Page 18 - Atlas of Sedimentary Rocks Under The Microscope
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Terrigenous clastic rocks 23, 24,25
Rock f r a g ments
(continued)
The variety of igneous rock fragments found in sediments
��·as great'" the variety of igneous rocks themselves and
lad or �pace prcvcnb the inclusion of more than " few
examples. Reference to the Atlas ()(igneous rocks and their
te\turl.'.llllay as�ist in the identification of fragments. The
ferro-magnesian minerals which arc common in basic
igneous roch arc relatively unstable in earth surface
conduion� and often show alteration. making identifica
uon or fragments difficult.
23 and 24 \hO\ \ a \olcaniC rock fragment in the centre
or the field of' IC\\. It con�ist� of plagioclase I at h' set in an
altered groum.lmass '' hich •� too finc-gntined fo r it!>
con\lltucnts 10 be identified at the magnification shown.
1\ 'econd rod fragment to the right of centre i� composed
of quart/ Cr)Stals SCI in a birefringent matrix probabl) of
cht) m1ncrals. The sedimem also contains separate f e ld
spar grains, some of which arc multiple-twinned plagio
dasc. and both monocrystalline and polycrystallinc
qu;u·v. The matrix or the whole rock contains hi refring
ent day or mien minerals.
2S and 26 show two diOercnt igneous rock fragments.
To the lcl't and above the centre of the field of view is a
fine-grained. probably volcanic. basi�: rock. It consists of
microphcnocrysts of plagioclase feldspar set in a ground
mn -.s or fcldspa r. very small pyroxene crysta Is and
opaque-.. Pale green chlorite occurs. possibly tilling
original \CSich.:s. Thi!> chlorite is black in the XPL view
l)Wing to it-. \Cry low birefringence. The lower part of the
held of viC\\ i� mostly occupied by a coarse-grained
pluwnic rock fragment consisting mainly of plagioclase
fcld,par and pyroxene. The simplc-t\\ inned fcld!>par� ma)
be ;�lk<tli f e ldspar. although in this case no difference in
rcfra�o:ti\C 1ndcx between them and the muhiplc-l\\inncd
pklglocla'c feld!>par� could be detected. This illustrate�
the dlflicuh) 111 prCCI\C Identification of igneou:. rod ..
fragment-. In adduion to �maller rock fragment\ the
,edimcnt cont<un� ,ubangular quart? grains and on the
nght hand edge a single cr) stal of a f e rro-magnesian
mmcral. probabl) amphibole. !>hO\\ing an orange inter
f e rence colour 1n the XPL \ icw. The relatively fresh
1gneou� rock fragmenb and ferro-magnesian ouneral
grain' suggest that thi!> sediment underwent lillie tran>
pon al'ter cros10n from source rocks.
Chert f ragml:nts arc quite common in sedimentary
rock-. \illl.:l: chert is stable and resistant to weathering.
Plate� 27 and 28 show a thin section of a conglomerate in
which the largl: rounded fragments arc chert. The view
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