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Author Topic: What's the exact meaning or the formal name of "SiO2(a)"  (Read 426 times)

Smith.Z

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What's the exact meaning or the formal name of "SiO2(a)"
« on: November 11, 2021, 06:31:06 AM »
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,
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dlparkhurst

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Re: What's the exact meaning or the formal name of "SiO2(a)"
« Reply #1 on: November 11, 2021, 03:16:18 PM »
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.

Code: [Select]
  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


Code: [Select]
solution
-units mg/kgw
Si 100
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Smith.Z

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Re: What's the exact meaning or the formal name of "SiO2(a)"
« Reply #2 on: November 12, 2021, 01:44:32 AM »
dlparkhurst, thanks for your full and  detailed explanation.
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Smith.Z

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Re: What's the exact meaning or the formal name of "SiO2(a)"
« Reply #3 on: November 17, 2021, 02:14:08 AM »
Dear all,

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

Thanks in advance

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dlparkhurst

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Re: What's the exact meaning or the formal name of "SiO2(a)"
« Reply #4 on: November 17, 2021, 02:31:22 PM »
https://phreeqcusers.org/index.php/topic,1668.msg5525.html#msg5525

and this note has been added in phreeqc.dat:

Code: [Select]
#(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.
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Smith.Z

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Re: What's the exact meaning or the formal name of "SiO2(a)"
« Reply #5 on: November 18, 2021, 02:29:13 AM »
Thank you very much!
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