NEW YORK (AP) -- Minerals Technologies Inc. has sweetened its offer to buy out Amcol International Corp. to about $1.38 billion in hopes the minerals and oilfield services company will be persuaded to nix its proposed sale to a French company.

Mineral Technologies, which produces mineral and synthetic mineral products, said that it sent a letter to Amcol's board on Monday detailing a binding offer to buy all of the company's outstanding shares for $42.50 a piece in cash.

That's up from the $42-per-share bid it announced on Feb. 14. But Amcol's stock, which had been trading higher than the proposed offer, slid 5.6 percent to $42.95 in after-hours trading Monday.

Minerals Technologies' bid followed the offer from France's Imerys. On Feb. 12, Amcol's board had approved a buyout offer of $41 per share, or about $1.33 billion, from the French minerals producer. Including debt, the companies put the deal's total value at roughly $1.6 billion. The companies said that they expected the deal to close in the first half of 2014.

The Hoffman Estates, Ill.-based company has about 32.5 million outstanding shares, according to FactSet.

Minerals Technologies, based in New York, said it has obtained committed debt financing to buy Amcol, noting it finished a review of the company's operations. That led Minerals Technologies to boost its offer price.

An email sent to Amcol was not immediately returned late Monday.