One for the Petrologists

[supernova1965]

I was looking for someone who could tell me about this rock I have all I know is I like it a lot it is almost like it has an animal in it:D

Dimensions are
width 6mm
hight 4.2mm
depth 3.5mm

[TrevorW]

Hi Warren, this site may help

http://geology.about.com/library/bl/blrockident_tables.htm

[supernova1965]

Thanks Trevor,


It seems the white bit is Sedimentry rock but that is far as I have got sofar the link is helpful thanks

[bkm2304]

Hi Warren,

Might be something like a tuffaceous chert - volcanic ash layed down in fine layers covering other rocks then metamorphosed. The little specks seem to be similar.

Richard

[TrevorW]

Is the dark brown a quartz or lava glass

http://en.wikipedia.org/wiki/Obsidian

or do you think it's a petrified animal

[supernova1965]

The white could be chert and the specks are silver in colour



No my animal comment was only to the shape it is waxy to feel and has no grain or layers that I can see I think it more like a quartz

[Doomsayer]

It's what is commonly known as a thunder egg.

They are usually formed within silica rich volcanic rocks such as rhyolite lavas as a sort of concretion. As the deposited lava cools trapped gases in bubbles or vesicles gradually solidify as the pressure and temperature conditions change during cooling of the lava.
Sometimes they can have very coloiurful crystalline minerals in their cores called spherulites.

guy

[supernova1965]

Thanks Guy they are very cool to look at:thumbsup:

[TrevorW]

Warren may have some value check this out http://www.coolrocks.com.au/aust/aust06.htm

[Ric]

Hi Warren

That definitely looks like a thunder egg to me, I remember that there was a place outside of Rockhampton where you could dig them up and they would cut and polish them for you.

Cheers

[renormalised]

Yep, that's a thunder egg, with red chalcedony in the centre. Very pretty rock. Will polish up nice too :):)

The rock itself is a rhyolite porphyry...those crystals you see in the rock are mostly plagioclase feldspar laths. You do rarely get laths of quartz, but acidic rocks like rhyolite normally don't form them because the silica content of the rock is too high (>69%)...they can't nucleate out of solution. Those metallic looking ones are biotite mica crystals (phenocryst is the technical term for crystals that grow large in the mineral groundmass of the rock). The chalcedony forms when silica rich groundwaters penetrate cavities in the rock and evaporates, leaving behind the silica. The silica itself forms a water rich cryptocrystalline to amorphous mass in the cavity. It's a very similar process to how opal forms and how gold nuggets form :)

It's not metamorphosed in the strictest sense....acid volcanics like rhyolite metamorphose to quartzite and quartz rich gneisses when put under heat and pressure. It may have undergone a form of metamorphism called diagenesis, but by the look of the rock I don't think so. The conditions which are typical of diagenesis normally won't affect volcanic or plutonic rocks, unless there's something atypical of the conditions. They usually just weather out and turn to soils in these conditions.

[DavidU]

I was waiting for Carl to nail this one.As he does.:thumbsup:

[renormalised]

It's a nice specimen :):)

[supernova1965]

Thanks all,

I just love it here you can put any subject up and find someone to help its just wonderful:D:thumbsup:

[ZeroID]

I think I understood about 1 word in 10 in that description.... :shrug: :rolleyes:

Well, no, not really but impressive knowledge and wonderful to have this resource on tap.
Fascinating .. :)

[AstralTraveller]

Did you buy it or visit Mt Hay yourself?

http://www.digthetropic.com.au/places/mthay.aspx

http://www.360tours.com.au/Mt_Hay/

http://www.mountminerals.com/17.html

[supernova1965]

we bought it and only paid $2 for it I don't remember where we got it though