FMF - Friends of Minerals Forum, discussion and message board

[trtlman]

I collect Scoria, Basalt. I know this sound weird but it is a fascinating rock, at least I think so. I enjoy looking at the pits under a microscope. One thing I have noticed since I purchased my microscope are glassy spots in the pockets of the basalt. It almost looks like it melted and hardened into an almost obsidian. Also I have noticed on all my basalts are little orange glassy spots. I do not know a whole lot about Basalt so I don't know what these are. Now that I have an imager for my microscope I can show you what I am talking about and hopefully get some information about the stone. Thank you.

[Peter Megaw]

Daniel...some may be glass...basalt chills very quickly and glass can be common.

However, it is also possible/likely that you're looking at partially altered olivine, which can also look glassy and has a greenish to brown color depending on how fresh it is. When it is very altered it goes to a quasi-mineral called "iddingsite" which is glassy and bright orange or red, which may be what you're seeing.

Olivine has very distinctive cleavage and fracture patterns that should be shown in sketches in any decent igneous petrology/petrography book..once you have an eye for the pattern it will make it easy to distinguish olivine from glass (which has conchoidal fracture only).

Finally, make sure the amygdules you're looking at (fancy word for the holes) were closed and you broke them open. There are a world of organic things that like to live in nice little protected holes and some of them can look glassy

[trtlman]

Thanks for the information. I was thinking Olivine but but thought it only came in green, and the holes were already open. What about when these glassy bumps, like in the first picture, are white?

[Peter Perkins]

trtlman,
I have a suggestion about the glassy bumps that are white. Although I must say that the white on the first picture isn't what I would expect. Lets go back to the formation - when the rock cools there are often cavities left behind which are called vesicles (hence Vesicular Basalt). The concept of a vesicle is common in other sciences; it is from the latin 'vesicular' meaning bladder, blister.
Often, but not always, these vesicles get filled or partly filled with other minerals which are often white in colour. Because they were thought to look like almonds (and often they do - just the right size and shape) they were named after the greek for almond - amygdale (hence Amygdaloidal Basalt). The minerals found to be forming the amygdales are zeolites, carbonates and silica, all of which can give a white patch on the black (dark grey) basalt.
Peter

[Mark Ost]

Hi Daniel
Would it be possible to get a picture of the whole rock? Thanks. Mark