What's the exact meaning or the formal name of "SiO2(a)"

[Smith.Z]

Dear all,

In Phreeqc.dat, there are equilibrium constants for quartz, chalcedony, SiO2(a), What's the exact meaning or the formal name of "SiO2(a)" ?

Thank you so much in advance,

Kind regards,

[dlparkhurst]

The "(a)" indicates amorphous, and represents a silica phase that might be the first to precipitate in a reactive environment. The phase goes all the way back to the original WATEQ program by Truesdell and Jones. The sources of thermodynamic data are documented in the manual for WATEQ4F (https://doi.org/10.3133/ofr91183).

Here are results from a calculation for a pure silica system with 100 mg/kgw, the approximate solubility of SiO2(a) at 25 C. The silica phases are reordered by log K.

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  Phase               SI   log IAP   log K(298 K,   1 atm)

  SiO2(a)          -0.07     -2.78   -2.71  SiO2
  Silicagel         0.24     -2.78   -3.02  SiO2
  Chalcedony        0.77     -2.78   -3.55  SiO2
  Cristobalite      0.81     -2.78   -3.59  SiO2
  Quartz            1.20     -2.78   -3.98  SiO2


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solution
-units mg/kgw
Si 100


[Smith.Z]

dlparkhurst, thanks for your full and  detailed explanation.

[Smith.Z]

Dear all,

 And what’s  the exact meaning or the formal name of "Sepiolite(d)" ?

Thanks in advance

[dlparkhurst]

https://phreeqcusers.org/index.php/topic,1668.msg5525.html#msg5525

and this note has been added in phreeqc.dat:

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#(a) means amorphous. (d) means disordered, or less crystalline.
#(14A) refers to 14 angstrom spacing of clay planes. FeS(ppt),
#precipitated, indicates an initial precipitate that is less crystalline.
#Zn(OH)2(e) indicates a specific crystal form, epsilon.


[Smith.Z]

Thank you very much!