A naturally-occurring mineral known for it's extreme temperature resistance and strength, as well as it's ability to be molded, machined, and woven into fabrics. It's also cheap, at $220 per metric tonne. Canada has huge natural deposits of asbestos, from which the United States imports 96% of its needs.

It is often used specifically as insulation for boilers because of its high emissivity and melting point. It just so happens that there is a good application for asbestos in space, for those exact reasons.

The world currently consumes about 14.2 terawatts of energy (from all sources). A photovoltaic array orbiting Earth would have to be 258 km in diameter to generate this much power.... But a quartz Fresnel lens orbiting the Sun at 2 million km distance and throwing a concentrated beam of sunlight toward Earth would only have to be 0.317 km in diameter. And an array of simple asbestos thermal panels (emissivity 0.94) orbiting Earth and heated to 959 deg.Kelvin by the beam of sunlight would only have to be 40 km in diameter to satisfy all the world's energy needs.

Asbestos!