FMF - Friends of Minerals Forum, discussion and message board

[jordanlowe1089]

[jordanlowe1089]

Orange red Dolomite in Quartz Geode

[John S. White]

I have been contacted by someone who has challenged the identification of copper in these geodes. He states that copper has not been found in them and he believes that the brown film is probably an iron oxide alteration of pyrite.

[Jordi Fabre]

[Bob Harman]

Jordan, I must laud you for getting youngsters interested in rock collecting, but at the same time, I must admonish you for not demanding that the youngsters wear hard hats while collecting at that roadcut. It is not the large rock that falls and squashes anyone collecting at roadcuts, it is the small pebble sized stone that falls and puts a gash in the head requiring an ER visit with stitches in the head.

[Bob Harman]

Jordan,
Now that I am registered, please permit me to give you a primer on what you are finding.
You are finding Mississippian age sedimentary geodes. They have nothing to do with igneous types. Certain layers of geode bearing Mississippian age limestones come to the surface (roadcuts) or near surface (quarries) in north central Tennessee as in your Woodbury roadcut, central Kentucky, as in the Hall's Gap roadcut.....famous for its millerite, and south central Indiana as in the Harrodsburg roadcuts south of Bloomington.....famous for many hi quality Indiana geodes. Also there are geode bearing roadcuts and quarries in northern lower Michigan and west central Illinois and, especially, the Keokuk region of SE Iowa, Missouri and Illinois.

The geodes that you are finding are basically quartz with certain well documented secondary minerals. Commonly found are calcite and dolomite; occasionally found are aragonite and Baryte; relatively infrequently found minerals include pyrite and marcasite. Rarely to very rarely found minerals include fluorite, millerite, malachite and chalcopyrite. Several other minerals have been "reported to occur" but reliable reports or pictures are not well documented. Other minerals, including some that you have mentioned (native copper) are not known to occur in Midwest sedimentary origin geodes as these are not from the proper geologic environment. Some roadcuts produce differing % of the above minerals such as the Hall's Gap cut producing many millerite geodes while other cuts rarely produce any millerite. Secondary coatings and iron staining is very common; your examples show the iron staining.

Maps of the Midwest geode producing areas, pictures of the roadcuts, numerous pictures of hi quality geode finds at these roadcuts and quarries, and discussion and descriptions of all this can be found in several sites online. Also several folks have had geode displays at many regional rock and mineral shows. So I urge you to take some time and review these. Some of the finds and displays are quite incredible. CHEERS.......BOB

[jordanlowe1089]

[Jordi Fabre]