Rocks & Minerals B/C

[inluvwyth_WANTED]

Does anyone know what almandine looks like under rotating polarizers? That was on our state test and I'm just curious.

[gneissisnice]

Does anyone know what almandine looks like under rotating polarizers? That was on our state test and I'm just curious.

Really? They asked about minerals under cross polars? I'm surprised that came up, that's college level Mineralogy.

When you look at mineral grains under a polarized microscope, you'll see something pretty cool; they'll appear to be a different color and if you rotate the stage, they'll slowly turn black and then turn back into that color, every 90 degrees. When they're black, you say that the mineral is "extinct", and that happens because the polarizing lens on allows light to pass through at two directions perpendicular to each other, and when the mineral is rotated, the crystal structure will block light in those exact two directions, making the mineral appear black. This technique is very useful for identifying small mineral grains in rocks, as no two minerals look the same under cross polars and they have distinct colors and patterns.

One thing I should mention is that these minerals show color under polarized light because they are anisotropic. That means that they do not exhibit equal optical properties in all directions, and so will change depending on the direction they turn. Garnet, however, is an isotropic mineral. It has the same properties in all directions due to its cubic crystal system. What this means is that garnet (and other isotropic crystals) will ALWAYS be extinct under cross polars because they only let in light in the direction blocked by the cross polars, no matter which way you turn it. That means that it will always appear black, no matter what.

TL;DR: Garnet is isotropic and will appear black under rotating polarizers.

[inluvwyth_WANTED]

Thank you so much for answering! I just got totally unrelated searches or material that I didn't understand. Thanks again.

[gneissisnice]

Thank you so much for answering! I just got totally unrelated searches or material that I didn't understand. Thanks again.

No problem. I'm just surprised it came up, that seems like it's way beyond the scope of the event. I mean, I love mineralogy and I like educating people about it, but having microscopy coming up in a jr. high event seems weird.

[piimasta314]

There's a lot of cool looking minerals encrusted on this rock. I don't know what the white stuff is from the picture alone...Where did you find it?

It doesn't matter where I got it from, because at regionals, my teammate found a stash of rocks & minerals a school obviously dumped onto the dirt (who knows why?). Well there was about 10 of us digging through snow and weeds finding topaz, jasper, many quartz, etc. Like I said before, I can answer questions about its physical properties.

Sorry I was unclear. I was just curious to what park you were in when you found that rock because it looks cool.
I'm really bad at identification so I can't help you much beyond this point...

[Cade]

There's a lot of cool looking minerals encrusted on this rock. I don't know what the white stuff is from the picture alone...Where did you find it?

It doesn't matter where I got it from, because at regionals, my teammate found a stash of rocks & minerals a school obviously dumped onto the dirt (who knows why?). Well there was about 10 of us digging through snow and weeds finding topaz, jasper, many quartz, etc. Like I said before, I can answer questions about its physical properties.

Sorry I was unclear. I was just curious to what park you were in when you found that rock because it looks cool.
I'm really bad at identification so I can't help you much beyond this point...

Thanks for the help. I am certain malachite is in it, but you are right about the various minerals. I think I might do some research to see what they are.

[sciencegeek100]

This one's a tough one. I found this mineral on a trail two weeks ago, and I still don't know what it is. I am leaning towards malachite, but what does anybody else think it is? I will answer questions about its property.

clear looks like quartz

green is probably beryl or tourmaline

[darkdeserthighway]

This one's a tough one. I found this mineral on a trail two weeks ago, and I still don't know what it is. I am leaning towards malachite, but what does anybody else think it is? I will answer questions about its property.

clear looks like quartz

green is probably beryl or tourmaline

I agree with the quartz, but I feel the green mineral in the 3rd picture is malachite.

[sciencegeek100]

It might be amazonite... I am pretty sure malachite does not form with quartz

but amazonite definitely does
https://www.google.com/search?q=malachi ... 29&bih=666

https://www.google.com/search?q=malachi ... 29&bih=666
when you google "quartz with malachite" no good pictures show up

[Ctes1357]

Michigan is known for copper and copper forms with quartz so im gonna say its that i have a piece just like that one but with a little more copper and not a bunch of quartz