Glencore Plc revised its bid to buy Teck Resources Ltd. after investors of the Canadian miner balked at the idea of owning the Swiss commodity giant’s thermal coal assets.

Glencore Chief Executive Officer Gary Nagle hosted private calls with Teck shareholders last week to sell them on the US$23 billion deal and get feedback, according to people familiar with the matter. In those calls, much of the concerns centered on investors’ opposition to combining Teck’s steelmaking coal with Glencore’s vast thermal coal operations, the dirtiest of fossil fuels.

“What Glencore was trying to figure out was where Teck investors stand,” said Jon Case, a CI Global Asset Management portfolio manager who participated in a conference call with other investors. “Most of their questions were around coal — how it works, why their structure is better than Teck’s.”

Major investors have become increasingly resistant to owning thermal coal because of its harmful environmental impact. BlackRock Inc., one of Teck’s largest shareholders, already pledged to eliminate companies that generate more than a quarter of revenue from thermal coal from its actively managed investment portfolios. Steelmaking coal, by contrast, hasn’t received the same level of criticism.

Glencore presented a revised plan Tuesday, giving Teck shareholders the option to receive up to $8.2 billion in cash instead of shares in the spun off coal company. Nagle is making his pitch in person this week, meeting Teck investors at a Thursday luncheon in Toronto hosted by RBC Capital Markets.