Scoriated vesicular rock

[Jonny74]

 

I have been trying to identify what the rock in the attached photos is & what processes may have caused the vesicular interior & base.

While I believe that the vesicles are there as result of heat, I do not believe that it is the result of volcanic activity, I come to this conclusion because of where the rock was found - there has been no volcanic activity in this region for millions of years & the lack of any secondary mineralization in the vesicles would suggest that the rock is geologically recent. Also, scoria, cinder or other vesicular volcanic rock does not look like the item in the attached photos, the domed exterior is way to smooth for vesicular volcanic rock - 

The vesicles are quite small (<2mm), & are elongated/orientated towards the flat surface & begin to form along the interior of the domed area. The vesicles become more abundant & open out into chambers as they near the center of the flat outer edge. On the way towards the flat outer edge, wheel like structures (something like the mag wheels on a bmx bike), begin to appear, which I believe function as a mechanical aid to move gas from the interior of the rock, to the exterior, I believe to facilitate out gassing due to the heat that the rock was exposed to at some point. 

It has been suggested that the rock is in fact bone, & it does somewhat resemble the interior spongy bone associated with the trabecular bone. However, spongy bone does not contain the structures that are present in this rock.

Other suggestions have been coral rock, I presume some sort of fossil coral sediment rock, or something of that nature. I think that this association is made purely on the resemblance between the wheel like structures & the structures that are present in some forms of coral.

Some notable characteristics on the exterior would include brown translucent like glass that partially covers some areas, (I can only describe it as glass because that is what it looks like). Fine cracking (somewhat like crazy paving) throughout the surface, although this cracking is only visible under magnification.

Any ideas?

 

 

 

 

 

 

[Jonny74]

Here is a magnified photo of the surface which shows the very fine cracking that is inherent throughout the domed exterior surface -

 

[d_day]

My money is on some variety of slag.

[Shokmelt]

Have you checked to see if it's calciferous?..A couple of drops of dilute HCl and it will fizz if it is.

If calcified; probably a fossil sponge or bone...or possibly coral;...if silicified; more likely bone.

[Jonny74]

6 minutes ago, Shokmelt said:

Have you checked to see if it's calciferous?..A couple of drops of dilute HCl and it will fizz if it is.

If calcified; probably a fossil sponge or bone...or possibly coral;...if silicified; more likely bone.

 

I did an acid test on the surface for which there was no reaction. 

[Shokmelt]

Intriguing find !  Have you compared it with the fossilized bones of birds or related dinosaurs?

[Jonny74]

7 minutes ago, Shokmelt said:

Intriguing find !  Have you compared it with the fossilized bones of birds or related dinosaurs?

 

On the surface it does look somewhat like the head of a femur but that is only in cross section, as in the above photo. I would not rule out some sort fossil but what I find interesting is the lack of secondary mineralization, the find comes from a very damp region & I would think that there would be some sort if infilling in the exposed vesicles, but nothing, it is sterile.

 

 

 

 

 

 

 

[Shokmelt]

I see your point. Good point! ...A rock that was slowly heated by magmatic or volcanic contact metamorphism 

maybe? Otherwise--

[Shokmelt]

I used poor terminology in the above post, sorry. --"Contact Metamorphism" is actually a form of solid state

recrystallization, that wouldn't allow for vesicles to form. What I meant, was near contact with magma or lava.

[Jonny74]

6 minutes ago, Shokmelt said:

I used poor terminology in the above post, sorry. --"Contact Metamorphism" is actually a form of solid state

recrystallization, that wouldn't allow for vesicles to form. What I meant, was near contact with magma or lava.

The problem with a volcanic origin would be that the most recent & closest volcanic activity in proximity to the find site occurred about 60 million years or so ago.  Something of that age would certainly be infilled with minerals.

 

 

 

 

 

 

[Shokmelt]

30 minutes ago, Jonny74 said:

The problem with a volcanic origin would be that the most recent & closest volcanic activity in proximity to the find site occurred about 60 million years or so ago.  Something of that age would certainly be infilled with minerals.

 

 

 

 

 

 

I agree. Did you by any chance perform even a cursory investigation as to there being other similar rocks in the area?

[Jonny74]

3 minutes ago, Shokmelt said:

I agree. Did you by any chance perform even a cursory investigation as to there being other similar rocks in the area?

I did indeed but so far nothing. It's pretty much a mish mash of different rock types to be found here, glacial deposits consisting mainly of sediments, limestone & arkose sandstone, to name a few.
 

[LipCa]

Sure looks like fig. 247 of this link:  http://www.theodora.com/anatomy/the_femur.html

[Jonny74]

3 minutes ago, LipCa said:

Sure looks like fig. 247 of this link:  http://www.theodora.com/anatomy/the_femur.html

I agree, it does look like the ball of a femur, however I believe that is only on the surface & the correlation between the two is based purely on the domed shape. Here is another photo of the rock prior to cutting. The cut that I made in the rock goes from the lower front visible point across the back & to the opposite lower point giving the domed shape that you see in the above photo. The specimen is almost symmetrical, with two of the lower points, one that is visible in the photo, & one almost out of view, a tail formation to the rear & a nose to the front - what does it remind you of?

 

 

 

 

 

[Shokmelt]

Question: of the two "whole-stone" photos, which one is closer to true color. The one in your original post, or this

last one?  I'm guessing the former, yes?

[Jonny74]

14 minutes ago, Shokmelt said:

Question: of the two "whole-stone" photos, which one is closer to true color. The one in your original post, or this

last one?  I'm guessing the former, yes?

You are correct, the photo that you refer to was taken outdoors on a sunny day, a slight adjustment in exposure was made to compensate for glare. The 2nd whole photo is taken indoors under a bright white fluorescent lamp & again a filter is applied to compensate for glare.

[Shokmelt]

Have you scratch tested the exterior surface? For that matter, the interior as well, for comparison purposes?

[Jonny74]

8 minutes ago, Shokmelt said:

Have you scratch tested the exterior surface? For that matter, the interior as well, for comparison purposes?

I have a cut piece that I used for scratch testing, the interior is exposed at an angle so both ends were tested on glass for scratch, where no scratch was detected, I had to apply considerable pressure to the glass, which at that point the specimen began to fragment, but still no scratch. It did however scratch copper & steel, so based on that I would say 5/6 Mohs.

 

 

[Shokmelt]

When I look at the close-up image, I can't see much, if any, discernible weathering transition between the

exterior rind and the interior. Would you say that is the case wherever an ext. / int. contact is visible?

[Shokmelt]

I realize that a weathered transition can be seen at the edge of the cut slice, but it's interesting that it 

doesn't show up on the broken surface.

 

 

[Jonny74]

15 minutes ago, Shokmelt said:

When I look at the close-up image, I can't see much, if any, discernible weathering transition between the

exterior rind and the interior. Would you say that is the case wherever an ext. / int. contact is visible?

To best answer your question I will need to post another photo - this image shows a clear rim running around the base & clearly defines the smooth top dome & the vesicular base. The top dome is completely smooth, except for a number of breaks in the dome where lighter material is visible. These are breaks in the dome which contain hairline fractures. There is no sign weathering between this rim or the interior.

[Jonny74]

14 minutes ago, Shokmelt said:

I realize that a weathered transition can be seen at the edge of the cut slice, but it's interesting that it 

doesn't show up on the broken surface.

 

 

When you mention the broken surface, are you referring to the area in the following photo? Here is a close up, you can see the domed shape coming into play, while the detail in the exterior is more visible -

[Shokmelt]

Yes, that's the area. Now I can see a normal-looking weathering transition. The photo exposure was

whiting-out the actual color. Are parts of this rock considerably more porous than others, or is that

too an illusion?

[Jonny74]

7 minutes ago, Shokmelt said:

Yes, that's the area. Now I can see a normal-looking weathering transition. The photo exposure was

whiting-out the actual color. Are parts of this rock considerably more porous than others, or is that

too an illusion?

The exterior dome is solid, except for a couple of vesicles that can be seen in the previous photo, which are situated left of center. Other than that its the vesicular interior & flat underside.  

[Shokmelt]

The big problem with trying to figure out what kind of rock this is, without a much more sophisticated 

analysis, is that glacial deposits could easily contain a whole host of different materials that have

been transported great distances; including volcanics. Just out of curiosity, mainly for the purpose

of distinguishing between mineral staining and mineral composition, have you tested it's magnetic 

response? With a strong magnet preferably.