friday flashback — dwars river chromitite conundrum
Poor Mbele's little bus has broken. He heroically kept it running all day and safely got his charges to Gethlane Lodge near Burgersfort. Now it won't go, he's uncomfortable in the company of his grateful passengers who just want him to be one of us but struggles with the artefacts of Apartheid. Perhaps it was for the best that his bus isn't taking us further. From here on, the "roads" are potholes with little strips of ashphalt just to make the puzzle of how to drive on it worse.
Those grim roads led, eventually, to the Dwars River Heritage Site. Here the river cuts straight down through spectacular 2 billion years old white and black striped rocks. There isn't just beauty here. There are layer upon layer of conundrums. Yes conundrums, they say, is the plural and quite possibly because it doesn't even have a Latin root.
First of all this section has layers. But this is an igneous intrusion and these layers are both very thin and very long. Fractional crystallisation and sedimentation is simple enough to understand. Once a fluid containing chemical gets to a critical temperature or composition insoluble fractions will crystallise out and if they are more dense, they may settle. All good so far. Except these layers alternate: anorthisite, chromitite, anorthosite and so on. The chemical and physical properties must happen over and over again to get the same result. That didn't happen in a simple body of cooling, fractionating liquid which dropped out phase after phase as a mineral fell below its solidus composition, temperature and pressure.
If all that phase stuff wasn't bad enough there's evidence here of physical processes akin to those commonly seen in sedimentary rocks. There appears to be clasts of anorthosite picked up, fractured and moved about in the chromitite. How do you even conceive of something like chromitite, made up mostly of FeCr₂O₄, with a high density and melting point around 2200°C coexisting with anorthosite made up mostly of CaAl₂Si₂O₈ and a density maybe half that of chromitite and a melting point of around 1500°C? I give up. Nobody really knows. Just look on in awe knowing that it did happen even if we don't have a clue.
friday flashback — dwars river chromitite conundrum
Poor Mbele's little bus has broken. He heroically kept it running all day and safely got his charges to Gethlane Lodge near Burgersfort. Now it won't go, he's uncomfortable in the company of his grateful passengers who just want him to be one of us but struggles with the artefacts of Apartheid. Perhaps it was for the best that his bus isn't taking us further. From here on, the "roads" are potholes with little strips of ashphalt just to make the puzzle of how to drive on it worse.
Those grim roads led, eventually, to the Dwars River Heritage Site. Here the river cuts straight down through spectacular 2 billion years old white and black striped rocks. There isn't just beauty here. There are layer upon layer of conundrums. Yes conundrums, they say, is the plural and quite possibly because it doesn't even have a Latin root.
First of all this section has layers. But this is an igneous intrusion and these layers are both very thin and very long. Fractional crystallisation and sedimentation is simple enough to understand. Once a fluid containing chemical gets to a critical temperature or composition insoluble fractions will crystallise out and if they are more dense, they may settle. All good so far. Except these layers alternate: anorthisite, chromitite, anorthosite and so on. The chemical and physical properties must happen over and over again to get the same result. That didn't happen in a simple body of cooling, fractionating liquid which dropped out phase after phase as a mineral fell below its solidus composition, temperature and pressure.
If all that phase stuff wasn't bad enough there's evidence here of physical processes akin to those commonly seen in sedimentary rocks. There appears to be clasts of anorthosite picked up, fractured and moved about in the chromitite. How do you even conceive of something like chromitite, made up mostly of FeCr₂O₄, with a high density and melting point around 2200°C coexisting with anorthosite made up mostly of CaAl₂Si₂O₈ and a density maybe half that of chromitite and a melting point of around 1500°C? I give up. Nobody really knows. Just look on in awe knowing that it did happen even if we don't have a clue.