What is a Hexagonal close packed lattice and what is sign it?"

Dolomite has a trigonal crystal structure which in turn has a hexagonal lattice structure.

a=2r

where a is horizontal distance of hexagonal system

c= (1.633*a)

or

C=(4r)*(SQRT(2/3))

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A metallic lattice consists of positive ions in a 'sea' of outershell negative electrons which are delocalised and mobile through the metal structure. The lattice is held together by strong forces of attraction between the mobile electrons and the positive ions.

Both face and hexagonal are the most efficient, so they are equal. Then it's body, then simple cube.