Cavechat.org

why do some formations tend to be more redish brown( do to the iron I known) on the out side,and then you see ones that are broken and they are white in the middle and redish on the outside....
is it due to the fact the iron didn't get a chance to oxides,because of rapid growth?

Excellent question. I'd say that due to the fact that formation growth is generally very slow(in human terms,anyway..),the chemistry of the groundwater changes over many centuries/millenium.
These differences in dissolved mineral content cause the different colors in the speleothems.

In my neck of the woods,it is not uncommon to see pure white to transparent speleothems mixed among larger,dark red formations,that range from ruby red to almost black.

While Iron and Manganese are usually the primary agents,there are a few cases where I would like to see some MS,or,x-ray diffraction tests done.

Good points CKB69.

Also, it could be that at one time, the white/clear calcite that deposits in caves is just calcite dissolved from the limestone. Then, maybe some regolith (dirt) is introduced into the area of dissolution, and is deposited along with the calcite.

Just an idea.

Hi,

True - iron and dirt are often argued to be linked to changed in speleothem colour. More specifically, there is more organic particulate matter in darker calcite. But nothing is ever so simple though.

You have probably heard that there can be annual banding in the calcite at a microscopic and sometimes if they grow real fast, at the "visible to the naked eye" resolution. The banding in speleothem calcite is in part due to seasonal and inter-annual (decadal, century, etc) changes in the hydrology, climate above the cave, etc etc. In addition to the particulate matter, some aspects of the changes in colour over time (the banding) is due to a real soup of complex organic acids leached out of the soil and organic matter above the cave as the water percolates down and this varies of course over the seasons in temperate environments.

The colour of speleothem calcite is therefore a really great subject, as you would expect the colour to be different depending on the hydrology (fast or slow dripping water), and climate (how much evaporation/evapotranspiration and so concentration of the organic molecules in the drip water) and of course, the type and density of vegetation which produces the organic matter in the first place. If you have significant change in the landscape/ecosystem, then these changes should be reflected in the drip water chemistry, but also the colour of the calcite formed by that drip! If you are looking at a broken stal and you can see a big change in colour or really noticeable rings, then you are probably looking at evidence of big changes.

You mention that different colour speleothems can be right beside each other in the same cave/chamber. I have seen this too. I am curious though if you have noticed which colours have water actively dripping on them, and if there is a distinction between colour / drip rate (fast-slow).

I am also curious if anyone out there knows of documented colour changes in the speleothem calcite before and after a specific landuse change - maybe when a parking lot is put on top of the cave, or the area has been clear cut logged.

Cheers
Trish Beddows.

i like all the answers that you guys have given me,..if anyone else has any as well,please feel free to share them. Thanks for you time.
Andrew

Tannic and humic (organic) acids cause reddish-brown colors in calcite as well as the oxidized iron in dolomite and some iron rich limestone bedrock regolith (clay left after carbonate removal).

I concur with much of what Trish says. If you are looking for documentation of speleothem change, show caves with 100 years or more of history would be a good place to look for a record. I know of one cave where in the last 50 years the brown part of a a major column has migrated (grown larger), although the land use change over the cave is negligible.

I'm partial to the muddy water staining scenario, myself. Of course most of my experience has been in dolomite caves. We have someone who did a cave sediment analysis of bright red phreatic clay, and turns out the actual iron content, although present, is negligible.

Of course out west here wet active formations are prone to have the ever present dust stick to them.

CKB69 wrote:While Iron and Manganese are usually the primary agents,there are a few cases where I would like to see some MS,or,x-ray diffraction tests done.

X-ray diffraction will generally only be able to tell you what type of material you are analyzing. It won't give you a quantitative chemical analysis. If you are trying to find the relative amounts or Fe or Mn and their relationship to the color of a stal, you would be more likely to use X-ray flourescence.

I remember reading an article in the Journal of Cave and Karst Studies (The Genesis of the Tennengebirge Karst and Caves, Dec 2002, pg. 155. That was a pain to track down.....) which stated that transparent calcite is indicative of having grown under a wooded covering (forest on the surface, I assume). The paper that is cited for this is "La haute montagne calcaire" by R. Maire. If you can track down this paper (and get it translated, assuming you don't speak french), it may have something to say about what affects the color of calcite in caves.

Here in Middle Tennessee, it is common to see broken speleothems that are clear or white on the inside, but brown on the outside.

It just seems very likely that when the land was forested, or at the least fairly undisturbed, cleaner water moved downwards and produced cleaner formations.

Once farmers moved in and the trees were cut down and the soil was plowed up, lots of soil moved downwards and now we have muddy-brown cave formations, at least on the outside.

Larry E. Matthews
Professional Geologist
NSS #6792-F