Mary Ellen Jasper A beautiful rock, made of life, consisting of a banded iron formation 1.85 billion years old made of microcrystalline silica containing red haematite inclusions sandwiched between the grey haematite of the precipitated iron. What makes this rock so interesting is that the red jasper is the fossilised remnant of stromatolites, some of the earliest complex organisms in geological history. They were formed by cyanobacterial microbial mats, growing sinuously over and through layers of precipitated sediments in an ever higher stack as the sediments fell rhythmically, creating the lovely shapes of the red jasper. They come from the Mary Ellen mine in Minessota, in a layer of rocks called the Biwabik iron formation. Originally precipitated in a shallow sea, the chemical reason for the richness in iron started with the very photosynthetic bacteria that formed the stromatolites. As their lifestyle released oxygen into the sea, the reduced iron oxidised, and being insoluble, precipitated on the sea bottom, accompanied by plenty of silica that fossilised the mats into jasper as they grew. The wiggling of the columns is thought to represent the tracking of the sun's position through the seasons, showing that the strategy adopted by sunflowers has along evolutionary history. Similar formations elsewhere have been used to prove the inference that the number of days in the year has shrunk through time as the Earth's rotation has slowed. These rocks also testify to one of the major events in global history, when free oxygen gradually appeared and poisoned off the existing ecosystem (who remain as some extremophile bacteria), thus paving the way for the evolution of the complex oxygen breathing life that is the most familiar feature of today's biosphere. Once again, these rocks demonstrate the ever spiralling links through geological history between chemistry and physics, rock, sea and life that are the theme of this year's AGI Earth Science week.....for more detail on these links see our past post on mineral evolution at http://tinyurl.com/ovd7w2l. Our past post on banded iron formation http://tinyurl.com/p5ob9a6 and stromatolites http://tinyurl.com/nh48h3z Loz Dear Readers, Image credit: Captain Tenneal http://www.agatelady.com/photo-gallery-mineral-of-the-month-january-2012.html