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                                                                   EXHIBIT 10.51

                                 PROMISSORY NOTE


US$ $100,000.00                                                December 18, 2002

                                    ARTICLE I
                               PAYMENT OBLIGATION

         FOR VALUE RECEIVED, the undersigned The UniMark Group, Inc., a Texas
corporation ("Maker"), unconditionally promises to pay to the order of M & M
Nominee, LLC or its assigns ("Payee"), the sum of One Hundred Thousand Dollars
(US $100,000.00), together with interest on the unpaid principal balance from
time to time outstanding at a rate per annum equal to ten percent (10%). The
principal of and accrued interest on this Note shall be due and payable in
lawful money of the United States on the earlier of (a) June 10, 2003, (b) the
occurrence of a Qualified Equity Financing (as defined below) or (c) upon demand
following the occurrence of an Event of Default (as defined in Article III) or
as otherwise provided in Article III hereof. Interest hereunder shall be
calculated on the basis of a year of 365 days and the actual number of days
elapsed. As used herein a "Qualified Equity Financing" shall mean the sale by
Maker of (x) equity securities of Maker or (y) debt securities which may be
convertible into equity securities of Maker or (z) any other securities
convertible into or exercisable for equity securities of Maker, in each case
with net proceeds to Maker in an amount at least equal to the outstanding
principal amount of this Note.

         Maker shall have the right to prepay the principal of this Note in full
or in part at any time and from time to time without premium or penalty,
provided, however, that accrued interest shall be due and payable
contemporaneously with such prepayment of principal.

                                   ARTICLE II
                              AFFIRMATIVE COVENANTS

         So long as any of the principal of and accrued interest of this Note is
outstanding, or until the Payee consents to the contrary and the Maker receives
prior written approval to the contrary from the Payee, Maker will comply, or
cause compliance, with each of the following covenants:

         1. QUALIFIED EQUITY FINANCING. In the event that Maker issues or sells
any equity securities, securities convertible into or exercisable for equity
securities or debt securities convertible into equity securities of Maker, in
any transaction which is not a Qualified Equity Financing, the proceeds of such
issuance or sale shall be first applied to pay the outstanding principal of, and
accrued interest on, this Note. In the event Maker consummates a Qualified
Equity Financing, the net proceeds from such Qualified Equity Financing will
first be applied to prepay immediately the principal of, and accrued interest
on, this Note in full, and the remainder of such proceeds will applied as
determined by Maker's Board of Directors.

         2. LIMITATION ON INDEBTEDNESS.. Maker will not incur any indebtedness
senior or pari passu in right of payment to the prior payment in full in cash of
the outstanding principal of and accrued interest on the Note, except for (a)
indebtedness of the Maker owed on the date hereof and interest accruing thereon
or (b) to provide working capital necessary for the proper conduct of the
Maker's business or (c) to provide capital expenditure necessary for the proper
conduct of the Maker's business.

                                   ARTICLE III
                                EVENTS OF DEFAULT

         Any of the following shall constitute Events of Default ("Event(s) of
Default"):

         1.       default in making due and punctual payment of the principal of
                  the Note, or any interest accrued thereon, or any other amount
                  hereon, when and as such payment shall become due and payable,
                  whether at maturity or by acceleration or otherwise if such
                  payment is


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                  not made within five (5) days of written notice received by
                  Maker specifying such default; or

         2.       the failure by Maker to comply with the covenants set forth in
                  Article II of this Note; or

         3.       the sale by Maker of all or substantially all of its assets;
                  or

         4.       Maker shall admit in writing its inability to, or be generally
                  unable to, pay its debts as such debts become due; or

         5.       Maker shall (i) apply for or consent in writing to the
                  appointment of, or taking possession by, a receiver,
                  custodian, trustee or liquidator of itself or of all or a
                  substantial part of its property, (ii) make a general
                  assignment for the benefit of its creditors, (iii) file a
                  petition seeking to take advantage of any law relating to
                  bankruptcy, insolvency, reorganization, winding-up, or
                  composition or adjustment of debts, (iv) fail to controvert in
                  a timely and appropriate manner, or acquiesce in writing to,
                  any petition filed against it in an involuntary case under any
                  law relating to bankruptcy, insolvency, reorganization,
                  winding-up, or composition or adjustment of debts, or (v) take
                  any action for the purpose of effecting any of the foregoing;
                  or

         6.       a proceeding or case shall be commenced, without the
                  application or consent of the Company, in any court of
                  competent jurisdiction, seeking (i) Maker's liquidation,
                  reorganization, dissolution, or winding-up, or the composition
                  or adjustment of its debts, (ii) the appointment of a trustee,
                  receiver, custodian, liquidator or the like of Maker or all or
                  any substantial part of its assets, or (iii) similar relief in
                  respect of Maker under any law relating to bankruptcy,
                  insolvency, reorganization, winding-up or composition or
                  adjustment of debts, and such proceeding or case shall
                  continue undismissed, or an order, judgment or decree
                  approving or ordering any of the foregoing shall be entered
                  and continued unstayed and in effect for a period of sixty
                  (60) days; or an order for relief against Maker shall be
                  entered in an involuntary case under any law relating to
                  bankruptcy, insolvency, reorganization, winding-up, or
                  composition or adjustment of debts.

         If an Event of Default occurs under paragraphs 1, 2 or 3 above, then
and in any such case Payee may declare the outstanding principal amount of the
Note to be due and payable immediately, upon written notice to Maker and, upon
any such declaration, the outstanding principal amount of the Note, and the
interest accrued thereon, together with all costs of collection, including
reasonable attorneys' fees if collected by law or through an attorney at law (as
further set forth in Section IV(4) hereof), shall immediately become due and
payable. If an Event of Default occurs under paragraphs 4, 5 or 6 above, then
the outstanding principal of and all accrued and unpaid interest on this Note
shall automatically become immediately due and payable, without presentment,
demand, protest or other formalities or notice of any kind, all of which are
hereby expressly waived by Maker.

                                   ARTICLE IV
                                     GENERAL

1.       NOTICES. Any notice or demand given hereunder by the holder hereof must
         be in writing to be effective and shall be deemed to have been given
         and received (a) when actually delivered to the address of the party to
         be notified if delivered in person, or (b) if mailed, on the earlier of
         the date actually delivered to the address of the party to be notified
         or (whether ever so delivered or not) on the third Business Day
         (hereinafter defined) after it is enclosed in an envelope, addressed to
         the party to be notified, properly stamped, sealed, and deposited in
         the United States mail, certified mail, return receipt requested. As
         used herein, "Business Day" means every day other than a Saturday,
         Sunday, or legal holiday in Dallas, Texas or New York, New York.


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2.       RENOUNCIATION. Maker hereby renounces to any presentation of this Note
         and to any notice or refusal, protest or notice of protest and to delay
         of grace.

3.       WAIVER. This Note evidences the payment obligations of Maker to Payee.
         No failure on the part of Payee to exercise and no delay in exercising,
         and no course of dealing with respect to, any right, power, privilege
         or remedy under this Note shall operate as a waiver thereof, nor shall
         any single or partial exercise of any right, power, privilege or remedy
         under this Note preclude any other or further exercise thereof or the
         exercise of any other right, power, privilege or remedy. The remedies
         provided herein are cumulative and not exclusive of any remedies
         provided by law.

4.       EXPENSES. Maker agrees to pay or reimburse Payee for (a) all reasonable
         out-of-pocket costs and expenses of Payee (including, without
         limitation, the reasonable fees and expenses of counsel to Payee), in
         connection with any amendment, supplement, modification or waiver of
         any of the terms of this Note and (b) all reasonable costs and expenses
         of Payee (including the reasonable fees and expenses of counsel) in
         connection with any default or Event of Default and any enforcement or
         collection proceedings resulting therefrom. The obligations of Maker
         under this Section (IV)(4) shall survive the repayment of the principal
         amount of this Note.

5.       AMENDMENTS. This Note may amended or modified only by an instrument in
         writing duly executed by Maker and Payee. Any amendment or modification
         effected in accordance with this Section IV(5) shall be binding upon
         Maker and Payee.

6.       SUCCESSORS AND ASSIGNS. This Note shall be binding upon, and inure to
         the benefit of, Maker and Payee and their respective successors and
         assigns. Maker may not assign its rights or obligations under this Note
         without the prior written consent of the Payee. Payee may assign or
         transfer this Note or any rights or obligations hereunder without the
         prior written consent of Maker.

7.       HEADINGS. The section headings appearing herein are included solely for
         convenience of reference and are not intended to affect the
         interpretation of any provision of this Note.

8.       SEVERABILITY. If one or more provisions of this Note are held to be
         unenforceable under applicable law, such provision(s) shall be excluded
         from this Note and the balance of this Note shall be interpreted as if
         such provision(s) were so excluded and shall otherwise be enforceable
         in accordance with its terms.

9.       USURY. Maker covenants (to the extent that it may lawfully do so) that
         it will not at any time insist upon, plead, or in any manner whatsoever
         claim or take the benefit or advantage of, any stay or extension law or
         any usury law or other law that would prohibit or forgive Maker from
         paying all or any portion of the principal of or interest on the Notes,
         wherever enacted, now or at any time hereafter in force, or which may
         affect the covenants or the performance of this Note.

10.      GOVERNING LAW; CONSENT TO JURISDICTION; WAIVER OF JURY TRIAL. This
         Agreement shall be governed by and construed in accordance with the
         laws of the State of Delaware applicable to contracts to be performed
         in such state. THE PARTIES HERETO AGREE THAT VENUE IN ANY AND ALL
         ACTIONS AND PROCEEDINGS RELATED TO THE SUBJECT MATTER OF THIS AGREEMENT
         SHALL BE IN THE STATE AND FEDERAL COURTS IN AND FOR WILMINGTON COUNTY,
         DELAWARE, WHICH COURTS SHALL HAVE EXCLUSIVE JURISDICTION FOR SUCH
         PURPOSE, AND THE PARTIES HERETO IRREVOCABLY SUBMIT TO THE EXCLUSIVE
         JURISDICTION OF SUCH COURTS AND IRREVOCABLY WAIVE THE DEFENSE OF AN
         INCONVENIENT FORUM TO THE MAINTENANCE OF ANY SUCH ACTION OR PROCEEDING.
         SERVICE OF PROCESS MAY BE MADE IN ANY MANNER RECOGNIZED BY SUCH COURTS.
         EACH OF THE PARTIES HERETO IRREVOCABLY WAIVES ITS RIGHT TO A JURY TRIAL
         WITH RESPECT TO ANY ACTION OR CLAIM ARISING OUT OF ANY DISPUTE IN
         CONNECTION WITH THIS AGREEMENT OR THE TRANSACTIONS CONTEMPLATED HEREBY.


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         IN WITNESS WHEREOF, the undersigned have executed this Note as of the
date first above written.

MAKER:

THE UNIMARK GROUP, INC.

By:  /s/ David E. Ziegler
     --------------------
     David E. Ziegler
     Chief Financial Officer


PAYEE:

M & M NOMINEE, LLC

By:  /s/ John F. Brown
     -----------------
     John F. Brown
     Attorney-in-Fact