He passed away at age 40 from a massive heart attack, leaving my grandmother with 4 children to raise on her own. She never remarried. Clara would write to him in her journals, and I know the day she passed away (39 years later) was probably the happiest of her life, as she could finally see him again.
The last few years she suffered from Alzheimer's. It was a truly sad love story.
Many years later, my son was born the day my grandfather died.