New Mexicans for Science and Reason present


Mystery Rock

This rock was found in a planter at a restaurant in Albuquerque.

What Is It??


Readers' Suggestions

Jean in CA:

Iron concretions. Ferruginous concretions
(my archeologist husband knew, not me!)

Laurie (geologist at UNM):

my guess is concretions. From the luster.. I would guess manganese oxides, if sand grains are included it could be a concretionary sandstone (these grains would be easily visible with a hand lens. It could also be a varnished Fe-calcite cemented concretion. I have millions of these. Obviously they look like the blueberries from Mars.

Nick from Berkeley:

Well, I can't figure it out in 15 seconds sitting here at my computer, so it must be intelligently designed.

Matt in Albuquerque:

Are the samples unusually heavy? They look like some type of iron concretion to me.

Richard from Ohio:

They look a little like some of the formations one would see in the clinkers from old furnace burning low-grade coal that I used to have to carry out during Minnesota winters a long time ago.

Gary from California:

They are fragments of pharyngobranchials (toothed fish bones derived from the last branchial arch).

These particular bones remind me of either the family Labridae (particularly examples related to genus Semicossyphus, or one of the many Embiotocidea.

Byron from Delmar, NY:

This is the standard inch (each ball is 1/10TH of it) and it must have been stolen from an Anasazi site.

Cheers, Byron

Steve in Colorado:

I can give it a shot, even tho planters in Albuquerque are a bit out of context! Context is often very important in identifying rocks/minerals.

These look like a type of iron-oxide concretions or concentric mineral growth. Because they are not magnetic, you're correct that they don't consist of the mineral magnetite. Another possibility is hematite, or other, related Fe-oxide minerals that aren't exactly hematite. There's a whole family of these Fe-O minerals. In purest chemical form, they might be botryoidal hematite. Here are examples of the latter:

They form by precipitation of iron from ground waters, growing outward in concentric layers sort of like an onion. People are enamored of them, collect them, and place them in various spots for viewing, such as planters (!?). A sample like this should be pretty dense, probably around 3.5-4.5 due to the iron. Rocks such as Sandia granite should have a density of 2.8-2.9.

When they have a rougher surface texture, some folks can confuse them with meteorites, but that's way off (and not at all like the typical, thin melt crusts on metal meteorites).

Structures like these have been in the news in the last year. It is likely that groundwaters in sediments on Mars have resulted in little concretions like these. They were described as blueberries with a mineral composition of hematite. Rovers provided amazing images and spectroscopic analyses. Similarly, I could also show you three sandstone formations (Tertiary; Cretaceous; Permian) in Colorado Springs with small, iron-rich concretion structures such as these. Sometimes the concretions are hollow inside with a rusty powder, which disappoints folks hoping for spectacular geodes or other riches. Still, they are very popular.

I'm not familiar with a formation in N.M. with these. (there are dead-ringers around here that I can pinpoint)

Features like this are an important part of how groundwaters can change the appearance and properties of sediments. Besides just gluing together sedimentary fragments, groundwaters can do very messy and interesting things with rocks.



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