Iron-oxide concretions and nodules 2

page 2

This grape-cluster texture is known as “botryoidal.”  Google “botryoidal hematite.” 
A very nice bunch of hematite grapes. Thanks to Heather and her daughter from Texas for the photo.
More botryoidal hematite 
This is a cluster of pyrite crystals beginning to oxidize to hematite. Note the cubic crystals on upper left.
This concretion appears to have started as a mass of pyrite crystals (iron sulfide, cubic) and is in the process of oxidizing to iron oxide.
This photo was sent to me by a fellow who found it while gardening in his yard in Kauai, Hawaii. I asked a colleague who lives in Kauai whether or not hematite concretions were found there. He said that limonite concretions did occur. Limonite is a mixed iron oxide-hydroxide. It is possible that some of the other concretions depicted here contain limonite.
Technically, this one may not a “concretion,” but it is rich in hematite.
This one was sent to me from Iowa. It makes a red streak and does not attract a magnet, but the specific gravity is only 4.1. Pure hematite has a specific gravity of 5.3, so this rock must contain some low-density mineral like quartz or a carbonate in addition to hematite.
I suspect that this one, from Delaware, also contains mineral phases other than hematite. I have received reports of chemical analysis of several meteorwrongs like this that were, for example, 20% SiO2 and 80% Fe2O3 – hematite with quartz sand.
Two examples of (what I call) “dog-turd” concretions