Grain boundary map

[Albert2000]

Hello,

I'm intending to produce grain boundary maps from microphotographs of (partially very fine-grained) schists.

So far I have applied a variety of procedures to my images e.g. filters, thresholding, skeletonizing, dilate, etc. Here's my problem: The grain boundaries (in black) in many cases aren't connected to each other or have gaps. Is there an operation that can connect the grain boundaries and close the gaps?

Alternatively: Can someone recommend a sequence of procedures that might be suitable, or recommend literature that is dealing with that exact topic (grain boundary maps).

Greetings