While Glencore and Xstrata work hard to get their merger plan past Europe's antitrust body, it is China, the biggest buyer of the materials they mine and trade, whose watchdog might bite hardest.

China's Ministry of Commerce (MOFCOM) is the newest and least predictable of the world's heavyweight regulatory bodies.

It also has a broader remit than most, and so could be the one to watch, even though it was Europe that kicked off the global hurdle race this week by flagging its desire to scrutinise a deal that will create the fourth largest miner and a leading producer of zinc, copper and coal.

For China, the deal offers an opportunity to extract reassurances from a key supplier of the raw materials it needs to keep its factories working and its economy growing.

(China's) MOFCOM is one authority where you can never be quite sure what's going to happen, said Frank Schoneveld, a partner at antitrust law firm McDermott, Will & Emery seconded to Shanghai.

Other big jurisdictions such as the European Union and the U.S., particularly if it's a big commodity deal, are more predictable. MOFCOM has tended to take longer than everybody else and can come up with rather unexpected undertakings.

China's place as workshop of the world and as a major consumer is central to its concerns about raw material supplies.

The case is a very sensitive area for MOFCOM because it's related to the...supply of raw materials in (the Chinese) steel industry, Zhan Hao, head of the antitrust department at law firm Grandall in Beijing.

This is a very concentrated industry, so the Chinese parties don't have much power to negotiate prices. So when they review the case, MOFCOM will take this situation into account.

China's anti-monopoly laws came into force in 2008. It has blocked only one deal, Coca-Cola's planned $2.4 billion (1.5 billion pound) purchase of juice maker Huiyuan in 2009, but has imposed conditions, including price and supply conditions, on almost a dozen cases.

Reviewing the acquisition of Russian potash producer Silvinit by rival Uralkali last year, MOFCOM fretted the merged entity, as the second-largest producer of potassium chloride and a key supplier to China, could have excessive weight.

It imposed conditions on supply and ordered the group to maintain its current sales procedures and price negotiations.

Behavioural remedies -- as opposed to remedial disposals -- are far less favoured by Western antitrust authorities.

Such conditions are tough to police, with virtually no precedent to go by and plenty of scope for discord down the line, for example, when players seek to renegotiate or raise prices.

As well as being the world's largest thermal coal exporter, Glencore and Xstrata combined would be the largest producer of both zinc and ferrochrome.

In thermal coal, the type used by power stations, it would be the largest exporter, but would still export less than 10 percent of the global total, well below the threshold eyed by most antitrust authorities.

But MOFCOM has other measures. It represents a domestic industrial policy interest that includes protecting the security of commodity supply, as well as traditional competition.

This means they will not offer a green light just because others have done before them, as in the case of the acquisition of car parts manufacturer Delphi by General Motors.

That deal last year was cleared by both the U.S. and European authorities.

But Beijing imposed conditions requiring the combined firm to supply Chinese automakers on a fair basis, and prohibited the merger partners from exchanging confidential information about Chinese automakers.

They are very independent, they don't feel they have to follow the decisions of other authorities, if they feel there is a point to be addressed in China, said one veteran lawyer with years of experience in China. It is very dangerous to assume European and U.S. norms apply in China.

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MUSCLES IN BRUSSELS?

While hardly anyone is betting the deal will be blocked, industry sources and antitrust lawyers say it is also unlikely to be an easy -- or a swift -- win for the partners.

And the $90 billion tie-up, the sector's largest deal to date, will have to jump through competition hoops in all major jurisdictions -- European Union, United States, South Africa, Australia, as well as China.

Glencore, the world's largest diversified commodities trader, already owns a 34 percent stake of Xstrata. For competition purposes, that is potentially enough to exert control -- and it already markets a third of Xstrata's output.

Yet after almost four decades as a private company, Glencore is a juicy target for regulators from all corners of the globe, and Xstrata's own chief executive, Mick Davis, has said he expected a lengthy process in jurisdictions especially China.

Even the European Union, which considered Glencore and Xstrata one company for the purposes of the miner's 2006 acquisition of miner Falconbridge, could prove more complex than expected, with steelmakers agitating and one industry source already referring to dozens of lawyers preparing for battle.

They'll look at the figures and see there are questions to be answered, said Gordon Moffat, director general of steel trade body Eurofer, which has already urged Brussels to look into the effect of the merger on zinc, nickel and thermal coal markets and is preparing a formal submission within weeks.

(Additional reporting by Victoria Howley in London; Editing by Andrew Callus)