what is this?

[clutch]

Hi all,
found this in a 14 yr old lava tube, any thoughts as to what it is?
Thanks!

 

[Roger W]

How big?

 

[clutch]

Quite small, probably about 20mm across. There were other similar features nearby as well.

 

[southpembscaver]

Hard to tell without a closer look but it looks like a type of mica, don't know why it would have formed in isolation - perhaps all of the other crystals were washed out when the tube was still active. Also it would have had to cool very slowly to grow to this size meaning that the tube must have been active for a fair length of time.

 

[langcliffe]

Hard to tell without a closer look but it looks like a type of mica, don't know why it would have formed in isolation - perhaps all of the other crystals were washed out when the tube was still active. Also it would have had to cool very slowly to grow to this size meaning that the tube must have been active for a fair length of time.

I know sod all about the mineralogy of lava tubes, but I would have thought this to be unlikely.

1. Mica is fragile and wouldn't have survived if "all of the other crystals were washed out when the tube was still active".

2. The wall of a lava tube cannot cool slowly when the tube is active since by definition it is in contact with 1,000+ degree magma.

It looks like something may have leeched out of the rock as a result of weathering. I do know that elephants lick salts from the walls of lava tubes in Kenya, but I am sure that such tubes will be considerably older than 14 years old.

 

[langcliffe]

Hard to tell without a closer look but it looks like a type of mica,

Having thought about this a little more and applying what little I do know about the mineralogy of lava, it is not a mica. Lava tubes will only form where a hot fluid magma is being extruded, which implies a mafic or ultramafic magma the mineralogy of which is dominated by pyroxene, olivine, and plagioclase feldspar, with both the micas being conspicuous by their absence.

Lava is actually fairly porous, and in conjunction with the fact that the minerals in lava are not that stable at normal temperatures and pressures, they readily leach out minerals which are then redeposited on the cave walls by evaporation. The feldspar is the most unstable so the dominant deposits tend to be calcium and sodium based (e.g. salt, gypsum, and calcite) - hence the elephant licks in the Kenyan caves.

Someone who knows a little about mineralogy may be able to recognise the crystals.

 

[Brains]

My thoughts go two ways...
If secondary, I suspect it may be a gypsum deposit, like the desert rose crystals, but the colour seems to negate this
The general morphology and colour is reminiscent of the tiny "opaques" seen in thin section and reflected light, so if a primary feature it could be a crystal of iron! More realistically, ilmenite or the like is a possibility.
A quick XRD or XRF of a sample would soon settle the issue, if a sample could be obtained...

 

[dpegg]

It looks unlikely to be ilimante as the crystals don't tend to grow out like that, i agree with the iron as it looks quite like a native metal. Its quite hard to tell from that picture; were there any obvious crystals shapes in it, possibly like looking like <<<
if so then its probably gypsum.

 

[clutch]

Fantastic, many thanks!
Here is shot showing a bit more of the setting.

It was located almost at the end of the tube about 5m before a lava seal and about 0.5m high on the wall. There appeared to be some gypsum powdery deposits elsewhere in the tube but not there. 

Are these types of deposit common in lava tubes?

 

[langcliffe]

Are these types of deposit common in lava tubes?

We haven't established what "these types of deposits" are yet!

In general, there are two ways for deposits to form on the walls of lava tubes. The first has already been discussed - percolating water weathering the minerals, mainly feldspars, which are then deposited on the walls by evaporation.

The second is less obvious, and  accounts for the deposition of insoluble minerals. When still active lava tubes are full of both magma and hot gases. When the tube is drained and the gases cool, mineral deposits can condense out of them to form crusts on the walls as well as individual clusters of crystals.

 

[Brains]

According to Deer, Howie and Zussman "An introduction to the rock forming minerals" on the paragenesis of haematite (p410):
"In igneous rocks it occurs... as a late stage product of volcanic activities as thin crystals sublimated on to earlier material."

In addition, the blue colour of engineering bricks (think railway viaduct...) can be seen in thin section and reflected light to be due to large leaf-like crystals of iron oxides laying sub parallel to the brick face.

I surmise, probably in error, that the crystals are haematite grown during the cooling phase of the emptying tube. Admittedly this seems rather a short time frame for such a large crystal, but brick firings are even quicker so I guess the time will be sufficient.

As before, XRD/XRF of the material would be definitive

 

[clutch]

v.interesting, many thanks. Unfortunately, didn't collect a sample.
Apologies for badly worded question, was just intrigued to know if anyone had come across similar features in a lava tube.

 

[braveduck]

It looks a bit like photographs of Native Copper I have seen.

 

[braveduck]

Yes just done a Google image seach,Native Copper does take many forms.
But some DO look very similar to your pictures.
Both form and colour.
Don't let the scrap men know the location.

 

[braveduck]

Hi all here we go again,just been on mindat.org and am absolutly sure it is Dendritic Copper.
Don't tell anyone where this is because,international mineral collectors will travel anywhere
on the planet to collect specimens like that.