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re: Non-crystalline material?
Nick Hoffman — (archived)

Robert,

Do me a favour and enroll in Geology 101. Make that a New Year Resolution for 2005.

In return, I'll resolve not to give sour replies, providing you don't make stupid comments. :-)

As regards this one:-

Sure, the "internal" character looks almost squishy - but remember it's been planed-off with a diamond-hard cutter, so it's really much harder than it looks.

What matters is the "external" shape of each inclusion. They have definite sharp edges - look at how jms and I trade descriptions such as "arrow" "keyhole" "wedge". We're seeing cross-sections of what were original crystals in this rock. They may have subsequently been altered or entirely replaced as the original minerals broke down. But the shapes of their boundaries are preserved in the rock.

In Geology, we use crystal shape as a diagnostic for the chemistry of the crystals. You learn hundreds of different minerals, each with characteristic shape, colour, hardness etc. On mars we're a bit limited and only have shape and greyscale colour to go with, but we can make a number of sound deductions from the evidence, IF we use our brains:-

The crystals must have been early-formed crystals because they are big (lots of time to grow in a cooling magma if you start early) and are well-formed - if you start early you grow in a free volume. Later crystals have to fit between earlier ones.

This favours mafic minerals like Olivine , Hornblende, etc. Metamorphic minerals are possible but we haven't seen any evidence for metamorphism on Mars, yet.

They can't be vesicles - those are fossil gas bubbles and would be roundish and crystals in them would be spiky aggregates growing in from the walls like a geode.

Go learn some Geology, then come back with your brain engaged.

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