15556 is a very vesicular, olivine-normative mare basalt. The vesicle size varies across the sample (4-8mm), but the grain size of the minerals doesn’t vary. Pyroxene and plagioclase feldspar are the main two species. Olivine occurs as a minor constituent forming ragged crystals (see centre of rotation 1). Accessory minerals include cristobalite, ilmenite, spinel, troilite and metallic iron.
The sample weighed 1542.3 grams before analysis. It has been dated at 3.4±0.1 billion years (K/Ar).
Further details of this and other Apollo samples are here: http://curator.jsc.nasa.gov/lunar/
The Apollo 15 landing site was in the Apennine Highlands, and close to Hadley Rille — a long, narrow winding valley. Approximately 76 kg of lunar material, including soil, rock, core-tube and deep-core samples, were returned to Earth.
This mission was the first flight of the Lunar Roving Vehicle which allowed the astronauts to venture further from the Lunar Module than in previous missions. During three periods of extravehicular activity, or EVA, on July 31st, and August 1st and 2nd, Scott and Irwin completed a record 18 hours, 37 minutes of exploration, travelling 17.5 miles, in the first car that humans had ever driven on the Moon.
Apollo 15 was launched on 26 July 1971.