ID help & geology question

[idontfollow]

What came first? The crystals or the rock? I found this little beauty in the caprock south of Lubbock, TX (go Raiders!) As you can see in the pics, crystals run clean through it and protrude from both top and bottom.
1) is this selenite?
2) how did this form? Crystals or rock came first?
Thanks in advance....

 

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[MrLee]

Nice piece, good question. Sorry I can't help.

 

[TRice]

The rock and crystals came (relatively) at the same time. It formed out of hyper-saturated water by precipitating out. The crystals are there simply because there was the space available for them to grow. Can't tell for sure if it's selenite, if you can scratch it with a fingernail and it's clear - it's selenite.

 

[austin]

Very nice sample. I went to grad school at Tech, but I never found anything that cool around the caprock. Remember hunting around post, but nothing that cool. Suspect you may be right about selenite, but I'm no expert...

 

[Eu_citzen]

Might also just be "etched" quartz. Seen similar samples in a quarry close by.
Selenite is actually just gypsum, slightly water soluble.
That and it getting scratched by the finger nails are good pointers that it is in fact gypsum.

 

[idontfollow]

Thank you for your answers. Very helpful. Austin, if you go south from Post you can stop at almost every road cut out on 84 and find amazing stuff. Thanks again. I enjoy the geo-interaction.