<PAGE>

                                                                   Exhibit 10.66

                   (English Translation of Original Document)

STOCK PURCHASE, DEBT ACKNOWLEDGMENT, MORTGAGE, AND TERMINATION OF OBLIGATIONS
AGREEMENT (THE "AGREEMENT") ENTERED INTO BY:

(i) NATIONAL FINANCIERA, S.N.C., IN ITS CAPACITY AS FINANCIAL TRUST INSTITUTION
CAPITALIZATION AND INVESTMENT FUND FOR THE RURAL SECTOR ("FOCIR"), as Seller,
represented herein by Francisco Javier Delgado Mendoza;

(ii) The mercantile company organized and existing under the laws of the United
Mexican States "SIERRA RANCHOS, S.A. DE C.V." ("PURCHASER"); represented herein
by Mr. Jakes Jordaan, in its capacity as Sole Administrator.

(iii) The company "THE UNIMARK GROUP, INC." organized under the laws of the
State of Texas of the United States of America ("UNIMARK"); represented herein
by Mr. Jakes Jordan, as legal representative.

(iv) The mercantile company organized and existing under the laws of the United
Mexican States "GRUPO INDUSTRIAL SANTA ENGRACIA, S.A. DE C.V." ("GISE" or the
"JOINT OBLIGOR"); represented herein by Mr. Jakes Jordaan, as legal
representative.

Pursuant to the following Recitals, Representations, and Clauses:

R E C I T A L S

1. On February 21, 2000, UNIMARK, the company GMARTINEZ, S.A. DE C.V. ("GM") and
JOSE MARTINEZ BROHEZ ("JMB"), collectively, the "Private Shareholders", entered
into, along with FOCIR, a Management Partnership Agreement (Convenio de
Asociacion Empresarial) (hereinafter "CAE"), through which FOCIR and the Private
Shareholders agreed to contribute to the equity interest of GISE, for the
purpose of creating the Investment Project described therein.

2. Pursuant to the terms of the CAE, the participation in the equity interest of
GISE on the part of FOCIR increased to $48,000,000.00 (Forty-eight million and
00/100 pesos).

3. As provided in the CAE, pursuant to the general ordinary shareholders meeting
of GISE, held February 21, 2000, formalized in public deed number 7452 before
Blanca Amalia Cano Garza de Bello, registered in the Public Notary Office number
187 of Ciudad Victoria, Tamaulipas, FOCIR took out 26,179,127 representative
shares of working capital of GISE without assigning a nominal value, paying as
consideration, on February 25, 2000 an amount of $30,000,000.00 (Thirty million
and 00/100 pesos).

4. In the general shareholders meeting referenced in the preceding paragraph,
the shareholders of GISE agreed to the increase of the variable working capital
of GISE up to the amount of $68,000,000.00 (Sixty-eight million and 00/100
pesos), authorizing the Board of Directors to issue the titles representing the
increase in working capital, and maintaining for such effects any unsubscribed
shares in the treasury of the company.

5. Pursuant to the above-referenced issue of shares, on June 30, 2000, FOCIR
contributed $13,116,000.00 (Thirteen million, one hundred sixteen thousand and
00/100 pesos) to the equity interest of GISE and on November 16, 2000
contributed $3,917,971.00 (Three million, nine hundred seventeen thousand, nine
hundred seventy-one and 00/100 pesos) comprising almost the total amount of
contributions to the equity interest of GISE committed under the CAE, and
increasing the share participation of FOCIR in GISE to $47,033,971.00
(Forty-seven million, thirty-three thousand, nine hundred seventy-one 00/100
pesos).

<PAGE>

6. Pursuant to the above-referenced contributions of FOCIR to the equity
interest of GISE, the Fund subscribed and paid for a total of 26,003,444
ordinary shares, nominally represented in the variable capital of the company,
of the 160,069,799 shares that represent the total of the equity interest
subscribed and paid of GISE as of the date hereof.

7. Pursuant to clause Fourth of the CAE, the participation of FOCIR would be
temporary and would remain effective for a period of nine (9) years, beginning
from the date of the first share contribution by FOCIR in the equity interest of
GISE, i.e. its equity participation would conclude on February 25, 2009.

8. Pursuant to clause Fourth of the CAE, the Private Shareholders have the
option to purchase in advance the shares repreentative of the capital stock of
FOCIR. Therefore, so as to accommodate the interest of the parties to the CAE
and party hereto; FOCIR, GISE, and UNIMARK executed a "Letter of Intent with
Terms and Conditions" (Carta de Intencion con Terminos y Condiciones) in which
they expressly, but not exclusively, set forth the terms and conditions by which
they agreed to implement and eventually to formalize the resolutions of each and
every obligation contained in such, as well as an acknowledgment of a debt
resulting from the purchase price of the representative shares of the equity
interest of GISE, property of FOCIR. A copy of such Letter of Intent is attached
hereto as Exhibit A.

9. Pursuant to an agreement by the General Manager of FOCIR, pursuant to the
powers granted by the H. Techical Committee, the disinvestment by FOCIR in the
equity interest of GISE, in the amount of $47,034,000.00 (Forty-seven million,
thirty-three thousand and 00/100 pesos), was authorized which represents the
"historic" value of the share participation of this Fund, under the credit
conditions set forth herein.

10. On August 2, 2004, GISE held at a General Shareholders Meeting, whereby,
among others, they adopted the legal and statutory resolutions necessary for
GISE and the Private Shareholders to execute this Agreement, as evidenced in
public deed number 13,344, granted before Mr. Francisco Hugues Velez, Public
Notary No. 212 of the Federal District.

RE P R E S E N T A T I O N S

1.    FOCIR represents that:

(i) It is a Public Trust, organized by the Ministry of Finance and Public Credit
and Nacional Financiera, S.N.C. (Secretaria de Hacienda y Credito Publico y
Nacional Financiera, S.N.C.) under the laws of the United Mexican States, fully
authorized in accordance with its purpose to enter into this Agreement.

(ii) The legal representative has the legal capacity and sufficient authority to
execute this Agreement, which have not been revoked, modified or restricted in
any manner as of the date hereof.

(iii) Based on the representations and warranties of GISE and the Private
Shareholders, it is prepared to enter into this Agreement, on the terms and
conditions set forth herein.

2.    THE PURCHASER represents that:

(i) It is a mercantile company duly organized and existing pursuant to the laws
of the United Mexican States, fully authorized in accordance with its corporate
purpose to execute this Agreement, as evidenced in public deed number 13,348
dated August 4, 2004, granted before Mr. Francisco Hugues Velez, Notary Public
number 212 of the Federal District, pending its registration.

(ii) Its legal representative has the legal capacity and sufficient authority to
execute the present Agreements, which, as of the date hereof, have not been
revoked, modified, or restricted in any manner, as evidenced in the
aforementioned public deed .

(iii) It agrees to execute this Agreement, on the terms and conditions set forth
herein.

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(iv) As set forth in the letters attached hereto as Annex "B", "GM", and "JMB"
have expressly and irrevocably waived (i) any right of first refusal granted to
them under article XIII of the By-laws of GISE to acquire, on a pro rata basis
to the number of shares held by it in GISE, the shares property of FOCIR,
representative of the capital stock of GISE, to be transferred in favor of THE
BUYER, as well as (ii) the procedure set forth in article XIII of the Bylaws of
GISE for the transfer of shares representative of its capital stock.

3.    GISE represents that:

(i) It is a mercantile company duly organized and existing under the laws of the
United Mexican States, fully authorized in accordance with its corporate purpose
to execute this Agreement, as evidenced in public deed number 953 dated August
2, 1988 granted before Blanca Amalia Cano Garza de Bello, Notary Public
registered with the Public Notary Office number 187 of Ciudad Victoria,
Tamaulipas, registered on August 29, 1988 in the Public Registry of Property and
Commerce of Ciudad Victoria, Tamaulipas under number 65, Book 57, Commerce
Section.

(ii) Its Legal Representative has the legal capacity and sufficient authority to
execute the present Agreements, which have not been revoked, modified or
restricted in any manner as of the date hereof, as evidenced by public deed
number 13,348 dated August 4, 2004, granted before Mr. Francisco Hugues Velez,
Notary Public number 212 of the Federal District, pending its registration.

(iii) It agrees to enter into this Agreement, on the terms and conditions set
forth herein.

(iv) Its Federal Tax Registry is: GIS-880802-GH6.

(v) Its Shareholders Meeting dated August 2, 2004 authorized the transfer of
shares referenced to in the Clause Second hereof, so as to create an obligation
as set forth in the CAE, duly subscribed and signed by the Private Shareholders
and by GISE.

4.    UNIMARK represents that:

(i) It is an American company, duly organized under the laws of the State of
Texas on December 31, 1991 and registered on January 3, 1992, in the Corporate
Section of the Secretary of State of Texas.

(ii) Its legal representative has the legal capacity and sufficient authority to
execute the present Agreements, which as of the date hereof, have not been
revoked, modified or restricted in any manner.

(iii) It agrees to execute this Agreement with FOCIR, pursuant to the terms and
conditions set forth herein.

(iv) It agrees to terminate the Trust referenced in Clause First hereof, as set
forth herein.

(v) It expressly and irrevocably waives (i) the right of first refusal granted
by article XIII of the by-laws of GISE, to acquire, on a pro rata basis to the
shares held by it in GISE, the shares owned by FOCIR representative of the
capital stock of GISE to be transferred in favor of PURCHASER, as well as (ii)
the procedure set forth in article XIII of the by-laws of GISE for the transfer
of shares representative of its capital stock.

      Given the foregoing recitals and representations, the parties acknowledge
the capacity in which they act and agree and submit to the following:

C L A U S E S

FIRST. DEFINITIONS

The terms used herein and set forth below will have the following meanings,
whether plural or singular, capitalized or in lower case:

<PAGE>

Shares. Means the shares representative of the capital stock of GISE to which
Clause Second hereof refers to and that the BUYER buys from FOCIR.

Debt. Means the amount set forth in Clause Third Hereof, which represents the
amount that THE PURCHASER acknowledges is owed to FOCIR for the purchase of the
Shares, not including interest, expenses, premiums or any other amount generated
by the Debt itself.

CAE. Shall mean the Convenio de Asociacion Empresarial referred to in Recital 1
hereof.

CCP. Means the last rate (Costo de Captacion a Plazo de Pasivos en Moneda
Nacional) that Banco de Mexico deems representative of the collective of
multiple banking institutions, published in the Official Federal Gazette (Diario
Oficial de la Federacion) for the ordinary interest to the date on which each
Interest Calculation Period begins and for the moratory interest to the date on
which occurs the delay, recognizing that that both the ordinary and moratory
interests with vary monthly together with the variations of the CCP.

Agreement. Means this Stock Purchase, Debt Acknowledgment, Mortgage, and
Termination of Obligations Agreement.

Business Day. Means any day not considered a holiday in accordance with
applicable provisions of the Mexican Banking System.

Payment Date(s). Mean(s) the last business day of the month of October of each
of the years from 2004 to 2011.

Trust. Means the Irrevocable Guarantee Trust Agreement (Contrato de Fideicomiso
Irrevocable de Garantia) entered into on February 21, 2000 by and among the
Private Shareholders, GISE, and FOCIR with Banco Santander Mexicano, S.A.,
Institucion de Banca Multiple, Grupo Financiero Santander Mexicano, as trustee.

Notes. Mean the negotiable instruments to be subscribed by the BUYER duly
guaranteed by GISE in favor of FOCIR to evidence the Debt.

Interest Calculation Period. Means the period of time for which the interest
that accrue on the Indebtedness is calculated, the first Interest Calculation
Period commencing the day of execution hereof and ending the last date of such
month and each subsequent Interest Calculation Period beginning immediately
after the completion of the previous Interest Calculation Period and ending the
last day of such month and so on, recognizing that the computation thereof
includes all the days of the Interest Calculation Period, provided that the last
Interest Calculation Period of Indebtedness will end exactly on the date on
which the last capital amortization payment is made.

Outstanding Indebtedness. Means the amount of capital of the Debt that has not
been covered by THE BUYER in a specific date.

Cetes Rate. Means the average of the annual yield, equal to the discount of the
Federation Treasury Certificates (Certificados de la Tesoreria de la Federacion)
(Cetes) in the first exchange issued for a term of twenty-eight (28) days or for
a substitute term in the case of non business days, corresponding to the
auctions held during the immediately preceding month on the first date of each
Interest Calculation Period or on the date on which the delay occurs,
recognizing that both the ordinary and moratory interest will vary monthly
together with the variations of the Cetes Rate.

TIIE Rate. Means the Interbank Balance Interest Rate (Tasa de Interes
Interbancaria de Equilibrio) published periodically by Banco de Mexico in the
Official Federal Gazette (Diario Oficial de la Federacion) on the last day of
the preceding month, as well as of the date on which the first Credit
Disbursement (Disposicion de Credito) as well as on the date on which the delay
occurs, provided that the calculation of ordinary and moratory interests, as the
case may be, will be made monthly based on the last Interbank Balance Interest
Rate published in the preceding month, regardless of the term for which it was

<PAGE>

determined and if on such date various Interbank Balance Interest Rates for
various terms are published the following will apply: (i) the Interbank Balance
Interest Rate set for the term of twenty-eight (28) days; (ii) failing that,
that Interbank Balance Interest Rate for term of greater than twenty-eight (28)
days that is closest to the length of such term; and (iii) failing that, the
Interbank Balance Interest Rate for a term of less than twenty-eight (28) days
that is closest to the length of such term, recognizing that both the ordinary
and moratory interest rates will vary monthly together with the variations in
the TIIE.

SECOND. PURCHASE

In accordance herewith, in order to accommodate the interests of Unimark, GISE,
and FOCIR, THE PURCHASER purchases for FOCIR, which, in turn sells each and
every of its shares representing the equity interest of GISE duly subscribed and
paid for by FOCIR (the "Shares") as set forth below, for the price of
$47,034,000.00 (forty-seven million, thirty-four thousand and 00/100 pesos).

The Shares acquired by the PURCHASER are those represented by the following
share certificate:

<TABLE>
<CAPTION>
CERTIFICATE             SERIES            SHARES REPRESENTED       AMOUNT
-----------             ------            ------------------       ------
<S>                     <C>               <C>                      <C>
     9                    B                   26,003,444            N/A
</TABLE>

In connection herewith, FOCIR will deliver to THE PURCHASER the stock titles
described in this clause, duly endorsed in blank in its favor, for the purpose
of delivering the shares in connection with the purchase.

FOCIR and THE PURCHASER expressly agree that the purchase price of the Shares
will be paid to FOCIR within a maximum period of seven years, three months,
accrued beginning with the signature hereof, made in seven annual partial
payments in consecutive installments on the Debt Payment Dates, pursuant to the
conditions set forth in the Clause Sixth hereof.

As a result the execution of this Agreement, FOCIR grants to THE PURCHASER the
broadest receipt allowable by law with respect to the fulfillment of the
obligation delivery of the purchase price, on the understanding that as of the
date hereof THE PURCHASER acknowledges that it owes to FOCIR the amount of Debt,
same that amounts to the purchase price.

In virtue of the foregoing, GISE agrees to register the transfer of Shares
referred to in this clause in its Shareholders Registry Book.

THIRD. DEBT ACKNOWLEDGMENT

As consideration for the Purchase of the Shares referenced in Clause SECOND
hereof, THE PURCHASER expressly and irrevocably acknowledges that as of the date
hereof it owes to FOCIR the amount of $47,034,000.00 (forty-seven million,
thirty-four thousand and 00/100 pesos) (the "Debt"), on the understanding that
the Debt will accrue ordinary and moratory interest, as the case may be, from
the date of execution hereof at the interest rate and under the conditions set
forth in Clause Fifth hereof.

The amount of the Debt does not include interest and expenses arising from this
Agreement.

FOURTH. TERMINATION OF THE TRUST

As a result of the purchase of Shares referred to in Clause Second hereof,
FOCIR, in its capacity as Trustee, and UNIMARK, in their capacity as settlor,
who must obtain the consent of the rest of the settlers (the "Settlors"), hereby
demonstrate their express and irrevocable intent to terminate the Trust.

Considering the foregoing, the parties agree to notify the Trustee of their
decision to terminate the Trust, so that said institution, together with FOCIR
in its capacity as Trustee, and the Private Shareholders in their capacity of
Settlors, and pursuant to the provisions on fraction I of article 392 of the
General Law of Negotiable Instruments and Credit Transactions (Ley General de
Titulos y Operaciones de Credito), will

<PAGE>

carry out the corresponding termination, as well as see to other necessary
matters for the formalization of said termination and to conform to the
provisions of article 393 of the General Law of Negotiable Instruments and
Credit Transactions (Ley General de Titulos y Operaciones de Credito), the
return being in favor of the Settlors of the pledged interests, recognizing that
any expense arising in connection with the termination of the Trust will be
charged to the Settlors.

FIFTH. DEBT INTEREST

The Debt will accrue ordinary and moratory interest from the date of execution
hereof, which will be calculated on a basis of a year of three hundred sixty
(360) days and of days which have effectively elapsed.

A)    ORDINARY INTEREST.

THE PURCHASER hereby agrees to pay to FOCIR on a quarterly basis, with no need
for a prior request, the ordinary interest accrued on the Indebtedness in effect
and calculated as set forth in this Clause, the last business day of the months
of October 2004; January, April, July, and October of each of the years from
2005 to 2011.

The Amount of Indebtedness in effect will accrue ordinary interest monthly,
which will be calculated to the last day of each Interest Calculation Period at
the rate obtained from adding 2.5 (two point five) percentage points to the
CETES Rate.

The foregoing is made on the understanding that the monthly calculation will be
obtained by multiplying the average of the Amount of Indebtedness in effect by
the Cetes Rate plus 2.5 (two point five) percentage points, and dividing the
result among 360 (three hundred sixty) days and multiplying by the number of
calendar days falling within each Interest Calculation Period.

The interest accrued in each period will be added and paid on the dates of
interest payment set forth in this Clause.

B)    MORATORY INTERESTS.

THE PURCHASER expressly and irrevocably acknowledges that in the event of a
delay in the timely and total payment of the amounts derived from the Debt, the
unpaid amount will accrue moratory interest from the due date until the day on
which it is fully paid. Such interests will be payable on demand, at the annual
rate obtained by multiplying the constant of 1.5 (one point five) (the
"Constant") by the ordinary interest rate set forth in letter A) above,
recognizing that the moratory interest rates will vary monthly together with the
variations of the Cetes Rate during the period of the duration of the delay.

In any event, while the delay exists and continues, the Debt will accrue only
moratory interest.

C)    SUBSTITUTE RATES:

I.    If the Cetes Rate ceases to exist, FOCIR will calculate the applicable
      ordinary and moratory interest, or the moratory interest applicable to the
      Indebtedness, in accordance with the following priorities:

(a)   The Outstanding Debt in effect will accrue ordinary interest at the annual
      rate obtained by adding 2.5 (two point five) percentage points to the TIIE
      Rate.

In an even of delay of the timely and complete payment of the amounts derived
from the Debt, the unpaid amount will accrue moratory interest from the date of
its occurrence until the date on which it is fully paid, payable on demand, at
the annual rate obtained by multiplying the Constant by the ordinary rate
specified in the preceding paragraph (a) above.

(b)   If the TIIE Rate ceases to exist, the Ioutstanding Debt in effect will
      accrue ordinary interest at the annual rate obtained by adding 2.5 (two
      point five) percentage points to the CCP.

<PAGE>

In the event of a delay in the timely and full payment of the amounts derived
from the Debt, the unpaid amount will accrue moratory interest from the date of
its occurrence until the day on which it is fully paid, payable on demand, at
the annual rate resulting from multiplying the Constant by the ordinary rate
specified in the preceding paragraph.

II. If not possible to determine the interest in accordance with this Agreement,
FOCIR may consider the debt payment term to be accelerated and in such case, the
PURCHASER, at the request of FOCIR, for a term of ten (10) calendar days there
from , will prepay the Indebtedness without penalty or premium, together with
the interest accrued and unpaid as of that date, determined pursuant to the last
calculable interest rate as set forth hereunder. If the PURCHASER does not make
such prepayment pursuant to the stipulations hereof, from that date moratory
interest will generate in favor of FOCIR, as of the due date of such term, same
that will be payable on demand, and will be calculated at the set fifth in
paragraph (b) above, which will be calculated at the last applied rate.

SIXTH. DEBT AMORTIZATION

The PURCHASER, with no need for prior request, agrees to pay to FOCIR the
Outstanding debt over seven (7) annual and consecutive amortizations, paid
exactly on the Debt Payment Dates and in the amounts set forth below and in the
relevant Note, recognizing that the PURCHASER will make the payment of the first
amortization of the debt in the month of October of 2005, as set forth in the
following table:

<TABLE>
<CAPTION>
AMORTIZATION                         PAYMENT DATE                                          AMOUNT
------------                         ------------                                          ------
<S>                        <C>                                                        <C>
    1                      Last business day of October 2005                          $  2,350,000.00

    2                      Last business day of October 2006                          $  4,700,000.00

    3                      Last business day of October 2007                          $  7,060,000.00

    4                      Last business day of October 2008                          $  8,230,000.00

    5                      Last business day of October 2009                          $  8,230,000.00

    6                      Last business day of October 2010                          $  8,230,000.00

    7                      Last business day of October 2011                          $  8,234,000.00

                           TOTAL                                                      $ 47,034,000.00
</TABLE>

SEVENTH. APPLICATION OF PARTIAL PAYMENTS

In the event that the PURCHASER makes partial payments under the Debt, the
parties agree that such payments should be applied by FOCIR in the following
priority: (i) expenses, (ii) moratory interest, (iii) ordinary interest due,
(iv) matured indebtedness, (v) ordinary interest in effect, (vi) Outstanding
debt applied to the next amortization and in the event that there are any
amounts remaining, they shall be applied in the reverse order of their due
dates.

EIGHTH. PREPAYMENT

The PURCHASER shall pay the amount of the Debt before it is due, either
partially or fully, without any penalty or premium.

In the event of partial or full payments referred to in this clause, and unless
FOCIR shall agree otherwise in writing, (i) the PURCHASER agrees to pay FOCIR
for all costs and expenses incurred in connection with acceptance of such
prepayment, which may not include fund breakage costs, (ii) the PURCHASER will
pay all interest accrued and unpaid on the Indebtedness, as applicable, that may
be prepaid, (iii) all prepayment shall apply in the same order set forth in the
Clause Application of Partial Payments.

NINTH. PLACE AND MANNER OF PAYMENT OF THE DEBT

All amounts payable to FOCIR in connection herewith, will be payable in National
Currency, on the date of maturity and with no need for a prior request, at the
domicile of FOCIR, located as of the date hereto at

<PAGE>

Av. Paseo de los Tamarindos, No. 400 B, Edificio Arcos 1, Torre Oriente, 10th
Floor, Colonia Bosques de las Lomas, Delegacion Cuajimalpa, Mexico, D.F., C.P.
05120, or at any other location or in any other manner which FOCIR may request
in writing of the person making the payment at least fifteen (15) calendar days
in advance of the next payment date.

All payments falling on a non business day shall be made on the immediately
preceding business day.

TENTH. MORTGAGE

In order to secure the fulfillment of all obligations in connection herewith by
THE BUYER, for the purpose of principle amount, ordinary and moratory interest,
legal fees and expenses, as the case may be (the #Secured Obligations"), GISE
creates a first priority mortgage (hipoteca en primer lugar y primer grado de
prelacion registral) in favor of FOCIR, on the real properties described in
EXHIBIT "C" hereto and deemed a part hereof (the "Real Estate").

The mortgage created shall extend to the natural accessions of the Real
Properties, to the improvements done by the owners of said Real Properties, to
the personal property permanently fixed to the properties that cannot be
separated without damaging the properties or the fixtures, to the new buildings
that the owners construct over the mortgaged Properties and the new floors that
are constructed in the burdened buildings, to the rents due and not paid on time
to demand the compliance with the Secured Obligations, as well as to everything
corresponding in fact or in law to the mortgaged properties, without reservation
or limitation, pursuant to articles 2896 and 2897 of the Civil Code of the
Federal District and its correlative articles in the Civil Code of the state
where each of the Real Properties is located.

The mortgage to which this Clause refers to, will not include the proceeds that
the mortgaged assets produce.

The mortgage referred to in this clause will secure the interest payable even
when they exceed three years for their prescribed time, which in due time shall
be noted with the corresponding Public Registry of Property. Also, the created
mortgage will exist until it is fully charged to FOCIR receives all that is owed
it in connection herewith, including principal amount, interest and any
incidentals related thereto.

Unless otherwise agreed to in writing by GISE and FOCIR, they expressly agree
that each of the mortgaged assets will secure in full, the total payment of the
Secured Obligations.

GISE also agrees that in the event that any of the Real Properties decrease in
value to the extent that they no longer cover the amount of Secured Obligations,
it shall grant additional guaranties, to the satisfaction of FOCIR, within a
term of sixty (60) calendar days commencing from the moment in which they are
aware of such decrease in value.

The parties unequivocally agree that noncompliance with any obligations on the
part of GISE, set forth in this Clause, will be sufficient cause for FOCIR to
have the power declare an acceleration event of the debt and its incidentals, by
simple written notice to the PURCHASER and/or GISE.

FOCIR and GISE expressly and irrevocably agree that as of the date hereof, GISE
shall have a period of sixty (60) calendar days to appear before the notary
public of their choice to formalize the mortgage created on the Real Properties,
as well as to register the corresponding deed before the Public Registry of
Property that they are the owners of said guarantees, on the understanding that
said registration should be granted to FOCIR as a first priority interest
(primer lugar y grado de prelacion registral), except as set forth in the
following paragraphs.

The parties also acknowledge that in the event of noncompliance on the part of
GISE with the obligation set forth in the immediately preceding paragraph, FOCIR
may solicit the notary public of its choice to formalize the mortgage created
over the Real Properties referenced herein, whose expenses will be charged to
GISE and/or THE PURCHASER, furthermore, GISE agrees to appear in the execution
of such deed.

<PAGE>

The parties expressly and irrevocably acknowledge and accept that as of the date
hereof, only the Real Properties identified in EXHIBIT "C" with numbers 4 and
11, evidence liens duly registered in the Public Registry of Property (the
"Liens"). The property identified under number 4, evidences two liens and the
one identified under number 11, evidences a lien. In virtue of the foregoing,
GISA expressly and irrevocably agrees to carry out all related necessary matters
within a period of a maximum of ninety (90) calendar days following the date of
execution hereof, and present to FOCIR sufficient evidence of the cancellation
of the Liens over the Real Property, provided that during the cancellation
process, the lien will be registered in the Public Registry of Property of the
corresponding location and with the relevant priority.

Finally, GISE agrees and FOCIR accepts that GISE will have 120 calendar days as
of the date of execution of this Agreement to create a first priority mortgage
in favor of FOCIR regarding the property identified as in Exhibit "D", same that
will secure the performance of the Secured Obligations in the same terms and
conditions set forth in this Clause. In the event that GISE can't grant the
mortgage over such property, within the abovementioned term, it agrees to grant
an additional mortgage in favor of and satisfactory to FOCIR.

ELEVENTH. INSURANCE

A. GISE agrees to contract with an insurance company acceptable to FOCIR and to
maintain during the term of this Agreement, insurance against any type of risk
in conformity with the common industry standards and pursuant to this Agreement,
over the Real Properties for its fair value. The irrevocable beneficiary of such
insurance shall be FOCIR.

In connection therewith, GISE agrees to inform FOCIR in writing, within thirty
(30) calendar days following the execution hereof of the type of coverage
provided by the contracted insurance, so that FOCIR certify its conformity
therewith. FOCIR may authorize amendments to its insurance programs during the
term hereof, for which a letter issued by FOCIR to GISE will be sufficient, who
shall carry out the actions necessary to put into effect such amendment.

In the event that the contracting of insurance proves to be financially unviable
further to a study done by GISE, he shall inform FOCIR of such situation as
FOCIR certifies what follows, on the understanding that FOCIR shall exempt GISE
from the contracting of insurance if it considers that such unviability is duly
evidenced.

Once FOCIR certifies the conformity of the insurance that cover the Real
Property, GISE will have a period of thirty (30) business days following the day
on which the insurance is obtained from the insurance company (but in any event
no later than thirty (30) calendar days following the date on which FOCIR
certifies its conformity with the type of coverage it is required to obtain).

B. GISE shall deliver copies of the respective insurance policies to FOCIR
within two (2) business days following the day on which the policies are
obtained from the insurance company (but in any event no later than thirty (30)
calendar days following the date on which FOCIR certifies its conformity with
the type of coverage it is required to obtain).

C. GISE agrees to confirm with FOCIR the payment of the premiums related to said
insurance with the corresponding payment receipts, within two (2) business days
following the receipt of the request by FOCIR.

D. Notwithstanding the provisions set forth in Clause of Acceleration Events
hereof, in the event that for any reason GISE does not comply with any of its
obligations set forth in this Clause, FOCIR will be expressly authorized to
contract such insurance in the name of GISE, as the case may be, and to pay all
amounts required to maintain such insurance in effect, in which case either of
GISE or THE PURCHASER are obligated to pay, on sight, to FOCIR the amounts
contributed by it for such purposes, recognizing that said amounts will cause
the moratory interest calculated by the moratory interest rate set forth in
paragraph B) of the Clause of Debt Interest, from the date on which the
contribution was made on the part of FOCIR and until the day on which they are
fully paid.

<PAGE>

TWELFTH. SOLE OBLIGATION

To secure the compliance with all Secured Obligations, in this act and by the
terms of articles 1987, 1988, and 1989 and others applicable of the Civil Code
for the Federal District, the mercantile company named GRUPO INDUSTRIAL SANTA
ENGRACIA, S.A. DE C.V. (GISE) becomes the Joint Obligor of THE PURCHASER under
FOCIR.

THIRTEENTH. CONDITIONAL DEPOSIT

GISE evidences that to the date hereof it has been carrying out a series of
measures so as to contribute various common use lands (terrenos ejidales de uso
comun), comprised of the Ejido Laguna del Mante, that together comprise the lot
known as "FLOR DE MARIA", to the working capital of GISE, with the subsequent
cancellation of the registration before the National Agrarian Registry, its
registration in the name of GISE, and its due registration before the Public
Registry of Property.

The parties who affix their signature hereto acknowledge that with respect to
the reciprocal support granted as a consequence of the management association
that has bound them as of the date hereof, FOCIR assumes the obligation to
appear with and to support GISE, so that both parties exercise their best
efforts so that the matters set forth in the preceding paragraph are carried out
successfully. To such effect, GISE will create a deposit of the minimum amount
of $8,4000,000.00 (Eight million, four hundred thousand and 00/100 pesos) under
a Conditional Deposit Agreement to be executed with a credit institution, for
the purpose of guaranteeing the funds of resources to accomplish the matters set
forth in this Clause.

FOURTEENTH. OBLIGATIONS OF THE PURCHASER (AND GISE)

Unless FOCIR otherwise consents in writing, so long as the Debt and accrued
interest are not fully paid, in connection herewith, GISE and THE PURCHASER, as
applicableagree with FOCIR to the following:

a) To provide FOCIR within thirty (30) calendar days following the end of each
quarter of each of its fiscal years, quarterly financial statements of GISE
(balances, income statements, cash flow) duly certified by its General Manager
and / or Financial Manager, as well as an insurance report.

b) To provide to FOCIR no later than the month of May following each of its
fiscal years, audited consolidated financial statements of GISE (balances,
financial statements) together with a report of its General Manager and / or
Financial Manager. The in-house auditor shall be previously approved by FOCIR.

c) To provide in writing to FOCIR:

(i) Prompt notification, no later than ten (10) calendar days following the day
on which it has knowledge thereof, of any event that constitutes or, that with
the passage of time will constitute, a reason for acceleration event, together
with a declaration that describing such event; as well as the proposed measures
to be adopted with respect thereto.

(ii) Prompt notification, no later than ten (10) calendar days following that on
which it has knowledge thereof, of the existence of any action, claim, suit or
proceeding against THE PURCHASER or GISE or any labor conflict, provided that it
can reasonably anticipate that said litigation or suit will have a material
adverse effect on its financial situation.

(iii) All reasonable and relevant information referring to THE PURCHASER and
GISE and their financial situation, provided that such information is produced
by them in their daily administration, that FOCIR directly or through a third
party requests with at least ten (10) business days in advance, provided that
FOCIR may request balances or accounting statements, facts or documents, when in
its judgment it is necessary to verify the compliance with the obligations
entered into by THE PURCHASER and / or the JOINT OBLIGOR in connection herewith,
and they shall be obliged to provide the necessary powers for such purposes and
to permit access to their company, without interfering with its say to day
business, for the people designated by FOCIR for such purpose.

<PAGE>

d) Preserve and maintain in full effect their corporate existence, as well as
all rights, licenses, permits, authorizations, certifications, registrations,
and approvals required to carry out its operations in each jurisdiction in which
it operates, throughout the life of this Agreement, provided that the lack
thereof could have a material adverse effect on its financial situation.

e) To allow, within business days and hours, and without such inspection
interfering with its day to day business, the persons designated by FOCIR to
inspect the Real Property or the personal property of the PURCHASER, at any time
with ten Business Days in avance.

f) To pay the fiscal debts and the fees of IMSS, of INFONAVIT, the fees of SAR,
contributions to AFORES, except for those that are contested in good faith in
appropriate proceedings and previous creation of the corresponding reserves and
for those whose failure to pay does not, nor reasonably can be anticipated to
have, a material adverse effect on its financial situation.

g) GISE agrees to inform FOCIR in writing, within ten (10) business days
following the day on which it occurs, or the day in which it becomes aware of
its eventual occurrence, whichever occurs first, should GISE do any of the
following:

(i) Reduce its working capital.

(ii) Modify its corporate purpose or change its commercial course.

(iii) Enter into a state of dissolution or liquidation.

(iv) Merge with another company or spin-off.

(v) To transfer in any manner its assets, provided that they are significant,
and that such transfer could have an adverse effect on its financial situation.

(vi) Change its principle shareholders, provided that they cease having majority
control over GISE.

(vii) Create a lien over its fixed assets, except for liens whose existence was
reported to FOCIR prior to the date hereof.

(viii) Grant securities, guarantees, pledges or assume other contingent
guaranteed obligations the compliance of obligations of third parties, except
for those that are granted to its employees or subsidiaries, or those companies
with which it has an economic relationship.

(ix) Lease, sublease, give a loan (comodato) or in any other manner transfer the
possession, property, use or operation of its assets to any third party, or
amend, alter, replace its assets.

(x) Acquire additional liabilities.

H) GISE shall carry out each and every of the actions necessary to cancel the
Liens.

FIFTEENTH. ACCELERATION EVENTS

THE PURCHASER and THE JOINT OBLIGOR expressly acknowledge and accept that the
execution hereof on the part of FOCIR is based on other factors, in the
obligation they acquire to comply with each of the obligations assumed on
connection with the execution hereof, and especially, on the obligations
referred to in Clause Fifteen and those contained in this clause, with which
they are in agreement that the noncompliance with any of such obligations would
be sufficient cause for FOCIR to declare an acceleration event with respect to
the payment of the Debt and its incidentals, by a simple written notice to
either of THE PURCHASER and / or the JOINT OBLIGOR.

In light of the foregoing, FOCIR can declare an acceleration event under this
Agreement for the noncompliance on the part of THE PURCHASER and / or the JOINT
OBLIGOR of any of the obligations

<PAGE>

contained herein, especially those contained in Clause Fourteenth, as well as
for any of the following reasons:

a) If the principal sum of the Debt, the interest thereon or any costs or fees
caused in connection herewith is not paid punctually,.

b) If any information provided to FOCIR by GISE or by UNIMARK in connection
herewith turn out to be false, or fraudulently incorrect or incomplete, so long
as the error is relevant.

c) If a proceeding against GISE is initiated for the purpose of declaring
bankruptcy, or subjecting it to collections (Concurso) unless said proceeding,
in the opinion of FOCIR, was grossly irrelevant.

d) If there is noncompliance with the provisions of the Clause Mortgage.

e) As otherwise provided by law.

SIXTEENTH. TERMINATION OF THE MANAGEMENT PARTNERSHIP

FOCIR, UNIMARK, and GISE, hereby expressly agree to terminate the CAE for all
existing legal effects, as well as to obtain the consent of all other parties to
the same, for which, as of this date, all rights acquired and obligations
assumed reciprocally between them and with respect to GM and JMB, for the
purpose of the CAE are extinguished.

FOCIR, UNIMARK, and GISE acknowledge not to not assume any obligation or any
loan among them with respect to the CAE. With the execution hereof by FOCIR,
UNIMARK, and GISE they grant reciprocally the broadest termination allowed by
law with respect to (i) the fulfillment of the obligations set forth in the CAE
and (ii) all obligations established in the by-laws of GISE, for which they
reserve no rights and / or shares among them in connection with the obligations
contained in said legal instruments and with respect to which they grant the
termination provided for herein.

SEVENTEENTH. NOTICES

In connection herewith, each of the parties designates the following as its
conventional domicile to receive all types of notices:

FOCIR:

      Paseo de los Tamarindos No. 400, Edificio Arcos 1, Torre oriente, 10th
      Floor, Colonia Bosques de las Lomas, Delegacion Cuajimalpa, Mexico, D. F.,
      C.P. 05120.

UNIMARK:

      Carretera Mexico-Laredo, Kilometro 128+150, Predio "El Cielo", Tramo
      Mante-Ciudad Victoria, Municipio Gomez Farias, Tamaulipas, Mexico. Tel:
      831-89-84-538.

GISE:

Carretera Mexico-Laredo, Kilometro 128+150, Predio "El Cielo", Tramo
Mante-Ciudad Victoria, Municipio Gomez Farias, Tamaulipas, Mexico. Tel:
831-89-84-538.

The parties agree that with respect to the notices to be delivered to GISE and
UNIMARK, such may be delivered simultaneously with a copy to "GALICIA y ROBLES,
S.C." with its domicile located in Blvd. Manuel Avila Camacho No. 24, Piso 7,
Torre del Bosque, Colonia Lomas de Chapultepec, C.P. 11000, Mexico, D.F.,
addressed to Mr. Ignacio Pesqueira Taunton, provided that delivery of such
notice will remain subject to the sole discretion of FOCIR, as a result, the
sole notice to GISE and/or UNIMARK will be necessary.

<PAGE>

      If the parties fail to notify in writing any change of domicile, the
        notices and other judicial and nonjudicial diligence made to the
        domiciles indicated above will be in full effect.

EIGHTEENTH. ASSIGNMENT OF DEBT

THE PURCHASER may not assign its rights or obligations hereunder or under the
Note.

The parties expressly agree that FOCIR has the authority to assign part or all
of its rights arising herefrom, including the unpaid expired interests, as
convenient to FOCIR, without any requirements beyond notifying THE PURCHASER
and/or GISE regarding the assignment, pursuant to applicable legal provisions.

Likewise, the PURCHASER and GISE expressly authorizes FOCIR to endorse, or in
any other manner, negotiate, even before the expiration of this Contract, the
Notes by which the Debt is documented, acting for this purpose as
attorney-in-fact of the holders of the Notes, for which purpose FOCIR will
continue to be responsible for monitoring and conserving the pledged securities.
For this purpose, FOCIR is authorized to assign or discount the debt so
documented.

NINETEENTH. WAIVER OF RIGHTS

No failure on the part of FOCIR to exercise the rights set forth herein shall
constitute a waiver thereof, nor shall a singular or partial exercise on the
part of FOCIR of any right derived hereunder exclude any other right, authority
or privilege.

TWENTY. AMENDMENTS TO THE AGREEMENT

No amendment or waiver to any provision hereof and no consent given to GISE or
UNIMARK to alter the Contract will be effective, unless in writing and signed by
FOCIR, THE PURCHASER, and GISE, and even in such case, such waiver or consent
will be effective only with respect to the specific purpose for which it was
granted.

TWENTY-FIRST. HEADINGS

The parties agree that the headings used in the Clauses hereof are for reference
only and in no way limit the content and interpretation thereof, which clauses
will be interpreted in all cases as agreed by the parties in said Clauses.

TWENTY-SECOND. APPLICABLE LAW

The parties agree that this Agreement will be governed and interpreted in
accordance with the laws of the United Mexican States and applicable statutes.

TWENTY-THIRD. JURISDICTION

For all matters related to the interpretation and fulfillment of the obligations
created hereby, the parties submit to the jurisdiction of the competent courts
of Mexico City, Federal District, expressly waiving the right to any other forum
that may apply by reason of its domicile of for any other reason.

The parties being informed of the content, reach and legal force of this
Agreement, sign four (4) counterparts, without any fraud, mistake, violence,
unjust enrichment, illegitimacy, injury, bad faith or any other misconduct that
affects the will of the parties, on this 4th day of the month of August of 2004
in the City of Mexico, Distrito Federal.

<PAGE>

FOCIR
NACIONAL FINANCIERA, S.N.C., FIDUCIARIA EN EL FIDEICOMISO FONDO DE
CAPITALIZACION E INVERSION DEL SECTOR RURAL
Represented by its Special Fiduciary Delegate

/s/ F. Javier Delgado Mendoza
-----------------------------
F. Javier Delgado Mendoza

"THE PURCHASER"
SIERRA RANCHOS, S.A. DE C.V.
Represented by:

/s/ Jakes Jordaan
-----------------
Jakes Jordaan

THE UNIMARK GROUP, INC.
Represented by:

/s/ Jakes Jordaan
-----------------
Jakes Jordaan

"THE JOINT OBLIGOR"
GRUPO INDUSTRIAL SANTA ENGRACIA, S.A. DE C.V.
Represented by

/s/ Jakes Jordaan
-----------------
Jakes Jordaan