[neilep (OP)]

I'm posting these piccys ion behalf of Bailey.

Hopefully more details will follow:

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[neilep (OP)]

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[Bailey]

thanks for posting the pics Neil...some more info on this rock..it was found near where i found the "concretion"(in an area with a lot of limestone caves south of Katherine)..it is very smooth in patches,but on the top it has been weathered by water i think ,and you can see different layers.the base has concentric rings,I'm guessing it is petrified wood..weird how it is so rounded though..

[JimBob]

frethak - where are you???

Now don't get all excited, Adam but this is a speilothem!!!  (http://en.wikipedia.org/wiki/Speleothem)

In common terms it is a stalagmite.

The concentric rings are built up as water drops in them and calcite is deposited on the cone. The runnels flowing down the side show where water has run off the stalagmite during times when more than normal water was available and taken some of the already deposited limestone with it.

This was probably dug up by one of the bulldozer blades you spoke to Neil about.

 

[Bass]

Bailey
Can you scratch the rock with a knife?
BTW Welcome to the forum

[JimBob]

Oh, oh. He's going to get get into it.

I can't be WRONG can I ?  (Besides, where are you going to find a "hard" rock near Katherine, Australia??? Huh, huh, huh? -- Oh, except for the gold mines.)

[Bailey]

thanks for the welcome bass,i suppose i could scratch it with a knife,but its outer surface is very hard,as is the rest of it,its deffinately not "chalky" in anyway.its quite a heavy"rock"...do you agree with jimbob?

[frethack]

How large is it?  Any pics with another object for scale?

[Bailey]

I'll take some shots so you can get an idea its size...also,it stands 150 mm high,about 105mm across the base and weighs 2.30kg..and the pic will be next to a standard size cd...

[Mazurka]

A few drops of acid (strong vinegar will do) should reveal all.

If bubbles form/ it effervesces then it is a carbonate rock and I would agree with Jim Bob that it was a speleotherm.

links to google images of El Soplao caves in Northern Spain, where the most incredible (and unlikely) forms of speleotherm are present
http://images.google.co.uk/images?hl=en&source=hp&q=soplao&um=1&ie=UTF-8&ei=DDoWS__6B5LbjQeh1PyHBg&sa=X&oi=image_result_group&ct=title&resnum=4&ved=0CCwQsAQwAw&safe=active

[Bailey]

ive tried the vinegar..no visible reaction on the base or the smooth surface,it hasent even made the thing 'cleaner' where ive rubbed it..

[neilep (OP)]

I'll take some shots so you can get an idea its size...also,it stands 150 mm high,about 105mm across the base and weighs 2.30kg..and the pic will be next to a standard size cd...

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[Bailey]

jimbob..why is it rounded so much?i thought it would be more conical ..like the stalagmites Ive seen in some caves

frethak - where are you???

Now don't get all excited, Adam but this is a speilothem!!!  (http://en.wikipedia.org/wiki/Speleothem [nofollow])

In common terms it is a stalagmite.

The concentric rings are built up as water drops in them and calcite is deposited on the cone. The runnels flowing down the side show where water has run off the stalagmite during times when more than normal water was available and taken some of the already deposited limestone with it.

This was probably dug up by one of the bulldozer blades you spoke to Neil about.

 

[Bass]

Bailey
one more time- when you scratch this with a sharp knife, does it leave a scratch on the rock?  Also try scratching it across the rings in the base and see if they are different hardnesses.
If this scratches fairly easily with the tip of a sharp knife, then it is probably some sort of carbonate, and I might agree with JB.  If you can't scratch it with a knife, it is siliceous, probably either a chalcedonic concretion or petrified wood.

[Bailey]

the outer surface does not scratch with a knife,it is too hard,but on the base about 7 or 8mm in from the edge the rock does scratch,leaving a white mark..some of the rings in the base are also too hard to scratch

[JimBob]

Well, blast and damn! I just might - MIGHT - be wrong!

WOE IS ME!

OK Bass - what is it?

[Bass]

Given the continuous nature of the concentric rings, my first impression is petrified wood- which would be consistent with the presence of the softer rings (not silicified).  I have seen trees petrified as carbonate that were later selectively replaced by silica. 
Don't know this area of Australia- are there any volcanic rocks nearby?

[Bailey]

there are lots of igneous rock outcrops in the region..and Ive been in limestone caves both north and south of Katherine..cutta cutta caves are the region where i found the stone.the little 'sphere'..was found about 2km north west of Katherine,where you will see some bigass dark gray rocks,i also found other spherical stones there..and they had'rings' like saturn but solid rock if you know what i mean

[Geezer]

Wow! It looks a lot like a head. Are you sure it's not somebody's headstone?

[JimBob]

Question Number 1

The area around Katherine is covered by mostly Mezoproterozoic and some Cretaceous clastic sedimentary rocks with Ordovician limestones and dolomites thrown in for good measure. There are also igneous rocks of all types.

I suspect this formed in an Ordovician carbonate and the addition of minerals from the hard rocks cased some silicate "agate-like" formation in the stalagmite.

Question Number 2

The top is round because of the way the water drops on it. If you look at an icecicle, the one on top is pointed. The place where the water drops is all lumpy. That is what is going on with the rock as it deposits limestone and other minerals. The drop of water falling from the ceiling splashes and the drop is distributed over a wider area, forming a dome, not a sharp point.