Dear Hakima

the one side of view is described by micro fold and the lamination is obviously observed in both prepared photos. Therefore it can be assumed that it is assigned to sedimentary rocks. The black layers may be organic matter and the light layers may be clastic (e.g. sand or fine clay). However the alternation of dark and light supports the alternation of sedimentary condition during deposition.

Regards

Massih

Dear Massih Afghah,

Thank you so much for your clarification, i thought that the laminations are sedimentary origin. But i didn't have any information about the material.

Now it is clear for me. Thank you.

Regards,

Hakima 

Dear Hakima,

The rock type can be even better understood,

by testing whether the matrix quartz and / or feldspar containing or carbonate.

For this hardness testing a glass surface is enough. Leaves the rubble after rubbing on the glass surface scratches, then it might be a quartz sandstone.

If the stone surface from slipping on the glass without leaving a trace, then it is likely to be a carbonate sandstone.

A test with dilute hydrochloric acid can clarify whether the rock contains calcite or not.

However, the best method of determining the rock type is the thin section.

The brown color is certainly caused by fine grained goethite.

Best redards,

Guenter

Dear Guenter,

Thank you so much for all ideas to studying this rock.

However, i will tell you about the reaction with HCL tomorrow. I think that it will be negative. And about the thin section in the future when i realise it.

Best regards,

Hakima

I agree with Mark,

probably a metamorphic rock, I guess  Hornfels

Dear all,

Thank you so much for your help.

The reaction with HCL is negative, so no CaCO3 in this rock.

There is no adamant in this rock caused by the biotite or feldspars, so why we can think  about the metamorphic origin for these laminations?

Best,

Hakima 

Dear Kenneth,

The rock is from Morocco picked up from the beach. As you said, it louks a long transportation.

Now, why Guenter proposes a goethite?

But i will prepare a thin section which it can answer many questions.

Thank you so much for your help.

Best,

Hakima

The rounded edges of this sample indicates that this is some piece of the cobble maybe in some alluvial fan because rounded edges are typical for rock pieces involved in longer transportation. It can be also a piece from some conglomerate. So, if this is a case, the primary rock latter involved in transport with surface water can be some metamorphic rock. Black minerals can be some primarily organic carbon or some minerals from piroxene group, while the white colored parts can be some of the feldspar minerals or from quartz origin. So, from the picture, it is heavy to determine mineralogical composition and primary origin of the rock.

 the aletrnace of black and white layers give at the  rock an aspect of   a metamorphic rock (perhaps gneiss??)   that had been eroded by  a lent alluvial transport!!

The rock may be a gneiss (metamorphic rock) due to its alternate bands of light and dark color minerals. Then, it was transported by by fluvial process to achieve its present rounded shape. The size of rock is cobble.

The rock appears to be a meta-sediment, probably a high grade quartzo-feldspathic schist or gneiss, derived from arkosic grits or sandy/siltstones.  If I am right and you examine it with a hand lens, you should be able to identify the following minerals: quartz and feldspars in the lighter layers and biotite mica with possible hornblende (amphibole) in the dark layers.  It is very similar to much of the Moine schists (Pre-cambrian) found in the NW Highlands of Scotland with which I am very familiar.  Hope this helps.

 Dear Geaorge,

When i examine this rock with a hand lens, I don't  found any quartz and feldspars in the lighter layers or biotite mica and amphibole in the dark layers. It looks as a sandstone with a lighter and dark layers. And the reaction with HCL is negative.

Best,

Hakima

Kenneth,

I agree with you, the dark layers looks as an organic carbon and the lighter layers might be silt. As you said making a x-ray diffraction can identify the major minerals. But unfortunately is impossible in our university. All can  i do, is preparing a thin section.

Best,

Hakima

Leonid,

You have true. When the thin section is ready, i will let you know.

Thanks for your time, help and interest!

Regards,

Hakima

It is unscientific and vague to record any observation of a rolled, transported pebble.It appears to be hard compact looks like"agate" not exactly lamination?

(Brijraj Krishna Das)