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Figure 5.17. Projection from H₂O. In the presence of water under pressure for all assemblages, the composition diagram for Mg₂SiO₄-H₂O-SiO₂ can be reduced from 2-dimensions to 1-dimension by projection from H₂O. A chemical projection can be done graphically by drawing a line from the projection point (H₂O) through the minerals being projected (Forsterite (Fo), Antigorite (Atg), Enstatite (En), Talc (Tc), and Quartz (Qz)) and finding where those lines intersect the 1-dimensional, 2-component line Mg₂SiO₄-SiO₂ .

According to the Mineralogical Phase Rule, for a 3-component system at a random T and P there can be a maxiumum of 3 phases in equlibrium. By projecting from water (H₂O), we are saying that one of the 3 phases is water, leaving a maximum of 2 other phases to determine. Thus, a 2-component, 1-dimensional assemblage diagram can be used to show assemblages and reactions.

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Click on the "Use H₂O Projection" button to see how the simplified composition diagram can be used to show equilibrium assemblages.

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