Marble tile - would rather not use grout

I am going to tile my shower surround with a dark brown (almost black) marble and when I imagine it in head, any color of grout would ruin it. I think it would look best if there was no gaps between the tile. I was thinking about butting the tiles up against each other when I was installing them and rather than you grout I was thinking about using clear silicone between the tiles. Is this acceptable? Also, should I just use regular mortar on the install? These tiles are awfully heavy and I am kind of worried about them falling down before the mortar dries. Should I use silicone adhesive to install in place of mortar? If I do use silicone like that does it just go directly between the tile and the cement backer board? I thought I read somewhere that is how the three piece surrounds are attached.