some last-minute discoveries:

1) the white powder does not fizz in vinegar - but it IS further
crushable to a fine white dust...

2) it does NOT taste like salt.

3) it has a dirty white appearance in the vinegar - like a white/brown

i should also point out that i live over an aquifer-fed water supply
in South Central Texas - the Edwards Aquifer.

historically, we have had heavy limestone deposits in our water
supply, but i've seen and drank the hard water, and this is NOTHING
like that...

my suspicions still lie with the plastic dip tube, except that large
white SOLID marble-sized pieces came out of the water heater when i
drained it 2 weeks ago - they looked like crystalline
formations...like "jacks" of yore...