Strange white spots inside oil tank, on bottom

Anyone ever come across this one. Strange white spots inside oil tank, on bottom curve, maybe ~1/4 inch ? in dia, I forget maybe ~50 of them.

50 Year old house in New Hampshire , original steel 275 gal oil tank, #2 heating oil, in semi-heated dry cellar ~ 60 degs from heat leakage, 44 inch side is vertical, .

Burner filter plugged after oil delivery , tank was almost empty. Figured tank sludge after 50 years.
Over summer let tank run dry and vacuumed out ~ 7 gallons of liquid, 2.5 of that was water (I assume 50 years of condensate) . Had 50 watt car headlight in tank. Saw strange white spots on tank bottom. I seem to think white spots were also higher than water level. What are the spots??

Though might be corrosion, so I tested for rust/corrosion. I ice pick stabbed entire outside bottom of tank looking for weakness, was none. Been using tank for 10 years since then, no problems or weeping. Just wondering what the white spots were , seemed rust would be brown. Tank walls seem 2X thickness of new tanks.