Page 39 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks

Introduction

Unlike terrigenous sediments. carbonate rocks comprise material relief between the two minerals. Results of the etching and staining
formed mostly at or ncar I he site of final accumulation of the sediment. process arc shown in Table 2. Details of the procedure are given in
Much of the material is produced by biological processes. Two Appendix 2.
carbonate minentls arc common in older limestones - cah·ife. CaCO,. The intensity of the stain colour is partly related to the intensity of
and do lomite, CaMg(C01}2. I n recent s h allow marine carbonate the etching with acid. Fine-grained crystal fabrics with many crystal
sediments the mineral aragonite, also CaC03• is abundant. However, it boundurie� etch more rapidly and thus show deeper stain colours than
is metastable under the normal conditions prevailing in sediments and coarse crystal f a bric� with few crystal boundaries.
i� u�uall� dl-,,ol\cd once a limc'>tonc is exposed to circulating meteoric Stain colours arc particularly weJI-iJlustrated in 100, 124. 131, 161
waters. Altcrnall,cJy 11 may 1nvcrt directly to calcite. Dolomite is and 165.
normally a secondar) mineral replacing calcium carbonate, although Other stams h;ne been used to distinguish between aragonite and
it ma) form m \Ctllments very soon after their deposition. Both calcite calcite and to idcntif) magnesian calcites: details are given in books on
and dolomite may contain some f e rrous iron. in which case the prefix technique� in sedimentary petrology such as that of Carver (1971).
/e rroan is used before the mmcral name. Carbonate roc"s may also be examined using acetate peels. These
The opllcal propertic'> of calcite and dolomite are similar and record an irnpre�sion of an etched rock surface (which also may be
therefore they can be difficult to distingubh optically. Simple chemical stained) on a thm sheet of acetate film. Acetate peels have the
�taining technique� arc often employed by carbonate sedimentologists advantage of being cheap and ca::.y to make. but because they arc
to distinguish calcite from dolomite and to distinguish f e rroan from isotropic. minerals cannot be identified by optical properties, such as
non-fcrroan minerals. relief and birefringence. Details of the procedure for making acetate
The dye Ali:arin !?ed S is used to diiTercntiatc calcite and dolomite, peels are given in Appendix 3.
whereaspotassiwn/('rricranide is used to diff erentiate f e rroan and non­
fc rroan minerals. The dyes arc dissolved in a weak acid solution. This Components
also helps to distinguish dolomite from calcite. as dolomite does not
react with cold dilute ucid whereas calcite docs. producing a contrast in
The three most important components of carbonate rocks arc
al/ochemical components, microcrystalline calcile. and sparry calcite.
Tahle l. Elchint: and stlliniiiK charaeleristics ol carbonate minerals
I. Allochcmical components or allochems. are organized aggregates
I Mineral Effect of St:lin Stain Combined of carbonate sediment which have fo rmed within the basin of
etthin,:: colour with colour with result
Alitarin I>Ota��iurn deposition. They include ooids. bioclasls. peloids. intraclasts and
RedS ferric) an ide oncoids and arc considered in detail in the fo llowing section (72 to
120).
Calcllc (non- COihltkr- rink to None 'Pink to red­ ., Microcr}stallinc calc1tc or micrite is carbonate sediment in the form
f c rroan) able (rdu:f I rcd-bnm n I brown of grains less than S11m in diameter. Much of it forms in the basin of
I reduced! deposition. either as a prcc1pitatc from seawater or from the
Calcttc Con\ldCr· Pmk to Pale 10 MaU\e to disintegration of the hard parts of organisms. such as green algae.
(fcrroan) able (rchcf rcd-bnm n <k c p blue blue The term ·carbonate mud· is also used for this fine sediment.
red uced) dcpcndmg on although stnctly mud mcludes material of clay- and silt-size (up to
tron content
6:2Jim). Micrite 1s illustrated in 84. 89. 111 and 157.
Dolomnc Ncghg1ble l None �one 'Colourless J. Sparry calcite. lf larite or .1par refers to crystals of 5 pm or more in
(non-fcrroan) (rehct I diameter. Much of it is coarse. with crystals commonly up to I mm
I m;unlaincd) in �iLe. It i'> usually a pore-filling cement and thus may form in a
Dolomite 1\icghgtblc None Very Very rod a long time after deposition of the original allochems and
(ferroan) (rdicf pale blue pale blue micrite. Spa rite is illustrated in 73. 82. 124 and 131.
maintatncd) (appears The clas�itkation of limestones involves the identification of
turquoise or
greenish in allochems and estimation of the proportions of micrite and spa rite (sec
thin scctio� p. 62).

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