Page 11 - Atlas of Sedimentary Rocks Under The Microscope

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Terrigenous clastic rocks
3, 4, 5
Quartz

(continued)

The three rounded grains in the ccntrc of3 and 4 are made
up of a number of quart; c1·y�tals in dillcrcnt orientations
and arc thu� colllfiO.Iil<' or polrl'rystal/in(' quartz. The
compo\ile nature or the grain� is clear only in the view
taken 111th polars cros,cd. Note that the boundaries
hct11ecn the Cl')\tal\ arc sulllrcd. Thi� is characteristic of
quart; from a metamorphic source. Composite quart7
from igncou� '>Ourcc� usuall)' has straighter crystal
houndanc\. The much liner '>cdimcnt surrounding the
compo,•tc quart; grain'> contam\ monocrystalline quart�:
and hnl\\ ni'h cla\b of fine-grained material which are
prohahl) 'hale or 'late fragmcnh.
5 sh011., a composite quartl grain viewed under crossed
polar,.m '' h1ch not only arc the crystal boundaries within
the grain sutured. hut aho the crystal' arc elongated in a
prcl'e�red direct ion. Such grains are called sheared q uarlt
or .1/rc•tc fwd 1111'/tllllorphic· quart;. In this type of quartz.
indi1·idual cry-.tals normally show undulosc extinction as
a re�ult of strain. Evidence l'or this in the example shown
comes from the non-unil'orm interference colours shown
hy many or the crystal:,.

3 wul 4. TrichmJ! I:Jedv. Silurian. Pomarllechau, D.1Jed.
ll'ale.1; IIIIIJ!IIi/imtion x 16:3 PPL. 4 XPL.
5: Carho11i/i'rous. Angle'sey. Wales: magnification x 41 .
.\'Pl..

5