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Rock Identification

 
 
Reply Wed 22 Mar, 2017 01:03 am
Hi All,

I know it's difficult to say too much without access to the specimen, as that and further diagnostic testing is necessary to make an accurate description of a rock.

However, I would appreciate your input on what you think regarding the links to the images which allow you to zoom in and see reasonable detail.

Link to image 1: https://drive.google.com/file/d/0B23LU48rz25cbzlUcmVzMlMzTEU/view?usp=sharing

Link to image 2: https://drive.google.com/file/d/0B23LU48rz25cYlpReHZqRk1mYmM/view?usp=sharing

Link to image 3: https://drive.google.com/file/d/0B23LU48rz25cbWZGZnNOTGh0SDg/view?usp=sharing

Link to image 4: https://drive.google.com/file/d/0B23LU48rz25cSVpDN3JWVENrVVU/view?usp=sharing

I am interested in description such as colour, mineralogy, grain size, sorting, sphericity, etc.. All descriptive input is welcomed.

Thanks for your time,

Chris
 
farmerman
 
  3  
Reply Wed 22 Mar, 2017 03:31 am
@ChrisHughes,
OK, ets use our powers of close observation.
1..You see a lot of sand grains that appar to be pretty unifom in size and (if you look at it under a 50 XPower scope) you should try to see whether the little pieces of cemented sand are rounded or angular

2 Then, when you look at it on edge (Your last picture) you see there are 3 distinct times recorded in the rocks makup
AQ. The red sndy stuff seems to have been laid down in a medium (You have to look closr to see whether its in a wind or water medium) I think its water.
3The color o the reddish sand grains indicates it was laid down in a terrestrial environment.

4 Then the "top" of the red layer appears to have been eroded a bit you can see the , sort of, "ripply patterns" on the top

5 Then, on top of that, is a new (Later" layer of sandy material that is gray (probably underwater slide or submerging area near a tidal or marine zone).

The story is really interesting and would make a neat sedimentary sample for teaching if you cut the sample on its cross section so w could see these structures and be able to read its story.
A geologist would use those observations to help locate the deposition environment and trace it to its sources area (Resources are associate with specific environments

SO, if its composed of grains and appears to be sedimentary, what do you think it would be called?

sandstone
sandrock
arkose
arenite
wacke

Perhaps all of the above?

As far as grain sizes and statistics, youre going to have to do that yourself.



ChrisHughes
 
  1  
Reply Wed 22 Mar, 2017 09:33 am
@farmerman,
My initial feeling is that it is a sandstone. Appears to be mostly quartz, but perhaps with plagioclase feldspar as the white mineral that is present.

For sorting, I was going to suggest it is moderately poorly sorted as it looks as though there are larger grains among the smaller grains, which I can't imagine could lead to being well sorted.

Grains appear sub-angular/sub-rounded.

Obviously I will need to inspect this diagnostically, but wanted to check an initial description prior to doing so.

What would the red grains be in the sandstone? I didn't even notice any red.
farmerman
 
  3  
Reply Wed 22 Mar, 2017 10:50 am
@ChrisHughes,
the picture you sent shows a pinkish hue, which is a probable coating of iron oxide and associated with arkosic sandstones. Do you have a scoppe with a counting reticle. I would compqre the clasts with a chart for sub rounding ubangulr degrees (if youre looking at doing an energy environment report). Did you notice the separate depositional environments? and separated by erosion? This is good stuff for a teaching specimen.
ChrisHughes
 
  1  
Reply Wed 22 Mar, 2017 01:06 pm
@farmerman,
I wasn't sure if that was colouring due to the presence of iron oxide, as my feeling is that the generally lighter colour is suggesting that it's predominantly quartz. And I thought arkose was where you had a higher content of feldspar?

What would you say what the percentage composition of quartz and feldspar in the sample?

Does it look as though there is a cement - perhaps silica?

farmerman
 
  3  
Reply Wed 22 Mar, 2017 02:40 pm
@ChrisHughes,
I cant see the individual grains so I went both arkose and arenite . Can you actually see any feldspar grains under a scope. Can you see the diagnoatic cleavage planes?

I think the big story of this hand sample i the three events in time
deposition 1
erosion
deposition 2

Are you in geology class or are you a hobbyist

ChrisHughes
 
  1  
Reply Wed 22 Mar, 2017 07:05 pm
@farmerman,
Geology class.

What would your suggestion be on the cement binding the likely quartz/feldspar grains?

It doesn't look to have much of a matrix, does it?
farmerman
 
  4  
Reply Wed 22 Mar, 2017 07:12 pm
@ChrisHughes,
It would vary .It can be silica or many things. The best way is to do a "Thin section. Hve you made thin sections in class or your petrology lab?.

There are chemical ways but they involve some fairly caustic things and if you havent had a "wet" lab yet, I wouldnt recommend using stuff like aqua regia or HF. Using a thin section you could put in a dye. The dye would select to inject itself between the sand grains and under a scope you could pick it out.
ChrisHughes
 
  1  
Reply Wed 22 Mar, 2017 08:15 pm
@farmerman,
I will try that tomorrow and let you know how I got on.

Am I correct in the bedform being cross-bedding?
0 Replies
 
ChrisHughes
 
  1  
Reply Wed 22 Mar, 2017 08:55 pm
@farmerman,
Obviously the grain size is 1/16 th mm - 2 mm. But what would I call the grains for the images linked? Medium, coarse, medium-to-coarse?
farmerman
 
  2  
Reply Wed 22 Mar, 2017 09:17 pm
@ChrisHughes,
do you have any sed lab books or apps? There are some neat diagrms re fine v med grain). Its been so long Id call it medium grained, (most of my work is geochem) When I have to do any field descriptions from samples, I just carry around one of those grain size cards and an AAPG grain size series of little capsules that hold a few grams each of many sizes and sortings of grains.
Look up a cc ofThe Diagenesis of Sandstones and Shales by Jonas and McBride . Its an oldie but still useful paper.



The lower unit looks fairly even grained
farmerman
 
  2  
Reply Wed 22 Mar, 2017 09:20 pm
@farmerman,
I assume youre in UK or OZ so you probably use M E Tucker's Sed Petrology no? Its your standard UK text and is a great source on all you need
ChrisHughes
 
  1  
Reply Wed 22 Mar, 2017 09:37 pm
@farmerman,
You've been most helpful. Thank you.

One last question: I know there is a reddish colouration, but in the third link posted, at the bottom of the rock, there is an obvious white speck alongside a yellow speck.

I figured the white speck was feldspar, but do you think the yellow may be a coating? And if so, what would cause that?
farmerman
 
  3  
Reply Thu 23 Mar, 2017 12:57 am
@ChrisHughes,
I cant really ee the yellow peck next to white one but they could be variations in the cement matrix or some variation in coatings. Yellow coatings in sandstones are often associated with certain ores like carnotite. But I am just guessing there.

If you want to assure yourself the matrix or some clasts are feldspars or even calcite, you are going to have to use a scope and look for the cleavage

Remember calcite=3 cleavage planes, feldspars=2
0 Replies
 
 

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