Page 65 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 140, 141, 142
N e o m o r p h i s m

Microspar, Pseudospar

The term neomorphism refers to all transformations
between a mineral and the same mineral, or another of the
same general composition. During diagenesis, aragonite
components of limestone may be transformed to calcite
without the development of significant porosity. There is
usually an acc ompanying increase in grain size (aggrading
neomorphism). In particular the micritic material of
limestones may be altered to coarser calcite. The terms
microspar and pseudospar are used for crystal mosaics of
neomorphic origin having a mean size of between 4 and
IOttm and > IOttm respectively. It is not always possible
to ditfcrcntiatc between neomorphic f a brics and fine spar
cements. or sediments composed of primary sih-Siled
part1cle�. As a guide. neomorphic spar generally has
irregular crystal boundaries and patchy grain size distri­
bution. often with relicts of micrite and 'floating' skeletal
grains.
140 shows a limestone in which the matrix is fine
pseudospar. It is cloudy and contrasts with the coarse
clear mosaic replacing the wall and infilling the chamber
of the mollusc on the right of the photograph. Its grain
si1c varies patchily and it is thus likely to be neomorphic,
having originally been a micritic sediment.
141 shows a very fi n e-grained limestone (note magnific­
ation) composed almost entirely of calcium carbonate
grains of microspar size. There appear to be no micritic
relicts and this fabric may be a primary one as a result of
deposition of carbonate mud of fine silt-sized particles,
rather than a product of neomorphism of micrite.
142 illustrates a limestone with a few dolomite rhombs
(dark-coloured) in a 'matrix' of pseudospar with patches
of microspar and micrite. ote how crystal size and shape
\ary irregularly throughout the mosaic. This is character­
Istic of a neomorphic fabric.

140: Swined thin section. Carboniferous Limestone. Llan­
gollen, North Wales; magnification x 43. PPL.
141: Stained acetate peel. Blue Lias. Loll'er Jurassic.
l.m·emo< ' k Point, South Wales: magnificmion x 72. PPL.
142: S/(/ined acetate peel, Woo Dale Limestone, Lower
Carhoniferous. Woo Dale. Derbyshire. England; magnific­
otion x 43, PPL.
A neomorphic f a bric is also shown m 161.