The main shapes are chance artefacts of the way the rock happens to have broken and exposed different layers selectively. The glittery bits are, indeed, mica; some layers will be richer in mica than others, and the rock as a whole would be prone to splitting along a particularly mica-rich layer.
With many fractures, they start at one place and propagate away. Sometimes that leaves a mark (a frondescent pattern or hackle marks) on the fracture surfaces. I think that is what you might be perceiveing as the veins of your 'butterfly'.