FMF - Friends of Minerals Forum, discussion and message board

[Kelly Jean]

Hello,

I'm so happy I got these Chihuahua Trancas geodes just the other day and had a lot of fun breaking them open. In the first picture I focused on the crystal formation in the front. In the second I got the blue mineral inclusion that I am not too sure of what it is ? Calcite ?

[Bob Harman]

Your geode is from the Trancas Estacion locality in Chihuahua State, Mexico.

They are very interesting, often having "twisted" quartz formations so if you want more, look for previously opened examples with these quartz formations.

As to the blue area, these geodes often are a mixture of crystalline quartz and microcrystalline quartz called chalcedony. The chalcedony usually has a blue tint so that area you note is probably chalcedony.
Calcite occasionally occurs. If you are curious, put a few drops of vinegar on the area. Calcite will fizz, then quickly wash it off, but I favor that area in your geode being chalcedony.

See my examples for reference. See the bluish chalcedony area in the lower right example and look closely, in the not so good photos, to find the "twisted" quartz formations. Bob

[Kelly Jean]

Thanks again, Bob :)

[Peter Megaw]

The Trancas chalcedony geodes have several different styles...but almost all are spectacularly fluorescent under UV