Those 'black lines' in the lower half of the diagram are guidelines to mark the occurrence of equilibrated plagioclase at 90, 80, 70 mol.% anorthite etc. Since this is a continuum, where we initially intersect the cotectic line with our initial 'yellow line', we use those 'black lines' as a guide to see the composition of equilibrated plagioclase. For example, if our initial line intersects the cotectic at the same part the An 80 mol.% black line intersects - out first plagioclase will be 80 mol% An - it's fairly confusing with the lines, but hope this helps.
in case for anyone else who might be confused, you draw a parallel line from the cotectic dipping towards Anorthite. Just like the fully shaded black line.
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Very well taught sir 👍
You made it easy and simpler. Thank you
Confused how to draw that blue line??
Those 'black lines' in the lower half of the diagram are guidelines to mark the occurrence of equilibrated plagioclase at 90, 80, 70 mol.% anorthite etc. Since this is a continuum, where we initially intersect the cotectic line with our initial 'yellow line', we use those 'black lines' as a guide to see the composition of equilibrated plagioclase. For example, if our initial line intersects the cotectic at the same part the An 80 mol.% black line intersects - out first plagioclase will be 80 mol% An - it's fairly confusing with the lines, but hope this helps.
Very helpful. Thankyou
in case for anyone else who might be confused, you draw a parallel line from the cotectic dipping towards Anorthite. Just like the fully shaded black line.