Can't get a good feel for it; can't tell if the white is silicate (~quartz/feldspar )or carbonate, and can't decide if I'm seeing a slightly schistose texture in the dark part; if I had to have a stab, I'd put it down as metasediment (which is a bit of a cop-out, I know).
Without being able to ID the component minerals and the microscopic textures, it will be difficult to get much better.
Looks somewhat cubic; I'd hazard a guess at fluorspar (rather than halite), though the cream colour doesn't rank as 'familiar' in my book. The dark matrix looks like silt, but what it really is may depend on its situation - eg. it could be rock flour if in a fault-vein cavity.
Again, testing really necessary to make a good ID.
You could try a streak test and scratch hardness test - might rule in/out some candidates.
And locality info would be useful. That might constrain the options to metamorphic, for instance.
Sorry, I agree with Mike that it is difficult to identify from the photographs. Please bring them into the Identification and Advisory Service in the Angela Marmont Centre, at The Natural History Museum, London.You can post them in if you wish.
Ah - I was getting the wrong impression of the bottom one - could be pyrite.
Not sure what to make of your comments on the top one.
Definitely best to let the folks at the AMC have a look-see.
Hi GillB - Sorry, I would not like to say from the photographs, please can you bring/ send your specimens to the Angela Marmont Centre for identification. The reason we need to see them is because ...
When examining any rock the questions geologist ask:
The answers help to name the rock and understand how it was formed.
Some of the tests to identify the minerals:
1. Crystal shape- the term 'habit' is used for the overall shape.
Tabular; habit = crystals are flat, plate-like or table-top shape
Acicular habit = long thin needle-like crystals
Bladed habit = long flat, blade-like crystals
Fibrous habit = thin, fine and hair like for e.g. asbestos
Reniform habit = kidney shape e.g. haematite.
2. Mineral colour
3. Mineral streak - colour of powdered mineral found by rubbing onto a porcelain tile.
4. Mineral lustre - metallic, vitreous (glassy) for example quartz; dull (no reflection at all); translucent; opaque
5. Mineral hardness (Moh's scale) http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
6. Mineral cleavage and fracture http://academic.brooklyn.cuny.edu/geology/grocha/mineral/cleavage.html
7. Mineral reaction to acid
8. Mineral specific gravity
9. Mineral magnetism
I hope this is of interest to you and hope to see your specimens in the AMC