. Quartz
with abundant vacuoles is usually derived from a source
of low-temperature origin, such as a hydrothermal vein,
and appears milky-white in a hand specimen. the photo­
graph also shows a green mineral in the matrix around the
quartl' grain, which is chlorite.

6 and 7: locality and age unknoll'n; magnification x 72: 6
PPL. 7 XPL.
8: Coal Measures. Upper Carboniferous. Lancashire.
Englmul: magn(fication x 72. PPL.
Undulose extinction can also be seen in 5.

6