I am currently doing core analysis and thin section analysis and I need some clarity on some aspects.?

What is the difference between intraclasts and rip up clasts? Also, how to differentiate between dolomite and calcite cement? The crystals are too small to distinguish the anhedral and euhedral shape.Thank you very much.

1 Answer

  • 7 years ago
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    A rip-up clast is produced by a specific depositional/erosional process. Intraclasts are simply clasts that came from syndepositional erosion of the rock. A rip-up clast can be either an intraclast or an extraclast. An example of an intraclast that is not a rip-up would be a clast formed by collapse of karst. The clast is intraformational, so it is an intraclast, but it was not formed by currents.

    "intraclasts are irregularly-shaped grains that form by syndepositional erosion of partially lithified sediment. Examples include mudlumps that are torn up from the bottoms of lagoons during storms, hardened desiccated mud flakes produced in intertidal and supratidal environments and fragments broken from cemented deep-sea crusts (above figure). Other intraclasts are aggregates of carbonate particles (figure below). These include grapestones and botryoidal grains. Grapestones are composite grains with an irregular shape that resembles a bunch of grapes, whereas botryoidal grains are similar but with oolitic coats enveloping the aggregate grain. These types of intraclasts from in shoal water environments with intermediate wave and current activity, where grains that are cemented on the sea floor are broken into aggregate fragments and lumps during storms."


    Dolomite and calcite cements are hard to differentiate in core, but easy to differentiate in thin section. With core you may try using a 5% acid instead of 10%. The 5% is not going to produce a strong reaction on dolomite, but will still cause a reaction on calcite. The 10% solution of acid is going to cause a vigorous reaction from both. This is still subjective, and requires some experience to judge. You may have to find some known samples to calibrate your observation skills.

    In thin section it is easy to tell the difference. First of all, staining with Alizarin Red is a common way to test for dolomite. Calcite and aragonite turn red, but the dolomite remains unstained.

    See: Scholle, Peter, 1978, Memoir 27: A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities, Tulsa, Oklahoma: AAPG, page 227

    Even without staining it is often possible to distinguish dolomite from calcite under a microscope. Here are some examples:


    Source(s): geologist
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