Blue, black and white

Today is a birthday. Not my birthday but my dear home country Estonia is 95 years old now. I wish her a prosperous future. May it be much brighter than the past.

Estonia’s national colors are blue, black and white and this post is inspired by these colors.

http://picasaweb.google.com/107509377372007544953/Rocks#5848481675389848706
This is a sample of hematite. Hematite is an oxide of iron. It is extremely widespread mineral, especially in sedimentary rocks. Many reddish rocks and minerals owe their color to hematite. Ironically, there is nothing red in this picture of a pure sample of hematite but it would be red if powdered.

The colors that dominate on the picture above are dark brown with white and bluish spots — the approximate colors of the national flag of Estonia. Dark brown is the natural color of hematite, white spots are reflections from the light source but what about the blue spots?

I have to say that I am really not sure. Some minerals possess a very beautiful play of colors called iridescence. Especially well-known example is a plagioclase feldspar mineral labradorite:

http://picasaweb.google.com/107509377372007544953/Rocks#5848488897488965714
Plagioclase crystals in this sample of anorthosite look bluish when viewed from a certain angle. Plagioclase crystals allow some light to penetrate into the crystal which is made of many alternating slabs of different composition which act like mirrors and reflect the light back. Reflected lightwaves combine (this is called interference) and create interesting bluish play of colors.

Is the same effect responsible for the bluish spots on hematite? No, I don’t think so because hematite is practically opaque mineral which does not allow light to penetrate into it. But is it still caused by some intrinsic property that is specific to hematite?

This is also not likely. I have seen very similar effect in other minerals and rocks and I even remember some comic books which I read when I was much younger. It was about Native Americans that always had pitch black shiny hair with bluish glare. I doubt that they really have hair like that but maybe some tribes with very dark hair in some conditions? Somehow artists had to get the idea.

http://picasaweb.google.com/107509377372007544953/Rocks#5847099160896443730
Smoky quartz is a silicate mineral that also reflects bluish light.
http://picasaweb.google.com/107509377372007544953/Rocks#5848481665432908098
Obsidian is a glassy volcanic rock with occasional bluish reflections.

Smoky quartz is a silicate mineral, obsidian is a volcanic glass, and hematite is an oxide of iron. They are all very different materials. We can therefore almost certainly rule out the possibility that the bluish color has anything to do with the internal structure of these materials.

However, the surfaces of these samples have something in common — they all reflect light well and they are very dark-colored, almost black.

This in probably the key to understand this phenomenon. Blue color may be a weak reflection which looks bluish because it comes from a very dark surface. This is my guess. I have not found any confirmation to this. I would be interested to hear your opinions if you have an alternative explanation.