[meteorite-list] OT: Rock type

From: MEM <mstreman53_at_meteoritecentral.com>
Date: Tue, 6 Aug 2013 19:28:08 -0700 (PDT)
Message-ID: <1375842488.89521.YahooMailNeo_at_web142403.mail.bf1.yahoo.com>

Fortunately I do not know who suggested these as some of these rock types/suggestions do not even exist. So I can caustically eviscerate some suggesters without anyone getting offended. ? One rule we used to have on this list is don't speak up on ID's if you aren't reasonably sure--sort of "the do no harm" concept borrowed from medicine.? I think the rule needs reinstating.? I've seen real meteorites identified by several list members as slag--I've made the mistake myself.? I try to only make IDs from an informed and experienced background.? When someone asks for an ID they aren't having a guessing contest--they want a realistic and ACCURATE answer.

First:? we really need a close up and preferably of a freshly fractured surface to improve the accuracy of our opinion.
Secondly:? tiny URls aren't permanent.? Any link you want to make click-able enclose it between "<" and ">" as this list doesn't use rich text formatting.? These tiny urls aren't showing up as clickable links on my emails. Maybe it is different for others.

Thirdly: It always helps to know where a rock comes from as most have already been identified on geological bedrock maps.

Ok once again there are a bunch of well intending met list members who really need to spend a few hours actually watching some very basic rock ID you tube videos.? There are a few dozen actual rock types and some have to get final ID in the lab but you should be able to distinguish at least a dozen by sight if you consider yourself an authority on rock ID.

Lets review the bidding:
-Basalt-- NOT? basalt would have hexagonal jointing like the Devil's Causeway save for in parts of Hawaii and Washington State in the US.
-Pillow lava(basalt), with a degree of metamorphosis--NOT and just what would a metamorphosed basalt look like?? Scale is wrong for pillows. You are mistaking exfoliation for pillows.
-Quartzite--A remote --very remote possibility as quartzite largely lacks banding.? AS one of the possibilities does include a sandstone on its way to becoming a quartzite, I give a partial pass.
-Limestone schist --No such thing? Limestone would show etching and pinicles in a coastal setting.
-Serpentinite (latter from gabbro or peridotite) Not even close in appearance. This IS NOT an igneous rock And gabbro--an igneous rock does not metamorphose to serpentite as this is hydrated olivine in origin (Mg2SiO3<--(0H)).? It takes a dunite/peridotite to form serpentine.
-Shale/Shale with concretions? Shale would show extreme stratification and would not be smooth to the degree this is. Shale is very soft and would be hammered into a scree pile in this type exposure.


To all these suggesters-- go back to the books and study the characteristics of the rocks you suggested then look again at the photo.? Spend some time with those online geology courses on Youtube and learn a little about rocks.

-Gneiss, with banded texture--This is the most realistic possibility when the photo is taken as a whole however gneiss generally doesn't separate along strata planes.? The question is are we seeing gneissic banding or alternating bands of sand and detrus made by seasonal runoff or storm action.? Note the blocks of banded stone in the lower left of the photo which have a curved fracture along a banded segment.? Without a better set of photos and a sample we are left with gneiss and standstone tied for 1st and second place.? There are some white calcite or quartzite veins representing secondary deposition/mineralization long after the bedrock formed at depths. These represent healed fractures and are common in most all rock sedimentary types.? More rare in schists and gneisses but can't be ruled out.

?Gneiss forms at depths where the rock is plastic and shows both deformation flow features and banding. Based on the fracture on the block in the left hand lower corner which broke along a band/strata I can't narrow it down further.? Sandstone whether or not lain down by water or wind action is often reworked by storm events as the deposit moves over the slope so it is common to find banding which is not straight but curved.

Elton
>________________________________
> From: Bjorn Sorheim <astrogeo at online.no>
>To: meteorite-list at meteoritecentral.com
>Sent: Tuesday, August 6, 2013 5:22 PM
>Subject: Re: [meteorite-list] OT: Rock type
>
>Thanks for sending me your reflections on this image.
>I got quite a variety of rock types suggested.
>So this seems difficult without more information.
>
>Here are the types you suggested:
>-Basalt
>-Pillow lava(basalt), with a degree of metamorphosis
>-Quartzite
>-Gneiss, with banded texture
>-Limestone schist or Serpentinite (latter from gabbro or peridotite)
>-Shale
>-Shale with concretions
>
>The rock layers in the first image have a? band of white marble (10-15 cm)
>overlain by a layer of dark grey clay containing much lime (about 50 cm), seemingly only little metamorphosed. While the layers over and under seems much folded - if that is what they are - these two layers are fairly straight. A lot of mostly whitish stones are visible in them. Seems a bit strange that the layers should be so straight while the layers under and above are undulating so much.
>An explanation could be that the undulating layers are felsic lava. Does
>this seem likely from what you see in the 3 images? And do the white stones suggest any clues?
>
>Two images of the layers:
>tinyurl.com/l4aqx83
>tinyurl.com/k6lzfh7
>
>Bj?rn
>
>
Received on Tue 06 Aug 2013 10:28:08 PM PDT