Monday, April 09, 2012

H is for Hematite



Hematite: the Other Red Metal.


Hematite is Iron Oxide.  It forms hexagonal crystals and is most often red-black or metallic, even in nature.  The streak is red.  This is why we do a streak test-- samples are often metallic but the streak is red to red-brown.

Crystals of hematite can be very thin blades to large blocky crystals that resemble a dark brown pyrite.

Often when pyrite psuedomorphs (rusts) it is changing to hematite.  It was thought for decades this was the case with the Pyrite psuedomorphs from Pelican Point in Utah County.  But, with recent research, it was determined they changed to Goethite.  Goethite is a polymorph of hematite, meaning that they are the same mineral with different chemical characteristics.

Utah has several good collecting areas for hematite.  The Thomas Range has several good sites.  There is the spectacular hematite at the top of Pismire Wash.  Here is where we find a small pink topaz in the hematite.  There are other sites in the Thomas range that produce “rosettes” of hematite up to 1", though they are rare.

Iron County is named for the amount of iron that comes from there,most was in the form of hematite.  Often in the hematite in this area you can find amethyst (purple quartz due to iron ) and lodestone ( iron that is divalent and hence magnetic). 

All the red sandstone in Utah (and the rest of the west) is caused by iron in the sand.  There is enough iron that it rusted causing the red.   This is the environment that caused the “Moqui Marbles” to form.  They are only bubbles of hematite that formed under the sandstone by ground water.

At a lot of the old copper/silver/lead mines in the western part of the state you find red dirt.  It is very crumbly and makes collectors very dirty when they dig in it.   This is usually limonitic gossan.  Gossan is hematite in limestone that was baked by super heated ground water when the ore deposit was being formed.

Most of the red in the agate & jasper & petrified wood in the West is caused iron staining them that color.

Not all the red in all these cases are caused by hematite.  However, most is related in one way or another.

The hematite beads that are so popular are not really hematite.  They melt hematite down and pour it into molds to make all those beads.  This is why they are so cheap.  If someone was cutting and polishing all those shapes they would be very expensive.

2 comments:

  1. Thanks for this interesting post. I'm certainly a novice when it comes to geology but I did enjoy reading this.

    Good luck with the rest of the challenge :)

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  2. Wow, what a cool theme!

    Visiting from A-Z Challenge!

    My A-Z

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