Activist investor Bluebell Capital Partners Ltd has asked miner and commodity trader Glencore Plc (GLEN.L) to separate its thermal coal business at a time when global mining firms have come under pressure to move to cleaner energy sources. Thermal coal is not only the most polluting fossil fuel, its prices have soared recently on the back of Chinese power shortages and a European gas squeeze. "Due to its coal business, Glencore is not an investible company for investors who place sustainability at the heart of their investment process," Bluebell said in a Nov. 8 letter to Glencore's top management. Glencore in June bought the remaining 66% stake in the Cerrejon thermal coal mine in Colombia it did not already own from rivals BHP Group (BHPB.L), (BHP.AX) and Anglo American (AAL.L) for $588 million. Glencore also owns thermal coal assets in Australia. read moreIts strategy of depleting its coal mines by mid-2040s rather than selling them, reflects a different path from other diversified miners such as BHP and Anglo American. "Mounting pressure on financial institutions to reduce their lending and cut bond and equity underwriting to the fossil fuel industry, also pose significant risk to your (Glencore's) future access to capital," Bluebell added. Responding to Bluebell's comments, Glencore said it engages regularly with its investors. The company has its annual investor day scheduled on Thursday. "We are confident that our business model is uniquely placed to produce, recycle, and market the materials needed to decarbonise energy whilst reducing our own emissions and delivering value for stakeholders," a Glencore spokesman told Reuters. Reporting by Aby Jose Koilparambil in Bengaluru, and Clara Denina and Zandi Shabalala in London; Editing by Shinjini Ganguli Our Standards: The Thomson Reuters Trust Principles.