<PAGE>   1

                                                                    EXHIBIT 99.1

                                LETTER OF INTENT

                     MEXICO STRATEGIC INVESTMENT FUND, LTD.




                                                           CONFIDENTIAL


                                                          June 22, 1998

The UniMark Group, Inc.
UniMark House
Bartonville
Argyle, TX  76226

Gentlemen:

         1. This letter is to confirm our agreement that we or an affiliate of
ours to be designated by us ("Purchaser") will purchase, and The UniMark Group,
Inc. ("you" or the "Company") will issue to Purchaser, 3,305,500 shares of
Common Stock (the "Primary Shares") of the Company for a purchase price of
U.S.$4.5375 per share, or an aggregate purchase price of U.S.$14,998,706.25, in
cash (subject to adjustment for stock splits, stock dividends and similar
transactions that occur between the date hereof and the date such transaction is
consummated). The purchase price for the Primary Shares will be payable in U.S.
dollars . The definitive purchase agreement for the Primary Shares (the
"Agreement") will grant to Purchaser and its assigns an option (the "Option") to
acquire 1,000,000 shares of Common Stock of the Company at an exercise price of
$4.5375 per share (subject to anti-dilution protections and adjustment), which
option will be exercisable on or before the first anniversary of the date the
Primary Shares are acquired.

         2. The Agreement will contain customary terms, including customary
representations and warranties, indemnities, conditions and covenants (including
covenants not to do the matters addressed on Annex A hereto, which negative
covenants will be applicable so long as Purchaser and its affiliates either own
(i) 10% or more of the outstanding Common Stock of the Company, or (ii) have not
transferred to third-parties 50% or more of the Primary Shares).

         3. The Agreement will include or be accompanied by an agreement
satisfactory to the Company and to Purchaser providing for customary
registration rights (with three demand registrations and unlimited piggyback
rights) for the Primary Shares, the Option and the Common Stock obtained upon
exercise of the Option (the Primary Shares and such Common Stock being
hereinafter referred to collectively as the "Shares"). The Agreement will also
provide that Purchaser will not sell the Option or any


<PAGE>   2

of the Shares for one year from the time the Primary Shares are acquired (other
than to affiliates, each of which shall be bound by the foregoing transfer
restriction).

         4. From the date hereof until August 1, 1998, the Company shall not,
and the Company shall cause its affiliates and representatives not to, initiate,
solicit, enter into or conduct any discussions or negotiations, or enter into
any agreement, whether written or oral, with any person with respect to the sale
of all or any significant portion of the stock or assets of the Company (other
than in connection with the transactions contemplated hereby).

         5. The Company shall pay all of our costs and expenses (including fees
and expenses of counsel and accountants) in connection with our due diligence
review of the Company, the negotiation, execution and delivery of this letter,
the Agreement and all other documents in connection therewith, and the closing
of the transactions contemplated hereby, up to a maximum of $100,000, as well as
our costs and expenses (including fees and expenses of counsel and accountants)
of our enforcement of the foregoing.

         6. You and we shall consult with one another before issuing any press
release or otherwise making any public statements with respect to the
transactions contemplated hereby. You and we will not issue any such press
release or make any such public statement without the prior approval of the
other except as may be required by law.

         7. Purchaser represents to the Company that this letter has been duly
authorized, executed and delivered by it and constitutes the valid, binding and
enforceable obligation of Purchaser and that no further authorization by
partners, directors or shareholders of Purchaser is required (by law, contract,
rule of any securities authority or exchange or otherwise). Purchaser will have,
at the time required by the Agreement, sufficient funds to effect the closing of
the transactions contemplated hereby.

         8. The Company represents to Purchaser that this letter has been duly
authorized, executed and delivered by it and constitutes the valid, binding and
enforceable obligation of the Company and that no further authorization by
directors or shareholders of Purchaser is required (by law, contract, rule of
any securities authority or exchange or otherwise). The Shares will be eligible
for NASDAQ NMS trading without further consents or actions (other than
registration under the federal securities laws).

         9. The Company further represents to Purchaser as follows: the Company
has timely filed with the Securities and Exchange Commission all forms, reports,
schedules, statements and other documents required to be filed by it (other than
the Company's proxy statement in respect of its annual shareholders' meeting to
be held in 1998) since January 1, 1996 under the Exchange Act (as defined below)
or the Securities Act (as defined below) (such documents, as supplemented and
amended since the time of filing, collectively, the "SEC Documents"). The SEC
Documents, including any financial



                                      - 2 -

<PAGE>   3

statements or schedules included therein, at the time filed (and, in the case of
registration statements and proxy statements, on the dates of effectiveness and
the dates of mailing, respectively) (a) did not contain any untrue statement of
a material fact or omit to state a material fact required to be stated therein
or necessary in order to make the statements therein, in light of the
circumstances under which they were made, not misleading, and (b) complied in
all material respects with the applicable requirements of the Exchange Act and
the Securities Act, as the case may be. The financial statements of the Company
included in the SEC Documents at the time filed (and, in the case of
registration statements and proxy statements, on the dates of effectiveness and
the dates of mailing, respectively) complied as to form in all material respects
with applicable accounting requirements and with the published rules and
regulations of the Commission with respect thereto, were prepared in accordance
with generally accepted accounting principles applied on a consistent basis
during the periods involved (except as may be indicated in the notes thereto or,
in the case of unaudited statements, as permitted by Form 10-Q of the
Commission), and fairly present (subject, in the case of the unaudited interim
financial statements, to normal, recurring year-end audit adjustments consistent
with past practice), in all material respects, the consolidated financial
position of the Company and its subsidiaries as at the dates thereof and the
consolidated results of their operations and cash flows for the periods then
ended. All projections previously (or hereafter) provided to Purchaser in
connection with the transactions contemplated hereby are (or will be) bona fide
projections of the Company and the Company believes that such projections are
reasonable. All other information previously (or hereafter) furnished by the
Company to Purchaser is (or shall be) true and correct in all material respects
does not (and will not) contain any material misstatements or omissions. No
subsidiary of the Company is subject to the periodic reporting requirements of
the Exchange Act or required to file any form, report or other document with the
Commission, any stock exchange or any other comparable governmental authority
(domestic or foreign). The "Securities Act" shall mean the Securities Act of
1933, as amended, including the rules and regulations promulgated thereunder.
The "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended
including the rules and regulations promulgated thereunder.

         10. You and we agree to use our respective best efforts to negotiate
and enter into the Agreement by July 15, 1998, subject to compliance with the
conditions referred to in this letter.

         11. Your obligation to sell the Shares and grant the Option referred to
in Paragraph 3 hereof is subject to the compliance by Purchaser of the terms of
this letter applicable to it (including the continued truth of the
representations and warranties of Purchaser contained herein) and to the
execution and delivery of the Agreement.

         12. Our obligation to purchase the Shares is subject to completion of
our due diligence review (and the results of which being satisfactory to us), to
our structuring the transaction in a tax-efficient manner, to compliance by the
Company and the Principals



                                       -3-

<PAGE>   4
(as defined below) with the terms of this letter (including the continued truth
of the representations and warranties of the Company contained herein) and to
the execution and delivery of the Agreement and the documentation referred to in
Section 13 below, as well as to the continued truth of the following: (i) since
the date of the last SEC Document prior to the date hereof, there has been no
material adverse change in the business, operations, results of operations,
assets, prospects or condition (financial or otherwise) of the Company and its
subsidiaries, (ii) since the date hereof, none of Mr. Rafael Vaquero Bazan and
Mr. Fernando Camacho Casas (collectively, the "Principals") has sold or
otherwise disposed of any of their respective holdings of Common Stock of the
Company, (iii) the Company has not, after the date hereof, done or caused to be
done any of the matters set forth on Annex A hereto without our consent, and
(iv) except as specifically disclosed in the SEC Documents filed prior to the
date of this Agreement, there is no suit, claim, action, proceeding, audit or
investigation pending or, to the knowledge of the Company, threatened against
the Company which, individually or in the aggregate, would reasonably be
expected to have a material adverse effect on the business, operations, results
of operations, assets, prospects or condition (financial or otherwise) of the
Company or a material adverse effect on the ability of the Company to consummate
the transactions contemplated hereby.

         13. The Principals shall not transfer or pledge any of their shares of
Common Stock of the Company (the "Principal Shares") until eighteen months
following the purchase by Purchaser of the Primary Shares. The Principals shall
enter into mutually satisfactory non-competition agreements with Purchaser and
the Company. In addition, the Principals and Purchaser shall enter into an
agreement pursuant to which at any time after the second anniversary of
Purchaser's purchase of the Primary Shares and for so long as Purchaser and its
affiliates are entitled to exercise the veto rights in Annex A, (i) the
Principals or Purchaser and its affiliates may propose a price per share of
Common Stock and (ii) the non-proposing group may elect either (x) to sell all
of its shares of Common Stock (including the Option and the shares obtainable
upon exercise of the Option) of the Company at such price per share or (y) to
purchase for cash the proposing group's shares of Common Stock (including the
Option and the shares obtainable upon exercise of the Option) of the Company at
such price per share.

         14. Purchaser shall be entitled to designate a percentage of the
Company's Board of Directors at least equal to the percentage ownership of
Purchaser and its affiliates of the outstanding Common Stock of the Company.

         15. This letter shall be governed by and construed in accordance with
the laws of the State of Delaware, without giving effect to any conflict of law
provision. You and we each hereby consent to submit to the exclusive
jurisdiction of the courts of the State of Delaware and of the United States in
each case located in Wilmington for any litigation arising out of or relating to
this letter and the transactions contemplated hereby (and agree not to commence
any litigation relating thereto except in such courts). You and we each hereby
irrevocably and unconditionally waive any objection to the laying of venue of
any such litigation in the courts of the State of Delaware or of the United
States of America in each case located in Wilmington and hereby further
irrevocably and unconditionally



                                       -4-

<PAGE>   5
waive and agree not to plead or claim in any such court that any such litigation
brought in any such court has been brought in an inconvenient forum.

         16. This letter may be terminated (i) at any time by mutual agreement
of the parties, or (ii) at any time after August 1, 1998 by either party (by
written notice to the other), provided that the terminating party is not in
material breach of the terms hereof. Notwithstanding the foregoing, the
provisions of Paragraphs 5 and 15 and this Paragraph 16 shall survive any
termination of this letter.


                                               Very truly yours,

                                               MEXICO STRATEGIC
                                               INVESTMENT FUND, LTD.


                                               By: /s/ Peter Streinger
                                                  -----------------------------
                                                   Name:    Peter Streinger
                                                   Title:   Attorney-in-fact

AGREED TO AND ACCEPTED 
as of the date first above written 
THE UNIMARK GROUP, INC.


By: /s/ Rafael Vaquero
   -------------------------------
   Name:  Rafael Vaquero
   Title: CEO and President



                                       -5-

<PAGE>   6

SECTION 13 AGREED
TO AND ACCEPTED
as of the date first above
written by the PRINCIPALS


/s/ Rafael Vaquero Bazan
-------------------------------
Rafael Vaquero Bazan



/s/ Fernando Camacho Casas
-------------------------------
Fernando Camacho Casas



                                       -6-

<PAGE>   7

                                                                         ANNEX A

o     (i) Incur or maintain any indebtedness other than bank line of credit for
      working capital not in excess of $18 million, (ii) be party to capital
      leases in amounts in excess of $30 million (in the aggregate), and (iii)
      at any time, allow the amounts of the indebtedness/obligations under items
      (i) and (ii) to be greater than $42 million 

o     Incur liens or encumbrances other than liens to secure the indebtedness
      permitted above

o     Incur contingent obligations or guaranties other than in the ordinary
      course of business 

o     Take actions which would result in the Company being classified as a
      United States Real Property Holding Company

o     Make or maintain any investment in or capital contribution or advance to
      any other person, other than investments in wholly-owned subsidiaries; or
      enter into joint ventures or partnerships or establish non-wholly owned
      subsidiaries

o     Enter into any commitment or amend any existing commitment with a value in
      excess of five percent of total assets (as set forth on the immediately
      preceding year's audited financial statements) or enter any commitment or
      transaction or amend any existing commitment or transaction with any
      affiliate or related party or significant shareholder

o     Incur capital expenditures in excess of 110% of the amount budgeted for
      such items (based on the budgets delivered to Purchaser)

o     Approve increases in any item in each year's annual budget in excess of 5%
      over the amount budgeted for such item during the preceding year

o     Appoint or remove any member of senior management

o     Amend the Certificate of Incorporation or the Bylaws

o     Change the number of directors (without maintaining at least proportionate
      representation by Purchaser on the Board)

o     Approve any merger or consolidation or similar reorganization or other
      fundamental change in the business of the Company

o     Acquire assets (other than in the ordinary course) or common stock, debt
      or other securities of any third party

o     Sell or dispose of assets of the Company or any of its subsidiaries having
      a value in excess of 10% per year of the value of the total fixed assets
      of the Company (nor shall the Company allow any facility of the Company to
      sell or dispose of assets having a value in excess of 5% per year of the
      value of such facility's total fixed assets)

o     Liquidate the Company

o     Issue any securities of the Company (including, without limitation, common
      stock, convertible securities, rights, warrants or options to one or more
      persons (including through a public offering)), other than pursuant to an
      employee benefit plan consistent with past practice

o     Register Securities under the Securities Act of 1933 (except as provided
      in Registration Rights Agreements disclosed to Purchaser) or grant any
      registration rights 

o     Declare or pay dividends

o     Effect any debt or equity repurchase or redemption or other restricted
      payment

o     Adopt or amend benefit plans

o     Enter into new businesses

o     Change the Company's independent accountants

o     Settle any material litigation

o     Permit the Company to fail to comply with customary affirmative covenants,
      including: (i) compliance with financial and other reporting requirements;
      (ii) compliance with laws; (iii) limitation on conflicting agreements;
      (iv) preservation of franchises and existence; (v) maintenance of
      insurance; (vi) payment of taxes and other charges; (vii) ERISA
      compliance; and (vii) access and inspection rights and maintenance of
      books and records.



                                      -7-