What is this rock?

[sjastro]

A challenge for geologists on the forum to identify this rock.

Background:
A sample weighing 3 grams and dimensions approximately 10mm X 10 mm X 10mm and several smaller fragments were found on a granite intrusion in South Eastern Australia.
The sample has a specific gravity of 2.9, does not produce a streak on a ceramic plate and is magnetic.
The sample is fluorescent under long wave (365nm) UV light.
General image:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Comparison_latestx.jpg
Microscope image:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Comparison_Closeup.jpg
Cross Section image of fragment:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Cut_Section.jpg

My first impression was the sample was a carbonaceous chondrite meteorite and the fluorescence was due to organic compounds in the matrix.
There is also indirect evidence of meteorite activity in the region as reported in a Geology Journal of a school boy almost being hit by an Australite in 1963.

“He told masters at the school that he thought someone was throwing stones when he witnessed the object whistle to the ground within five feet of his position on the road. The specimen was identified as an australite. The australite, supposed to have been a newly fallen tektite, was described in one newspaper as “cylindrical and just over an inch long, with a kind of equator in the centre”, and in another newspaper as being “about three-quarters of an inch in diameter and about three-eighths of an inch thick.”

Investigation:
The images were sent to Randy Korotev at Washington University who was quite adamant the samples were not meteorites as the surface did not look like a fusion crust and the cross section lacked chondrule structure.
When it came to an explanation as to what causes the florescence particularly in the black matrix for a terrestrial rock Randy gave a frank “I don’t know”.
The identity of the rock and an explanation for the florescence has drawn similar blank responses on other forums.
None of the mineral candidates for florescence in long wave UV fit the description.

Future:
Unless someone is able to confidently identify the rock, I might have to bite the bullet and investigate non destructive testing such as X-ray florescence spectroscopy for elemental analysis and reflectance FTIR for organic compounds.
Washington Uni will hopefully provide feedback if reflectance FTIR is even possible on a rock as transmission FTIR has been historically used on carbonaceous chondrites were the rock is destroyed.
The tragedy is my occupation gave me free access to this type of equipment 10 years ago and any form laboratory testing isn’t going to be cheap and whether it is worth the expense in the first place.

 

[DavidFirth]

I'm no geologist but that appears to be larvikite.

 

[juvenissun]

A challenge for geologists on the forum to identify this rock.

Background:
A sample weighing 3 grams and dimensions approximately 10mm X 10 mm X 10mm and several smaller fragments were found on a granite intrusion in South Eastern Australia.
The sample has a specific gravity of 2.9, does not produce a streak on a ceramic plate and is magnetic.
The sample is fluorescent under long wave (365nm) UV light.
General image:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Comparison_latestx.jpg
Microscope image:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Comparison_Closeup.jpg
Cross Section image of fragment:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Cut_Section.jpg

My first impression was the sample was a carbonaceous chondrite meteorite and the fluorescence was due to organic compounds in the matrix.
There is also indirect evidence of meteorite activity in the region as reported in a Geology Journal of a school boy almost being hit by an Australite in 1963.


Investigation:
The images were sent to Randy Korotev at Washington University who was quite adamant the samples were not meteorites as the surface did not look like a fusion crust and the cross section lacked chondrule structure.
When it came to an explanation as to what causes the florescence particularly in the black matrix for a terrestrial rock Randy gave a frank “I don’t know”.
The identity of the rock and an explanation for the florescence has drawn similar blank responses on other forums.
None of the mineral candidates for florescence in long wave UV fit the description.

Future:
Unless someone is able to confidently identify the rock, I mightuhave to bite the bullet and investigate non destructive testing such as X-ray florescence spectroscopy for elemental analysis and reflectance FTIR for organic compounds.
Washington Uni will hopefully provide feedback if reflectance FTIR is even possible on a rock as transmission FTIR has been historically used on carbonaceous chondrites were the rock is destroyed.
The tragedy is my occupation gave me free access to this type of equipment 10 years ago and any form laboratory testing isn’t going to be cheap and whether it is worth the expense in the first place.

First, if it is an xenolith in granite, then it is not a meteorite. And the fluorescence is not likely caused by organic substance.

Second, the density is less than 3, and the existence of abundance plagioclase (?) suggest that it is not mantle in origin.

My best bet is that it is an anorthosite. A chemical analysis should be able to confirm that. A XRF analysis (destructive) should not be expensive. Not sure if FTIR would help. FTIR needs a flat surface, so it would also be destructive. Since you already made a cross cut and have a thin section, it seems you can also afford a small piece for XRF. If you do XRF, be careful to take a piece which could represent the composition of the whole rock. You may also modify the thin section a little bit to do some microprobe analysis. It should not be expensive if you only ask for the detection of major elements.

 

[sjastro]

I'm no geologist but that appears to be larvikite.

Thanks for the feedback David.
I have been given some recommendations by the Department of Earth and Planetary Sciences Washington University on testing so hopefully the identity of this rock will ultimately be revealed.

 

[DavidFirth]

I hope you are able to identify it without having to spend a fortune. God bless.

 

[sjastro]

Thanks for your response.

First, if it is an xenolith in granite, then it is not a meteorite. And the fluorescence is not likely caused by organic substance.

PAH's (Polyaromatic hydrocarbons) have been found, ranging from sedimentary rocks to carbonaceous chondrite meteorites and are a source of fluorescence.

My best bet is that it is an anorthosite. A chemical analysis should be able to confirm that. A XRF analysis (destructive) should not be expensive. Not sure if FTIR would help. FTIR needs a flat surface, so it would also be destructive. Since you already made a cross cut and have a thin section, it seems you can also afford a small piece for XRF. If you do XRF, be careful to take a piece which could represent the composition of the whole rock. You may also modify the thin section a little bit to do some microprobe analysis. It should not be expensive if you only ask for the detection of major elements.

XRF is a non destructive test but has the disadvantage of typically not detecting elements below an atomic number of 13 which eliminates the carbon, hydrogen, nitrogen and oxygen.
A separate test for organics is therefore required.
The sample preparation for reflectance FTIR only requires a very small sample usually in milligrams which is squeezed onto a crystal surface (usually diamond) using a pressure arm device.
The success of the technique depends on how intimate the contact between sample and crystal.
I'm not sure if this works for rocks.

Interestingly enough last night I received an E-mail from the Department of Earth and Planetary Sciences Washington University and their tone has completely changed once I sent them UV images of the rock.
Initially their responses were dismissive, now they suggest I send the main sample to a specified laboratory making no reference to the sample possibly being a meteorite or that the University is ultimately behind the request so as to eliminate the possibility of bias.
The University will analyse the test results.

 

[DavidFirth]

Sounds cool. Go for it.

 

[Job 33:6]

A challenge for geologists on the forum to identify this rock.

Background:
A sample weighing 3 grams and dimensions approximately 10mm X 10 mm X 10mm and several smaller fragments were found on a granite intrusion in South Eastern Australia.
The sample has a specific gravity of 2.9, does not produce a streak on a ceramic plate and is magnetic.
The sample is fluorescent under long wave (365nm) UV light.
General image:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Comparison_latestx.jpg
Microscope image:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Comparison_Closeup.jpg
Cross Section image of fragment:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/Cut_Section.jpg

My first impression was the sample was a carbonaceous chondrite meteorite and the fluorescence was due to organic compounds in the matrix.
There is also indirect evidence of meteorite activity in the region as reported in a Geology Journal of a school boy almost being hit by an Australite in 1963.


Investigation:
The images were sent to Randy Korotev at Washington University who was quite adamant the samples were not meteorites as the surface did not look like a fusion crust and the cross section lacked chondrule structure.
When it came to an explanation as to what causes the florescence particularly in the black matrix for a terrestrial rock Randy gave a frank “I don’t know”.
The identity of the rock and an explanation for the florescence has drawn similar blank responses on other forums.
None of the mineral candidates for florescence in long wave UV fit the description.

Future:
Unless someone is able to confidently identify the rock, I might have to bite the bullet and investigate non destructive testing such as X-ray florescence spectroscopy for elemental analysis and reflectance FTIR for organic compounds.
Washington Uni will hopefully provide feedback if reflectance FTIR is even possible on a rock as transmission FTIR has been historically used on carbonaceous chondrites were the rock is destroyed.
The tragedy is my occupation gave me free access to this type of equipment 10 years ago and any form laboratory testing isn’t going to be cheap and whether it is worth the expense in the first place.

None of the links to your images work (they may require a login). Can you simply post pictures directly into this discussion?

 

[sjastro]

None of the links to your images work (they may require a login). Can you simply post pictures directly into this discussion?

Very strange...............
Here they are.
General Image:

Microscope Image:

Cross Section of Fragment:

 

[Job 33:6]

Very strange...............
Here they are.
General Image:
View attachment 241751
Microscope Image:
View attachment 241752
Cross Section of Fragment:
View attachment 241753

Do you have images of a thin section?

 

[Job 33:6]

Very strange...............
Here they are.
General Image:
View attachment 241751
Microscope Image:
View attachment 241752
Cross Section of Fragment:
View attachment 241753

Yea, i would put a thin section under polarized light and get an understanding of what specific minerals are present in the rock. Then i would look into any hydrothermal activity or historic activity associated with the area.

Looks like a meteorite to me, but as stated above, it doesn't appear to be partially melted in any way. The next best option might be minerals of an epigenic deposit.

PAHs may be found in association with hydrothermal deposits, as could iron. Which might explain its fluorescence, density, magnetism and lack of a streak. But you would have to look into the geology to get an idea of what options there may be for natural sources of the minerals present in the rock. Which means that you first should probably identify the minerals under polarized light.

@Astrophile Any ideas or suggestions?

 

[sjastro]

Yea, i would put a thin section under polarized light and get an understanding of what specific minerals are present in the rock. Then i would look into any hydrothermal activity or historic activity associated with the area.

Unfortunately I don't have the capability for producing thin sections.
My set up is purely home made and involves cobbling together a camera to a cheap stereo microscope and developing the capability for imaging in UV, IR and polarized light.

Perhaps the geological map of the area provides a few clues.
The granite intrusion is described as "coarse grained, porphyritic; large phenocrysts of alkali feldspar; abundant xenoliths of igneous and sedimentary origin".
The region has also been subject to glaciation with the surrounding area composed of "outwash fans consisting of coarse sand to fine gravel containing angular quartz and feldspar."
I am no geologist (confession I flunked first year Geology at Uni) but the sample does seem foreign to the environment it was found in.
A night survey of the granite intrusion with a UV torch reveals the only fluorescence is caused by fungi growth on the surface.

Looks like a meteorite to me, but as stated above, it doesn't appear to be partially melted in any way. The next best option might be minerals of an epigenic deposit.

PAHs may be found in association with hydrothermal deposits, as could iron. Which might explain its fluorescence, density, magnetism and lack of a streak. But you would have to look into the geology to get an idea of what options there may be for natural sources of the minerals present in the rock. Which means that you first should probably identify the minerals under polarized light.

Thanks for your comments.

 

[Job 33:6]

Unfortunately I don't have the capability for producing thin sections.
My set up is purely home made and involves cobbling together a camera to a cheap stereo microscope and developing the capability for imaging in UV, IR and polarized light.

Perhaps the geological map of the area provides a few clues.
The granite intrusion is described as "coarse grained, porphyritic; large phenocrysts of alkali feldspar; abundant xenoliths of igneous and sedimentary origin".
The region has also been subject to glaciation with the surrounding area composed of "outwash fans consisting of coarse sand to fine gravel containing angular quartz and feldspar."
I am no geologist (confession I flunked first year Geology at Uni) but the sample does seem foreign to the environment it was found in.
A night survey of the granite intrusion with a UV torch reveals the only fluorescence is caused by fungi growth on the surface.


Thanks for your comments.

Do you have coordinates for the location? And with your geologic map, do you have a cross section or just an aerial view that you could share?

 

[juvenissun]

Thanks for your response.

PAH's (Polyaromatic hydrocarbons) have been found, ranging from sedimentary rocks to carbonaceous chondrite meteorites and are a source of fluorescence.


XRF is a non destructive test but has the disadvantage of typically not detecting elements below an atomic number of 13 which eliminates the carbon, hydrogen, nitrogen and oxygen.
A separate test for organics is therefore required.
The sample preparation for reflectance FTIR only requires a very small sample usually in milligrams which is squeezed onto a crystal surface (usually diamond) using a pressure arm device.
The success of the technique depends on how intimate the contact between sample and crystal.
I'm not sure if this works for rocks.

Interestingly enough last night I received an E-mail from the Department of Earth and Planetary Sciences Washington University and their tone has completely changed once I sent them UV images of the rock.
Initially their responses were dismissive, now they suggest I send the main sample to a specified laboratory making no reference to the sample possibly being a meteorite or that the University is ultimately behind the request so as to eliminate the possibility of bias.
The University will analyse the test results.

The occurrence (an igneous inclusion) and the low density of this piece pretty much exclude the possibility of a meteorite, even a stony meteorite.

And, in order to give the rock a correct name, trace element measurement is not needed. Unless you have some other purpose.

 

[sjastro]

Do you have coordinates for the location? And with your geologic map, do you have a cross section or just an aerial view that you could share?

The sample was found at approximate grid coordinates 726958 or at the label "Big Rock".
Unfortunately the cross section is not for the region of interest.
Victoria is one of the few Australian states where the finders keepers law for potential meteorites applies.
Full resolution for map:
http://members.iinet.net.au/~sjastro/astrophysics/Club_Space_Rock/28120_youyangs_50_geol_ed1.pdf

 

[sjastro]

The occurrence (an igneous inclusion) and the low density of this piece pretty much exclude the possibility of a meteorite, even a stony meteorite.

While Randy Korotev offered many counterarguments, density was not one of them as the Carbonaceous Chondrites can have densities well below 3.0.
Meteorite Density (Densities of Meteorites by Classification) - Meteorites Australia

And, in order to give the rock a correct name, trace element measurement is not needed. Unless you have some other purpose.

In the context of meteorite vs terrestrial rock, element measurement is quite relevant as the concentration of Ni, Cr and La fall within different ranges for different meteorites against terrestrial rocks.
Chemical Composition of Meteorites
Chemical Composition of Meteorites
Chemical Composition of Meteorites

The objective is also to try to identify the type of meteorite if my sample happens to be one.
The unfortunate aspect if I decide to get the sample tested it will be destroyed in the process as the lab will conduct an extensive element analysis using ICP/MS.
4Litho - Lithium Metaborate/Tetraborate Fusion - ICP and ICP/MS - Geochemistry / Assay | Actlabs

 

[juvenissun]

While Randy Korotev offered many counterarguments, density was not one of them as the Carbonaceous Chondrites can have densities well below 3.0.
Meteorite Density (Densities of Meteorites by Classification) - Meteorites Australia



In the context of meteorite vs terrestrial rock, element measurement is quite relevant as the concentration of Ni, Cr and La fall within different ranges for different meteorites against terrestrial rocks.
Chemical Composition of Meteorites
Chemical Composition of Meteorites
Chemical Composition of Meteorites

The objective is also to try to identify the type of meteorite if my sample happens to be one.
The unfortunate aspect if I decide to get the sample tested it will be destroyed in the process as the lab will conduct an extensive element analysis using ICP/MS.
4Litho - Lithium Metaborate/Tetraborate Fusion - ICP and ICP/MS - Geochemistry / Assay | Actlabs

Good luck.