Nikon claims that fluorite cracks more easily than glass, and is more susceptible to heat
Both are true. However, do those facts have any practical relevance in terms of lens use in the field? A 10 M solution of sulfuric acid is more acidic than an eqimolar solution of hydrochloric acid - but if you annoy someone to the point where they push you into a vat of either there no practical difference - you'd be just as dead either way.
HCl, pK = -7, is a stronger acid than H2SO4, pK = -2. Although H2SO4 has two Hs, the second pK is 2 so only one of them is dissociated in a 10 M solution (pH = -1). Sorry, I am a bit of a chemist.
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