Educate Me On Dolostone And Limestone

[pinkpantherbeekeeper]

So I need help identifying Limestone and Dolostone from each other. I would like to be able to properly id, describe, catalog, etc finds. This also would help me date my finds.

From what I know from Wikipedia is basically dolostone, is limestone, that dolomite replaced the magnesium in limestone. Obviously limestone contains high calcium carbonate. Dolostone can come in brown, pink, and white colors. Also I have seen pictures of grey colors as well. This is confusing as what I know as limestone comes in white and grey. Dolomite also crystallizes? Would this make it appear to be more grainy like sandstone?

Can somenone break this down to me? Basically articles on the internet just tell me over and over dolostone contains dolomite! Arghh! Lol. Feels like I am banging my head againt a wall! Thanks for your help!!!

[Ridgehiker]

Not really all that defined. In geology in Alberta we prefer to use the term dolomite over dolostone. Both are used interchangeably even though they 'sort of' are different.

In the practical world we call limestone that visibly bubbles when hydrochloride acid is applied as 'limestone'...if it doesn't react then 'dolomite'. Fossil fuel deposits tend to be found in dolomite ( a big deal here and to do with crystallization and porous rock for deposits).

One of the issues with definition is one of precedence in the literature. Dolomite precedes dolostone so it's a bit like having a legal name on a birth certificate that was a mistake but nobody ever corrected. Regardless if dolostone or dolomite, for paleo purposes still often called 'fossil bearing strata, or whatever.

Also, don't get caught up in 'colour' except to locate a layer. Colour can change depending on the traces of magnesium, iron,etc. Colour is more a local variable.

[pinkpantherbeekeeper]

Thanks! That does help me believe it or not. One question still... will dolomite strata containing higher percentages of dolomite appear to be more "grainy," such as a sandstone?

[squalicorax]

No really

Limestone is pretty diverse in its texture and can appear grainy. Dolostone is definitely harder and more crystalline.

But yes the differences can be difficult

[Ridgehiker]

As squalicorax states it may. Depends on what the original minerals were that mixed with the organic deposits. Also, what minerals have permeated the rocks over the last 'x' millions of years. Often there are various quartz grains from silica sands or calcite crystals from Echinodermata, etc.

Bentonite clays, mudstone,sandstone, limestone, shales, etc are all 'local'. They are Different between formations and different between members in a particular formation.

Adding to confusion is some deposit or formation may be referred to as the 'Something' limestone or 'Something' shales, etc. whereas the reality is the deposits are often a mix of deposits like cake layers....mudstone, shales, limestone, etc.

[painshill]

Dolomite is a mineral. It has a defined chemical composition as calcium magnesium carbonate, ie CaMg(CO3)2. It is most usually found as “druzes” (coatings on or within cavities in other minerals or rocks) or masses/clusters of small white to tan to pink rhombohedral crystals with a “saddle”-like shape. But other colours are possible, depending on trace mineral impurities. It’s found in both sedimentary rocks (such as limestones and dolostones) and metamorphic rocks (such as marbles and metadolostones). It also forms by crystallisation in hydrothermal veins in many other rock types and in rocks that are serpentinized (typically hydration and metamorphic transformation of ultramafic rocks from the Earth’s mantle, as might be seen in ancient ocean floor rocks at tectonic plate boundaries).

Dolostone is a rock (or - more accurately - a family of rocks collectively called dolostones). It doesn’t have a defined composition. The conventional geological definition is that dolostone refers to any principally carbonate sedimentary rock within which the mineral content of dolomite is more than 50% by weight (or judged by eye under the microscope from the volume/area). The mineral composition of the remaining 50% can cover a huge variety of other carbonate forms and lesser amounts of other secondary minerals. As a geological term, it’s pretty meaningless unless accompanied by additional qualifications that describe the mineralogy or – at minimum – the stratum/location where it came from. As such, pretty much any colour is possible for a dolostone and such rocks may not reflect dolomite’s typical moderate hardness of 3.5 – 4.

The two terms are used interchangeably by non-geologists (and many lazy geologists).

[Addition]: Dolomite the mineral most certainly does react with acids to produce bubbles... just not very vigorously so compared to calcium carbonate minerals... and you normally need to do a bit of scraping with penknife blade to generate some powdery material. Same for dolomitic rocks.

Edited August 6, 2013 by painshill

[pinkpantherbeekeeper]

Thanks again guys! Hopefully this helps some other people as well.