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FAQ: QUESTIONS ASKED BY NOVICE USERS

(c) Australian Geological Survey Organisation 1999-2002
(c) Yuri Shvarov 1999-2002

Last updated: 3 May 2002

This file does not substitute for the comprehensive HCh
documentation. It only provides quick answers to some questions
asked by the package users. Please see HCh documentation for
further details. Your own contribution to this file in terms of
additional questions will be very much appreciated.

LIST OF QUESTIONS


1 HCh

1.1 GENERAL QUESTIONS

1.1.1 What does HCh stand for?
1.1.2 What are main areas of HCh application?
1.1.3 Can HCh be used to carry out mass transfer (reaction
path) calculations?
1.1.4 Can I use HCh for the construction of conventional
activity-activity diagrams (e.g., fO2-pH)?
1.1.5 What is the HCh T-P Range?
1.1.6 What is the HCh calculation range in terms of the ionic
strength? (NEW)
1.1.7 Where can I get the summary of the HCh
specifications?
1.1.8 What are advantages/disadvantages of HCh compared to
EQ3/6 in the field of "conventional" modelling in
hydrothermal geochemistry?
1.1.9 What are limitations of the free energy minimisation
approach employed by HCh from the practical point of
view?
1.1.10 Can I run HCh on a Macintosh computer?
1.1.11 How do I get assistance in using HCh?

1.2 HCH INTERFACE

1.2.1 What is the best way to get started for a devoted
Windows/Mac user?
1.2.2 Are there any plans to upgrade HCh to a special
Windows version?

2 UNITHERM USAGE

2.1 UNITHERM DATABASE

2.1.1 What is the guaranty that the included UNITHERM
database has no incorrect entries?
2.1.2 How to prevent inadvertent database changes made by
a user?
2.1.3 How can I add a new species to the UNITHERM
database?
2.1.4 Why do I have a wrong Clapeyron slope for a UNITHERM
mineral with phase transitions?
2.1.5 How to extend the default database?
2.1.6 How to create and use my own customised database?
2.1.7 What other models for aqueous species can I use
apart from those of Helgeson and Ryzhenko?
2.1.8 Why can't I read some extended database references?

2.2 UNITHERM REACTION MODE

2.2.1 How do I write a new reaction?
2.2.2 Why do I have apparently wrong pK values?

3 MAIN USAGE: PROBLEM DEFINITION & PROCESSING RESULTS

3.1.1 What is the best way to get started with modelling?

3.2 CREATING & EDITING FILES

3.2.1 Why do I have a message on database errors, but all
the HCh programs seem to work properly?
3.2.2 What is the best way to choose system components
(species)?
3.2.3 Can I add new input substances to an existing Blank
file?
3.2.4 What is the best way to define my rock composition?
3.2.5 Can I use mineral names to define my rock
composition?
3.2.6 Can I use brackets in mineral formulae?
3.2.7 Can I fit a long mineral formula in a short input
window?
3.2.8 What is the best way to define my fluid composition?
3.2.9 How can I delete "Initial values" from *.BL or *.IN
files?

3.3 PROCESSING GIBBS RESULTS

3.3.1 Within MAIN, how can I copy a GIBBS result file to
another directory?
3.3.2 How can I obtain fO2/fH2 values from my results?
3.3.3 What is the best way for plotting the modelling
results?

4 RUNNING GIBBS

4.1 CALCULATING EQUILIBRIA

4.1.1 Why do I have an error message "Restrictions
incompatible"?
4.1.2 How can I restrict the volume of mineral assemblage
for my isothermal-isochoric system (T, V = const)?
4.1.3 Can I calculate models with isoenthalpic boiling?
4.1.4 What factors affect the equilibrium calculation time
and how can I speed up my calculations?

5 EQUILIBRIUM MODELLING (MAIN + GIBBS)

5.1.1 How can I define pH and Eh of aqueous phase for
computing Eh-pH stability diagrams?
5.1.2 What is the easiest way to calculate solubilities of
minerals?
5.1.3 Can I calculate metastable equilibria using HCh?

--------------------------------------------------------------

1 HCH


1.1 GENERAL QUESTIONS


1.1.1 What does HCh stand for?

HCh stands for HydroChemistry. But you can calculate
absolutely "dry" systems as well.


1.1.2 What are main areas of HCh application?

- Hydrothermal geochemistry
- Hydrochemistry and hydrogeology
- Experimental geochemistry (aqueous)


1.1.3 Can HCh be used to carry out mass transfer
(reaction path) calculations?

Yes, HCh is IDEALLY suited for MASS TRANSFER
calculations. In fact, such calculations are probably
the main area of the HCh application. You should keep
in mind, however, that overall non-equilibrium PROCESS
is modelled using local-equilibrium approach without
any provisions for modelling of the process kinetics.

Speaking of the REACTION PATH modelling the answer
depends on its definition. You cannot model a selected
(specified) sequence of mass-transfer calculations that
follow predefined phase or reaction boundaries.
However, employing a sequence of mass-transfer
calculations (e.g., "titration" models), it is possible
to establish the direction of the chemical PROCESS and
the sequence of main reactions leading to a new
equilibrium state. Limitations imposed by the local
equilibrium approach would still apply.


1.1.4 Can I use HCh for the construction of conventional
activity-activity diagrams (e.g., fO2-pH)?

No, you cannot use HCh for this purpose directly.
However, the results of HCh calculations may be plotted
on such diagrams.

For a concise review of the relationship between mass-
transfer calculations and activity-activity diagrams
please refer to T. Wolery (1992):

http://www-ep.es.llnl.gov/www-
ep/esd/geochem/EQ36manuals/eq36pkg.pdf (page -4-).


1.1.5 What is the HCh T-P Range?

In UNITHERM, Gibbs free energies are tabulated up to 500ЬC
and 5kb. But it is stated in the package specifications that
the possible calculations involving an aqueous solution cover
the temperature range of 0 - 1000ЬC and pressure range of
1 - 5000 bar at water densities exceeding 0.35 g/cm3. Are the
calculations of HCh valid to 500 or 1000ЬC or does the upper
temperature limit vary phase by phase according to the
conditions pertaining to the basic data for each phase?

(a) A T-P grid defined in UNITHERM is not related to
the range of HCh calculations -- but the mentioned
density limitations should be met. For example, you
cannot calculate equilibrium composition for an aqueous
phase at 500ЬC and 500 bar where dH2O = 0.26g/cm3. The
calculations involving aqueous phase are limited by the
MHKF model for aqueous species.

You can modify the UNITHERM T-P grid at any time by
using the UNITHERM command (DefTab). Just follow
the syntax of the default table to create your own T-P
grid. You can also modify the default T-P grid by the
UNITHERM /TAB command-line option.

(b) The upper temperature limit for a phase (e.g.,
mineral) will not limit calculations. It will limit
stability only of this particular phase.


1.1.6 What is the HCh calculation range in terms of the ionic
strength? (NEW)

Activity coefficients of charged aqueous species are
calculated by HCh according to the extended Debye-Huckel
equation. As the Debye-Huckel equation does not work
properly at high ionic strength, HCh cannot check ALL
POSSIBLE states of your system.

To avoid overflow in the Debye-Huckel procedure during
calculations for concentrated solutions, the free energy
minimisation code of HCh (GIBBS) artificially restricts the
value of the ionic strength by 10. Thus, you should keep in
mind the following:

1. If the GIBBS output contains the aqueous phase with
ionic strength in excess of 10, THE RESULT IS INCORRECT!
Please use other geochemical programs that support
calculation of activities of aqueous species in brines.

2. If the GIBBS output contains the aqueous phase with
ionic strength less than 10, the equilibrium should be
found correctly. We believe that the free energy is a
convex function, and the local minimum should be found
in the reasonable area.

3. If the GIBBS output does not contain the aqueous phase
at all, but it is present in your System file as a
possible phase, this result can be either correct or not
depending on properties of brines that GIBBS cannot
calculate.

4. You are not affected by these problems if your System
file does not include the aqueous phase.

Note that definitions "correct" and "incorrect" in the
current context refer exclusively to your thermodynamic
model but not to the model agreement with nature.


1.1.7 Where can I get the summary of the HCh specifications?

Please refer to your User's Guide (page 48), GUIDE*.TXT
file, or the WWW page

http://www.agso.gov.au/geochemistry/HCh/hch_www_spec.html


1.1.8 What are advantages/disadvantages of HCh compared
to EQ3/6 in the field of "conventional" modelling in
hydrothermal geochemistry?

Presently there are no users with comparable experience
in using both packages. However, William McKenzie
(Univ. of California) contributed a preliminary answer
to this question.

We assume that you have at least partial access to the
HCh documentation (see Question 1.1.10). To learn more
about modelling codes, and EQ3/6 in particular, browse
the following WWW sites:

http://www-ep.es.llnl.gov/germ/WR-codes.html and

http://www-ep.es.llnl.gov/www-ep/esd/geochem/eq36.html#Manuals

The main differences in the usage of the HCh and EQ
packages result from the different algorithms employed
to calculate equilibrium compositions of chemical
systems. EQ3/6 employs a method based on equilibrium
constants, whereas HCh uses the free energy
minimisation technique. Bethke (1996), Zeleznik and
Gordon (1960, 1968) and Brinkley (1960) have argued
that both methods are computationally and conceptually
equivalent. However, they result at least in different
ways of setting your modelling problems.

EQ3 is an equilibrium aqueous speciation model. EQ3
solves for equilibrium by using the log Ks, aqueous
concentrations of species, activity coefficient models,
and mass and charge balance constraints. The resulting
set of equations (including non-linear ones) is solved
by the Newton-Raphson method.

To calculate speciation using EQ3, you need to specify
basis species (e.g., Na+, Cl-, SiO2(aq), etc)
representing all elements in the system. The chosen
basis species are used to write reactions for the rest
of aqueous species and for saturation of minerals. The
log K for each reaction is calculated at a given
pressure at 6 temperatures (25-300ЬC); log Ks at other
temperatures are interpolated using the calculated
constants.

The free energy minimisation code of HCh (GIBBS)
calculates equilibrium composition of chemical systems
"directly" without having to specify basis species,
write reactions, and calculate log Ks. GIBBS solves for
equilibrium by using mass and charge balance
constraints, free energy of components (species) at
standard state conditions, and activity coefficient
models.

Using HCh, you can specify any temperature-pressure
path for you calculations. In case of EQ3 you have to
use a different data base for each pressure or set of
basis species.

EQ6 is mass transfer model. After the equilibrium state
for the aqueous solution is determined by EQ3, EQ6
titrates a small amount of reactant (e.g., a mineral)
into the solution, and the new equilibrium state is
again determined by EQ3. The equilibrium system
continues to evolve by continuing to titrate small
amount of the reactant into the solution until the
fluid reaches saturation with the reactant or the
reactant is totally consumed. Aqueous composition,
mineral precipitation, mineral dissolution are
determined at each step of the titration.

In turn, HCh has its own tools for mass-transfer
calculations. Processes like titration or mixing can be
modelled by HCh very easily and effectively using the
means provided by the Control menu.

EQ3/6 includes some provisions for kinetic
calculations, including "real-time" calculations. In
contrast, the HCh usage is restricted only to local-
equilibrium models.

As to the databases, HCh can be easily made compatible
with EQ3/6 if necessary (at least using SUPCRT92/96
SPRONS files).


1.1.9 What are limitations of the free energy minimisation
approach employed by HCh from the practical point of
view?

Not all local-equilibrium processes can be directly
modelled using the free energy minimisation method. For
example, you cannot directly model processes taking
part under constant volume, or isoenthalpic processes.
See answers to Questions 4.1.2 and 4.1.3.


1.1.10 Can I run HCh on a Macintosh computer?

To the extent of our knowledge, HCh has been
successfully run on PowerMac G3 under MacOS 8.0 using
SoftWindows 2.0 (make sure your math coprocessor is
turned ON!). No problems should be expected with better
hardware and DOS/Windows emulation programs. To make
sure that the software runs on your system, try to
download and run the HCh DEMO version from

http://www.agso.gov.au/geochemistry/HCh/hch_www_demo.html


1.1.11 How do I get assistance in using HCh?

Please refer to the following files included in your
distribution package:

Installation: README.TXT
General information: GUIDE*.*
Running examples: EXAMPLES.TXT
Distribution and licensing: LICENCE*.TXT
Disclaimer: LICENCE*.TXT
Latest information: WNEW*.TXT
Frequently asked questions: FAQ.TXT (this file)

You may also consult the manual "HCh: a software
package for geochemical equilibrium modelling. User's
Guide. Australian Geological Survey Organisation,
Record 1999/25". This text is an extended hard-copy
version of the GUIDE*.TXT file. The manual can be
ordered from the Sales Centre of the Australian
Geological Survey Organisation (sales@agso.gov.au)

You can get some immediate help during your HCh working
sessions. In the main UNITHERM or MAIN windows you can
always press to obtain a general reference on the
available commands, keys, and menus. Normally, at the
bottom screen lines you can also find a tip(s) on a
currently possible action(s).


1.2 HCH INTERFACE


1.2.1 What is the best way to get started for a devoted
Windows/Mac user?

I am a devoted Windows/Mac user who is not accustomed to the
DOS environment or DOS shells. As a result, I am finding
using the program interface a little bit frustrating. What is
the best way around it?

PLEASE NOTE THAT AT THE BOTTOM SCREEN LINES THERE IS
NORMALLY A TIP(S) ON A POSSIBLE ACTION. In the main
UNITHERM or MAIN windows you can always press to
obtain a general reference on the available commands,
keys, and menus.


1.2.2 Are there any plans to upgrade HCh to a special
Windows version?

Development of the 32-bit Windows version of HCh is
currently in progress. Please contact us at the end of
2002 re the anticipated release date and pricing.


2 UNITHERM USAGE


2.1 UNITHERM DATABASE


2.1.1 What is the guaranty that the included UNITHERM
database has no incorrect entries?

The default UNITHERM database has been used at the
Department of Geochemistry, Moscow State University for
the last 10 years for research and industry-oriented
projects. All data were typed in and checked
"manually". However, see the DISCLAIMER in your
LICENCE*.TXT file.


2.1.2 How to prevent inadvertent database changes made by
a user?

Inadvertent changes made by the user can be blocked by
setting the Read Only attribute for any of the database
files. See DOS help for the ATTRIB command.


2.1.3 How can I add a new species to the UNITHERM
database?

The data can be added interactively in a UNITHERM
session. Use or commands to activate the
UNITHERM input mode. If you are working under Windows,
you can use Windows Copy and Paste commands to
facilitate data entry.

To add a new or alternative data for an already
existing species you may find useful the UNITHERM Copy
command, . By duplicating the record, you may
reduce a possibility of mistypes. Just edit the copied
record using the command.

Starting from the HCh version 3.4 you can use the
UNITHERM command-line option /IMPort to load data from
the text file DATA.TXT. This file is intended for data
interchange between different program packages and
databases.


2.1.4 Why do I have a wrong Clapeyron slope for a
UNITHERM mineral with phase transitions?

Many mineral data in the default UNITHERM database come from
the compilation of Helgeson et al. (1978). The format of
mineral data seems to be completely consistent with that of
Helgeson, especially if I use the J<->cal switch . Why
do I have an apparently wrong value for the Clapeyron slope
(dP/dT) for minerals with phase transitions (quartz, for
example)?

Please note that UNITHERM uses the reciprocal of the
Clapeyron slope expressed as dT/dP*1000, rather than
the "true" Clapeyron slope dP/dT. Just view your
mineral record again, paying attention to the exact
name of the relevant record field. The dT/dP
representation is more convenient from the
computational point of view.


2.1.5 How to extend the default database?

If I want to add a new mineral/phase to the default UNITHERM
database, where would I preferably obtain the relevant
thermodynamic data?

Generally, the database is YOUR OWN responsibility.
Presently, there are a number of quite comprehensive
compilations available (e.g., Robie & Hemingway, 95;
SUPCRT92; Holland & Powell, 98; etc). The choice of the
particular database will depend on your PARTICULAR
PURPOSES, PROBLEMS, and REQUIRED ACCURACY. Databases
themselves are generally tuned for particular
application areas or for particular chemical systems.
Please note that the result of simple "data mixing"
sometimes might be disastrous-you should make sure that
the data are reasonably consistent.


2.1.6 How to create and use my own customised database?

Your own customised database can be created in a number
of ways. In most cases it is expedient to copy the
initial UNITHERM database (files UT.DIR, UT*.BIN &
UT*.REF) into a separate directory, and modify the
database copy.

If you want to keep only a part of the initial records,
start UNITHERM from the new database directory with the
/SCOPY command-line option. Now you can select the
components that should be kept in the new database. The
database compiled from this selection will be saved in
the WT.DIR and W*.BIN. files when you finish your
UNITHERM session. To make the new database usable, copy
these files into UT.DIR and U*.BIN files over the
initial UT*.* files. After that you can work with
UNITHERM as usual. Refer to Question 2.1.3 how to add
new database components.

Use the same command-line option (/SCOPY) if you want
to create your own database starting from scratch. Just
deselect all the database components in your /SCOPY
UNITHERM session, and copy the empty database on exit.
Again, the new database will be saved in the WT.DIR and
W*.BIN files. Refer to Question 2.1.3 how to add new
database components.

You can use SEVERAL versions of UNITHERM databases in
your everyday work. Just (a) keep your relevant
database files (UT*.*) in separate directories (not the
UNITHERM program!), and (b) start UNITHERM (UT.BAT)
from an appropriate database directory.

However, make sure to specify the proper PATH to the
appropriate database for MAIN and GIBBS in a MAIN
session. Follow the steps summarised below:

Menu Bar -> Options -> Set Paths ->
...UNITHERM database -> Enter

Save this setup in the preferred directory:

Menu Bar -> Options -> Save Setup ->
Choose the preferred directory -> Enter


2.1.7 What other models for aqueous species can I use
apart from those of Helgeson and Ryzhenko?

You may include in your database aqueous species
defined through pK of exchange reactions using the
"complex" (Ryzhenko-Bryzgalin) format of the UNITHERM
database.

(1) For example, you can use an assumption that pKdiss
NaHS (aq) is approximately equal to pKdiss NaCl (aq).
This implies that pK of the exchange reaction

NaHS (aq) + Cl- = NaCl (aq) + HS-

is independent of T and P and equal to 0. In this case,
the complex NaHS (aq) must be defined as follows:

NaHS (aq)
B.Ryzhenko model
Coef. Basic species
1 NaCl (aq)
1 HS-
-1 Cl-

pK(298) 0.000
A(zz/a) 0.000
B(zz/a) 0.00
----------------------
Calculated from
the assumption:
pK(NaHS) = pK(NaCl)

(2) Using a similar approach, you can also use the
method suggested by Gu et al. (GCA, 1994). This method
implies that dGr(T,P) of an exchange isocoulombic
reaction is approximately constant. In this case,
temperature and pressure dependence of pKr(T,P) values
will be represented by a simple equation:

pKr = (298/T)*pKr(298,1),

and an ambient known value of pKr can be used to
calculate the reference pKr(298,1) value.

For example, Zn(HS)2 (aq) can be expressed in terms of
the exchange reaction

Zn(HS)2 (aq) + 2Cl- = ZnCl2 (aq) + Cl-

with pKr(373,PSat) = 10.43 (pKr(298,1) = 13.05). You can
enter in the database the following record:

Zn(HS)2 (aq)
B.Ryzhenko model
Coef. Basic species
-2 Cl-
1 ZnCl2 (aq)
2 HS-

pK(298) 13.054
A(zz/a) 0.00
B(zz/a) 0.00
----------------------
Calculated from
the approach by
Gu et al. (1995)


2.1.8 Why can't I read some extended database references?

I want to obtain an extended reference for a UNITHERM
component (), but instead have unreadable strings of
Windows symbols. Is this a program bug?

No. Some of the extended references in the primary
default UNITHERM database may be provided in Russian.
They can be read in normal Russian characters in a
conventional (non-Windows) DOS session or in a full-
screen Windows session ().


2.2 UNITHERM REACTION MODE


2.2.1 How do I write a new reaction?

Press enter when you have completed the reaction name.
Now specify the stoichiometry of your reaction. For
example, for the reaction

0.25 Magnetite + 1 H2S (aq) = 0.25 Pyrite +
0.5 Troilite + 1 H2O

type in

PyPoMt buffer
Component Coefficient
Pyrite 0.2500
Troilite 0.5000
H2O 1.0000
Magnetite -0.2500
H2S (aq) -1.0000

and press Esc. Note that the program calculates pK
values (= -logK) of reactions.


2.2.2 Why do I have apparently wrong pK values?

I tried to calculate pK (Delta G) values for a reaction
involving a gas species, but they do not make any sense. Is
this a program bug?

Make sure that you are using the "1bar" reference state
for gases to calculate the parameters of your reaction.
Just start UNITHERM with the /1BAR command-line option.
Otherwise, UNITHERM tabulates values g(t,p) =
g(t,1bar) + RTln(P), and by default will calculate pK
and Delta G values of your reaction on this basis.


3 MAIN USAGE: PROBLEM DEFINITION & PROCESSING RESULTS


3.1.1 What is the best way to get started with modelling?

Refer to the "Equilibrium Modelling" section of the
GUIDE*.TXT file or the HCh manual (especially, the
provided flow chart). The experience has shown that it
is a good idea to BROWSE THE HCH DOCUMENTATION, help
windows, and the provided examples, even if you are
extremely reluctant to do so!


3.2 CREATING & EDITING FILES


3.2.1 Why do I have a message on database errors, but all
the HCh programs seem to work properly?

When I create a new System file I get a message from MAIN
that the UNITHERM database contains errors. However, MAIN
seems to work normally, and UNITHERM cannot find any database
errors. What can be wrong?

You may have violated the HCh naming conventions for
aqueous species.

When MAIN reads your database to create a new System
file, it parses the chemical formulae of all database
components to obtain the list of the chemical elements
and species stoichiometry. The error message tells you
that there are some formulae that cannot be parsed
properly. However, these components are not shown by
MAIN and they will not be offered for your selection in
the list of the available species.

This error can occur only for aqueous species, where
the component names are represented by combinations of
a conventional chemical formula and an optional
comment. An optional comment must be separated from the
chemical formula either by the space or comma. The
error message tells you that you need to check the
names of the aqueous species that you have added to
UNITHERM.


3.2.2 What is the best way to choose system components
(species)?

Is it a good idea to include all the offered components in my
new System file when I create it?

From a point of view of an experienced user rather NO
than yes. If you want to avoid computational problems,
loss of computational time or strange results it is
better to limit yourself to the components that are (1)
likely to be stable or significant in your models; (2)
come from reasonably consistent datasets.

However, observing the condition (1) may require some
preliminary calculations. Observing the condition (2)
is the direct responsibility of a literate user. You
will be able to EXCLUDE excessive components later
through EDITING of your System file, but you will not
be able to ADD new (or excluded) components to an
existing System file. It might be a good idea to keep a
reference copy of your initial System file with the
longest list of components.


3.2.3 Can I add new input substances to an existing Blank
file?

You can add new input substances ONLY INSTEAD of
previously defined substances (the total number of
substances will stay the same). If you are not sure how
to define the input for your system, you may want to
reserve a few extra lines for the future use. Just
enter a few dummy substances allowed by your chemical
system within the bottom rows of your Blank file, for
example:

H O S
* * * Substance ..... Unit * * *
0. H2O kg
1. H2S moles
2. H2S moles
3. H2S moles
* * * End of file * * *

To specify the real content of H2S for your system in
the Input file, you will need to enter only one H2S
value, and set the rest of the "H2S" fields to zeroes.


3.2.4 What is the best way to define my rock composition?

What is the best way to define my rock composition in a Blank
file for the purposes of geochemical modelling?

Choose "...other substances" from the option list when
you create a new Blank file. Then you can specify the
composition of your rock either in terms of (1) oxides
and chemical elements, or (2) minerals, in grams or
moles. For example:

H O K Na Al Si
* * * Substance ..... Unit * * *
0. H2O kg
1. SiO2 g
2. K2O g
3. Na2O g
4. Al2O3 g
* * * End of file * * *

H O K Na Al Si
* * * Substance ..... Unit * * *
0. H2O kg
1. SiO2 g
2. KAlSi3O8 g
3. NaAlSi3O8 g
4. KAl3Si3O12H2 g
* * * End of file * * *

When you create an Input file to prepare Control files
for modelling, it will be often convenient to normalise
your rock composition per 1 kg.


3.2.5 Can I use mineral names to define my rock
composition?

I want to define my rock composition as a number of minerals,
can I just enter the appropriate mineral names in the
provided fields?

No. You must use an EXPLICIT CHEMICAL FORMULA of the
mineral. For example, if you want to use quartz, type
in SiO2. If you have started MAIN from Windows, you can
also start UNITHERM and use it for further reference
using Windows Cut and Paste capabilities if necessary.


3.2.6 Can I use brackets in mineral formulae?

I want to define my rock composition as a NUMBER OF MINERALS.
Can I use brackets in the mineral formula?

You can (e.g., AlO(OH)), but they consume space in long
formulae (see below).
You can also use square brackets [ ] and the asterisk
(*) (e.g., CaSO4*2H2O).


3.2.7 Can I fit a long mineral formula in a short input
window?

I want to define my rock composition as a NUMBER OF MINERALS,
but my formula won't fit in the provided window. Is there
any way around this problem?

The provided input box allows you to enter a chemical
formula of 14 characters. If you want to use a
mineral/substance with a longer formula, you can
"split" your mineral/substance in two. For example,
epidote Ca2FeAl2Si3O13H (15 characters) can be
represented within two separate lines as:

1. Ca2FeAl2O7H
2. Si3O6


3.2.8 What is the best way to define my fluid composition?

What is the best way to define my FLUID COMPOSITION in a
Blank file for the purposes of GEOCHEMICAL modelling?

Choose "...other substances" from the option list when
you create a new Blank file. Then you can specify your
fluid composition in terms of any convenient and
sensible substances:

H O K Na Al Si
* * * Substance ..... Unit * * *
0. H2O kg
1. CO2 moles
2. NaCl moles
3. HCl moles
4. H2S moles
* * * End of file * * *

We recommend you to AVOID definition of fluid
compositions using COMBINATION OF THE "...OTHER
SUBSTANCES" AND CHARGED AQUEOUS SPECIES (see Question
4.1.1).

If you intend to enter a solution composition obtained
by preliminary calculations with GIBBS (for example, if
according to your model the initial solution should
have an equilibrium composition), you can specify the
solution composition in terms of chemical elements. At
the same time, composition of rock can be better
specified in terms of minerals or oxides. Using the
"...other substances", you can combine a number of ways
to set the bulk composition of your system. However, if
you want to specify the chemical composition of your
solution in terms of chemical elements, make sure to do
it with the maximal accuracy. Otherwise you may face
the problems addressed in Question 4.1.1.


3.2.9 How can I delete "Initial values" from *.BL or *.IN
files?

Copy your file to a file with a new name, delete the
source file, and rename your copy back to its
"parental" name. The initial values will be deleted.
This operation is possible because initial values are
not copied.


3.3 PROCESSING GIBBS RESULTS


3.3.1 Within MAIN, how can I copy a GIBBS result file to
another directory?

Unfortunately, you cannot do it. However, to ensure a
convenient work environment, follow the steps
summarised below:

1. Create your own working directory for a particular
problem (or group of problems).
2. Start HCh from this directory.
3. Menu Bar -> Options -> ChDdir Request -> None ->
Enter
4. Menu Bar -> Options -> Save Setup ->
...in the current directory -> Enter

All your files and settings relevant for a particular
problem will be easily kept together. See the HCh
documentation about the HCH.BAT and UT.BAT files.


3.3.2 How can I obtain fO2/fH2 values from my results?

Unfortunately, GIBBS does not output fO2 or fH2 values.

Use log K values for the reactions

O2(g) = O2(aq) log K1, and
H2(g) = H2(aq) log K2

to recalculate your results, e.g.:

log fO2 = log mO2 - log K1.

However, make sure you use the proper reference state
for gases (T, P = 1 bar).


3.3.3 What is the best way for plotting the modelling
results?

Write your results in a binary file (*.RE). Process it
within HCh by means of the Result menu, and save the
selected results as an ASCII *.REX file. The data will
be ready for import by EXCEL and any other graphing
package. Just follow the chain of options summarised
below:

Menu Bar -> Result -> Binary file ->
Choose Result (*.RE) file ->
Choose Cross sections (graph) -> Choose Horizontal ->
... -> Press (save) -> Choose All (autoselect) ->
Choose table layout -> Edit a filename and/or press


Now your data can be imported by EXCEL. Use the REX.XLS
workbook to facilitate this import.


4 RUNNING GIBBS


4.1 CALCULATING EQUILIBRIA


4.1.1 Why do I have an error message "Restrictions
incompatible"?

The bulk system composition you have specified cannot
be defined in terms of the possible system phases. Most
likely something is wrong either with your speciation
model or the way you have specified the bulk system
composition. Go back and have a close look at your
Control, Input, or System files.

The most frequent cause of this message is the absence
of the charge balance in the aqueous solution.


4.1.2 How can I restrict the volume of mineral assemblage
for my isothermal-isochoric system (T, V = const)?

Equilibria in an isothermal-isochoric system (T, V =
const) can be calculated only by the minimisation of
the Helmholtz free energy (the Gibbs free energy is a
thermodynamic potential of a system only at constant T
and P). GIBBS does not calculate the Helmholtz free
energy.

A fundamental consequence of the existence of the
equation of state is that the system pressure is a
DEPENDENT function of the equilibrium state at the
specified temperature and volume, and cannot be set to
an independent value. Likewise, for the given
temperature and pressure, the volume is a function of
equilibrium, and cannot be independently specified.
The phase rule for the isothermal-isochoric system is
different from the phase rule for the isothermal-
isobaric system. Moreover, as was shown by Korzhinskii,
for some systems under a fixed volume, different phases
may be subjected to different pressures. All these
factors make it impossible to solve such problems by
the minimisation of the Gibbs free energy of the
system.


4.1.3 Can I calculate models with isoenthalpic boiling?

As GIBBS minimises the Gibbs free energy of the system,
processes in isoenthalpic systems cannot be calculated
directly. You need to estimate an isoenthalpic path for
your system independently in terms of temperature and
pressure. The resulting T-P curve can be specified in
your Control file.


4.1.4 What factors affect the equilibrium calculation
time and how can I speed up my calculations?

There are many factors that affect the calculation rate
for apparently similar problems. Clearly, the
calculation time will depend on the set of the phases
and components specified for your problem (the more
phases and phase components, the longer the calculation
time). However, the calculation time can dramatically
vary for the systems with the same set of the chosen
components. Here we discuss the most obvious factors
pertaining to this situation.

1. The bulk system composition.

This is one of the most critical factors affecting the
calculation rate. For example, for hydrothermal systems
involving a rock and an aqueous phase, calculations on
rock-dominated systems are generally faster than for
fluid-dominated systems. This is an intrinsic property
of the minimisation algorithm employed by GIBBS.
An unfavourable calculation case may occur when the
bulk composition of your system is close to the
saturation with an infinitely small amount of a new
phase. For example, in a gas-water system the bulk
composition of your system can match an equilibrium
between the aqueous solution and an infinitesimal
quantity of the gas phase. This situation may occur if
you model isothermal-isobaric degassing of a fluid
(e.g., according to an algorithm like [*] = [A] + k*[G],
where k <= 1). If the amount of the gas phase at a new
equilibration step is as small as the uncertainties of
the mass balance equations, the equilibrium might not be
calculated at all.

2. Redox equilibria.

Calculations on systems with redox equilibria are
inherently slower than on systems without them. We
recommend you to avoid calculations of this type in
closed systems without specifying sufficient quantities
of redox buffers.

For example, you may wish to calculate equilibria in
the system H2O-H2S. If you include in the solution
model only H2O, H+, OH-, H2S (aq), and HS-, you will
encounter no problems- calculations will be fast.
However, if you add other species such as O2 (aq), H2
(aq), HSO4-, and SO4--, the calculation time will
increase dramatically. Without a redox buffer (i.e.
sufficient quantity of a chemical element in different
oxidation states of relatively comparable amounts),
redox equilibria will be extremely sensitive to very
small variations in concentrations of species in
different oxidation states. If these variations are
comparable to the uncertainties of the mass balance
equations, the equilibrium might not be calculated at
all.

In geochemical modelling, try to avoid this problem by
setting reasonable redox constraints from the very
beginning. To this end you can explicitly specify redox
buffers in your rocks (e.g., coexisting Fe-oxide + Fe-
bearing silicate, or Fe2O3/FeO ratio). You should apply
the same approach to the fluids that are not initially
constrained by the fluid-rock equilibrium (e.g., by
specifying total H2 (aq) or H2S/H2SO4 ratio).

Of course, you cannot always avoid this problem-you
might be interested exactly in the redox equilibria in
the H2O-H2S system resulting from oxidation of H2S
through reactions with water.

If you are not interested in redox equilibria at all,
and they will not be significant in your models, you
might discard them COMPLETELY. For example, you might
be interested in solubility of calcite in CO2-dominated
aqueous solutions. In this case, make sure that the
components of your system comprise elements only in the
same oxidation state each (e.g., do not include H2
(aq), O2 (aq), and CH4 (aq) in your aqueous phase!).

Some of the factors affecting the calculation time are
related to temperature and pressure specified for your
calculations. This relationship results in an apparent
T-P dependence of the calculation rate.

3. Greater variety of low-temperature minerals.

If your problem includes a large number of solid phases
stable at low-temperatures, calculations for the low
temperature region will be slower. The problem of
choice of the stable mineral assemblage will require
more time as more possibilities exist in this region.

4. Behaviour of electrolytes and activity models for
aqueous species.

In aqueous systems the increase in the calculation time
might be related to calculation of the activity
coefficients of aqueous species. Generally, higher
temperatures and low pressures will promote association
of aqueous complexes, lower values of ionic strength,
and values of activity coefficients closer to unity.
All these factors make the aqueous solution closer to
ideal. The overall result is a faster calculation
convergence. The exact calculation time will be
affected by the exact models for the activity
coefficients of charged and neutral aqueous species
(e.g., the default GIBBS options vs /c=@1, /b=@3). For
a fluid-dominated system the fastest calculation time
should be expected for high-T, low-P region for
activity coefficient models favouring association of
ion pairs.

You can speed up your calculations for aqueous systems
in a "manual" calculation mode of GIBBS when using
Blank or Input files. To this end, calculate a number
of sequential equilibria in a row going along the
defined temperature/pressure gradient. In this case
GIBBS will use the previously obtained results for the
aqueous solution as the initial approximation for
further calculations, and the new results will be
obtained faster. Note that to take the advantage of
this mechanism you do not need to specify the GIBBS /s
option.

In the general case, you can speed up your calculations
by setting the GIBBS /s option (compute sequentially).
In this case the entire equilibrium phase assemblage
obtained at the previous calculation step will be used
as the initial approximation for further calculations.
Without this option GIBBS estimates the starting phase
assemblage by itself.

Note that in most cases of sequential calculations it
is expedient to go "down" the temperature; it makes the
usage of the /s option more effective.


5 EQUILIBRIUM MODELLING (MAIN + GIBBS)


5.1.1 How can I define pH and Eh of aqueous phase for
computing Eh-pH stability diagrams?

Unfortunately, you cannot do it. Physically, pH and Eh
are DEPENDENT variables and cannot define a state of
equilibrium. Some chemical programs indeed allow you to
specify these variables. However, strictly speaking
their results are not correct, as a "conditional"
equilibrium is not the true chemical equilibrium. To
fix pH and Eh values you can only use buffer
substances, like in real chemical system.


5.1.2 What is the easiest way to calculate solubilities
of minerals?

What is the easiest way to calculate solubility of minerals
if I already know the stable mineral assemblage?

You can use a system with the "perfectly mobile
components" (PMC). In this case, GIBBS does not need to
search for minerals that are already known to be
stable. For example, if you want to calculate
solubility of minerals in the Quartz-Calcite-
Wollastonite assemblage, follow the steps summarised
below:

1. Create your System file:

a) From the list of the possible phases, select
"Perfectly mobile components", "Individual phases",
and "Aqueous solution".
b) From the list of the chemical elements, select C,
Ca, H, O, and Si.
c) From the list of the PMC, select Quartz, Calcite and
Wollastonite.
d) From the list of individual phases, select all
minerals.
e) From the list of aqueous species, select aqueous
species as usual.

2. Create your Blank file:

a) Choose definition of PMC fugacities in terms of
lg(P) or ln(P).
b) Choose any option for definition of the bulk
chemical composition.
c) Define fugacities of the PMC as constant, and set
their log values to 0. Specify the measurement units
for water as kg.

3. Run GIBBS in the interactive mode:

a) Specify your Blank file as a source file.
b) Start GIBBS. Input temperature, pressure, and the
amount of water (1 kg) -- and calculate equilibrium.

If you obtain only the aqueous solution, it represents
the solubility of your specified mineral assemblage.
Precipitation of any new mineral(s) means that
dissolution of your assemblage is incongruent. If you
get the error message "Solution unlimited", your
specified mineral assemblage is NOT stable.


5.1.3 Can I calculate metastable equilibria using HCh?

The most general approach for calculating such
equilibria is using artificial "User Elements". Using
user elements you can specify the chemical substances
that cannot be re-equilibrated. For example,
thermodynamically unstable organic substances in
natural waters can survive within long periods of
geological time without any modification (see the
EXAMPLE.TXT and relevant HCh files).

In many cases, metastable equilibria can be calculated
in a more straightforward way. For example, you can
simply exclude from your system a potentially stable
mineral, thus preventing it from precipitation (e.g.,
you can "stabilise" amorphous silica versus crystalline
quartz).

Another example: if you want to calculate composition
of a natural water in equilibrium with the atmosphere
(including atmospheric oxygen and nitrogen) and include
all the relevant aqueous species from the UNITHERM
database, you will obtain a solution of nitric acid
with pH ~ 1. On the contrary, if you exclude from your
system NO3-, you will be able to calculate the
metastable equilibrium-aqueous solution with pH ~ 7.