Density Derivation Inquiry for Hematite and Goethite in "thermo_minteq.tdat" Database

[Xiaofan Pan]

    Hello, I am writing to inquire about the derivation of the densities of Hematite (Hm) and Goethite (Gt) in  "thermo_minteq.tdat" database.

    Recently, I have come across some discrepancies in the reported densities of Hm and Gt from different sources. According to “thermo_minteq.tdat” database, the densities are both 2.650 g/cm3 (as shown in the attached screenshot in a PDF file). However, other sources, such as the Powder Diffraction Files (PDFs, also attached) and some crystallochemical database (http://database.iem.ac.ru/mincryst/s_carta.php?HEMATITE+1919; http://database.iem.ac.ru/mincryst/s_carta.php?GOETHITE+1774), report different values of 5.273 g/cm3 and 4.256 g/cm3 for Hm and Gt, respectively. 

    To ensure the accuracy of the densities used in my study, I conducted a preliminary test and obtained value for the density of Hm, being 2.530 g/cm3, that were very close to those in the “thermo_minteq.tdat” database. Therefore, I would greatly appreciate it if you could provide me with details about how the densities of Hm and Gt were derived in "thermo_minteq.tdat" database. Your help would be invaluable to me, and I look forward to your response. Thanks!

thermo_minteq.tdat Goethite PDF#81-0464.pdf Hematite PDF#79-1741.pdf screenshot.pdf

[Brian Farrell]

Hi Xiaofan,

GWB thermodynamic datasets contain entries for the molar volume of minerals, in cm3/mol. A "0" value indicates that no data is available. Rather than using a "0" value, which implies an infinite density, the GWB modeling programs estimate the mole volume of such minerals assuming a density of 2.65 g/cm3. The formula is mole volume = mole weight/ density. 

The thermo_minteq.tdat dataset contains no values for mole volume because the program from which it was converted, Visual MINTEQ, does not use or contain such data. Thus, all densities you see in the Show dialog when that dataset are loaded are the same, 2.65 g/cm3. On the other hand, the GWB's default dataset, thermo.tdat, contains mole volumes for a large number of minerals. The values for Hematite and Goethite are quite close to those you report. This dataset originated from Lawrence Livermore National Lab.

You can view or edit any dataset in the TEdit program, or in a text editor, if you prefer. 

Hope this helps,

Brian Farrell
Aqueous Solutions LLC