Non-silicate

Crystal system: trigonal (-)

Colour in hand specimen: white, often tinged with yellow or brown 
Cleavage: rhombohedral 
Hardness: 3.5-4 

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: 0.179-(0.185); >4^th order 
Extinction: inclined 
Twinning: lamellar, common

M543 Dolomite, associated with siderite and copper pyrites 
Cerro Blanco, Chile 
Rhombohedral crystals of dolomite with curved faces, associated with siderite (iron carbonate) and copper pyrite 
White, slightly grey 
Hardness 3.5 – 4

L352 Limestone, dolomitic

dolomite

Ca₅(PO₄)₃(OH,F,Cl)

Non-silicate

Crystal system: hexagonal (-)

Colour in hand specimen: green, white, yellow, blue brown etc.
Hardness: 5

Colour in plane polarised light: generally colourless
Pleochroism: weak to moderate in coloured varieties
Relief: moderate
Birefrinence: 0.001-0.007; first order greys
Extinction: straight
Form: commonly hexagonal in cross-section

L100 Alkali granite 
L101 Granite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite)

apatite

Non-silicate

Crystal system: cubic

Colour in hand specimen: extremely variable: colourless, white, yellow, green, blue and purple most common
Cleavage: perfect octahedral, up to four visible at one time, breaks into dipyramids
Hardness: 4

Colour in plane polarised light: colourless, pale green or pale violet
Pleochroism: none
Relief: low

M560 Fluorite 
Derbyshire 
Cubic fluorite crystals 
Generally very variable; in this case white to purple 
Glassy lustre 
Hardness 4 
Octahedral cleavage

fluorite

Non-silicate

Crystal system: cubic

Colour in hand specimen: colourless or white when pure, more typically orange or red due to inclusions of iron compounds, and may also be grey, yellow or blue 
Cleavage: perfect cubic, three planes intersect at 90° 
Hardness: 2.5

halite

Cretaceous

Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida

'g' shaped coiled shell
Ribs
Involute
Ammonitic suture lines

Internal mould

Heteromorphic ammonites (such as Scaphites) diversified during the late Cretaceous. They may have lived as planktonic or benthic organisms depending on where the centre of gravity lies in the shell, which is much dependent on the soft parts. Sadly these soft parts rarely preserve.

Jurassic

Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida

Planispiral
Ammonitic suture lines visible on some moulds
Ribs

Most external moulds, few examples of internal moulds, pyritised

Jurassic

Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida

Planispiral
Evolute
Ribs
Ammonitic suture lines

Calcite has replaced aragonite in the skeleton

Beware! The specimen in set 2 has a dark circular hole, which you may be tempted to identify as a siphuncle. However remember that in ammonoids the siphuncle runs around the outer edge of the chambers, so this hole is in the wrong place. It is probably there due to eccentricities of the mineralisation process.

Why is it a dark colour when the rest of the shell is white? Generations of Earth Scientists have made the same mistake, their pencil marks fooling others into making the same error.

Jurassic

Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida

Planispiral
Evolute
Ribs

Mineral replacement of shell (aragonite to silica)