Page 66 - Atlas of Sedimentary Rocks Under The Microscope

143, 144, 145 Carbonate rocks
N e o m o r p h i s m

Bioclasts

Most aragonite bioclasts arc preserved as calcite casts
with n o trace of the original microstructure of the wall
(sec p. 40). Occasionally. however. aragonite bioclasts
in1crt to calcite in situ. This is a f orm of neomorphism.
143 illustrates parts of the shells of bivalves which have
been subject to neomorphism. The shells consist of a blue­
>tained fcrroan calcite -;parry mosaic. but there are lines
or mclusions cutting across cry:.tal boundaries and in­
�\1:\�� \\ c original f o liated structure of the shell. Many
\
cn'\.1' arc a\so brown-coloured because of their inclu­
''" content The sediment between the shells is muddy
1,, '17/ .n> itbund;wr quart7 (unstained).
JJ
/J.I;md 145 show sections through a colonial scleract­
mtan coral, origmally composed of aragonite and now
calctte. 144 shows the coral at low magnification. The
coral walls and septa arc inclusion-rich. non-fcrroan
calcite of which the detailed fabric is unclear. Pore-space
tS filled with an inclusion-free sparite cement, very pale
mauve-stained and hcn�e slightly ferroan. 145 shows the
colony at higher magnification in a section which has been
ground slightly thinner than is usual. The coral walls
comprise an irregular mosaic of crystals of varying grain
silCS and shapes which is neither the original micro­
structure nor ad rusy mosaic, but a product of ncomorph-
1\m, Note that some crystal boundaries cut across from
por e-filling sparitc to coral septa and thus some crystals
arc partly neomorphic and partly cement.

/4.l Stamer/ 1hin .1ec1ion. Wealden, Lower Cretaceous,
"'llfllt'm I:11Kimul: magnification x 21. PPL.
/U am/ 145: S/(/ined thin sec/ions, lgKui e/ Behar Form­
t/1/1111, (ppa Jura,\Sil'. lmou::er d e .v lda-ou- T a nane. Wes­
ltlll 1/igh Alias, Morocco: magn!ficmion 144 x 12: 145
x4.1 PPL.
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