You can explain that with Young's equation sigma[LG]*cos(theta) = sigma[SG] - sigma[SL] (S-solid, L - liquid, G - gas).

1. sigma[SL] vs. x[water]. Angle of 60 deg for water means positive affinity of water for the calcite (sigma[SL]  sigma[SG]). From the above two it is obvious that if one adds x = 5-10% water to EG, the surface of the calcite will be covered by an adsorption layer of water decreasing sigma[SL] steeply (EG will be displaced); from Young, this corresponds to steep decrease of theta with x[water].

(ii) sigma[LG]. If one increases x[water] further (e.g., 30-50-70%), the SL interface won't change much (it will already be completely covered with water). However, sigma[LG] increases monotonously with x for this mixture. Since sigma[SG] > sigma[SL] for x larger than e.g. 10%, the increase of sigma[LG] will decrease cos(theta), and will increase theta.

I tested a very simple model for sigma[LG](x) and sigma[SL](x) (with random numbers but qualitatively correct shape) to check the above, you can try it yourself:

sigma[LG]:= 72e-3*x+50e-3*(1-x)      (mixture-like dependence; you should be able to find the correct sigma[LG] in the literature)

sigma[SL]:= 50e-3+30e-3*(1-x)^9      (dependence ala strong adsorption)

sigma[SG]:=70e-3;                             (SG independent of x)

This leads to theta(x) of the same qualitative dependence as the one you describe.

What I would do is to use sigma[LG] from the literature, multiply it with the experimental cos(theta) and thus I'll get sigma[SL] - sigma[SG] vs. x (which will be monotonic if I am right). Then use Gibbs isotherm R*T*Gamma = -d(sigma[SL] - sigma[SG] ) / d(lnx) to determine the adsorption of water vs. x by numerical differentiation. It should be something close to Langmuir isotherm.

 Dear Radomir, Thank you very much for your answer! Do you know if this or similar explanation was published, so I can refer to?

Dear Alexander,

No, I don't know any, but I'm pretty sure you can find it. Try to search for studies that use data for contact angle to determine adsorption.