Dull luster, with 2 main layers:

TOP) about 1 cm thick, plate-like, dark coffee brown, bubbly melty looking texture. The top layer is harder (about a 5, scratched with a knife) and the edges are abruptly broken into long gentle curves with very distinct edges. I cannot easily break off a piece.

BOTTOM) about 4 cm think at thickest point, pale orange "redrock", sandy texture, softer than the top (about a 3, scratched with a penny) and smoothly rounded. If any edges existed in the past on this bottom part they have been worn away. Pieces do not break off easily, but the bottom layer scratches more easily than the top.

Weight seems about what I'd expect for a rock this size, maybe slightly lighter?

I'm very curious about the mineral nature and appropriate name for this kind of rock, but also what the possible processes were that could have made this rock look like it's burnt on top, but still raw on the bottom?

I found the rock in Southern Utah Red Rock country somewhere, but unfortunately I no longer remember the exact site. It was something I just picked up from the ground as is. I don't think it was unique to other rocks around it. There were several in the area with this same texture / combination. Although, I don't think all the other rocks were loose like this one, some were still attached to the mountain.


top of rock

side view

bottom of rock

closeup of top

closeup of bottom

  • 1
    $\begingroup$ Interesting rock. Looks like a sandstone that has been exposed to the elements. The top, dark surface looks like some concretions or trace fossils but difficult to be sure. A closeup picture of both surfaces could help with the identification. $\endgroup$ – Friddy Apr 16 at 17:50

I would give the following identifications and interpretations.

I. "TOP) about 1 cm thick, plate-like, dark coffee brown, bubbly melty looking texture. The top layer."

I would identify it as "ironstone." Basically, it consists of sandstone that is solidly cemented and even partially replaced by iron oxyhydroxides. Normally, this type of ironstone, especially in the Southwestern United States, results from the replacement of a siderite, iron carbonate," cement. In fact, the iron in the iron oxyhydroxides might have come from the dissolution of the siderite by the circulation of reducing fluids through the sandstone. The features that it exhibits appear to only concretionary features cementation along and the filling of former cracks in the sandstone.

Detail discussions of how this type of ironstone is formed:

Chan, MA, and WT Parry. Mysteries of Sandstone Colors and Concretions in Colorado Plateau Canyon Country. Public Information Series no. 77. Utah Geological Survey, Salt Lake City, Utah. (2002 http://geology.utah.gov/online/pdf/pi-77.pdf or http://archive.li.suu.edu/docs/ms130/AR/chan10.pdf

Chan, Marjorie, et. al. "Diagenetic Hematite and Manganese Oxides and Fault-Related Fluid Flow in Jurassic Sandstones, Southeastern Utah." AAPG Bulletin 84:9 (2000) 1281-1310. http://archive.li.suu.edu/docs/ms130/AR/chan3.pdf

Chan, Marjorie, et. al. "Red rock and red planet diagenesis: Comparisons of Earth and Mars concretions." GSA Today 15:8 (2005) 4-10. http://archive.li.suu.edu/docs/ms130/AB/chan9.pdf and http://www.geosociety.org/gsatoday/science/#2005

Loope, David B.; Kettler, Richard M.; and Weber, Karrie A., "Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A." (2011). Faculty Publications in the Biological Sciences. 198. http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1197&context=bioscifacpub http://digitalcommons.unl.edu/bioscifacpub/198/

II. "BOTTOM) about 4 cm think at thickest point, pale orange "redrock", sandy texture, softer than the top (about a 3, scratched with a penny) and smoothly rounded."

I would identify it as ferruginous (iron-cemented) sandstone. it is sand that cemented, but not replaced, by iron oxyhydroxides. the cement was precipitated by the circulation of reducing iron-rich fluids through the sandstone. The difference between the two rocks is mainly the degree to which the sand has been cemented and replaced by iron-rich fluids that circulated through them. For the details, see the above references.

  • $\begingroup$ Wow! Thank you so much! You are so full of fantastic information. I really appreciate the links too. $\endgroup$ – RHabtour Apr 18 at 0:00
  • $\begingroup$ Thanks @Paleocene. I am curious as to what the source of a reducing fluid would be. $\endgroup$ – Friddy Apr 18 at 17:26
  • $\begingroup$ Ancient oil and gas. - for, example - Beitler, B., Chan, M.A. and Parry, W.T., 2003. Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs: evidence of exhumed hydrocarbon supergiants?. Geology, 31(12), pp.1041-1044. archive.li.suu.edu/docs/ms130/AR/beitler.pdf archive.li.suu.edu/archive $\endgroup$ – Paleocene Apr 18 at 19:18

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