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I got the following info from a commercial site and would be happy to provide its URL via PM - just let me know. Under the definition provided, water IS a mineral.

"A mineral, by definition, is any naturally occurring, inorganic substance, often additionally characterized by an exact crystal structure. Its chemical structure can be exact, or can vary within limits. Elements that occur naturally are also considered minerals.

All minerals belong to a chemical group, which represents their affiliation with certain elements or compounds. The classified chemical groups are known as: Elements, Sulfides, Oxides, Halides, Carbonates, Nitrates, Borates, Sulfates, Chromates, Phosphates, Arsenates, Vanadates, Tungstates, molybdates, and Silicates. Some of these chemical groups have sub-categories, which may be categorized in some mineral references as separate groups.

All minerals belong to various crystal structure groups, classified according to the way the atoms of the mineral are arranged. Minerals also have distinctive properties, such as color, hardness, crystal habit, specific gravity, luster, fracture, and tenacity. Many of these properties can vary among a single mineral, within limits. Many minerals exhibit certain properties that others do not, such as fluorescence and radioactivity.

Minerals are an economic commodity; they are mined because of the need for a valuable element they contain or an intrinsic property they may have. Other minerals are mined for their beauty and rareness, thus giving many specimens an accepted worldwide value. There are about 3,000 different types of minerals, and new ones are constantly discovered. Most of them are not known to professional mineral collectors, because they are rare, have no economic purpose, and for the most part do not make good specimens."

I note also that there's a school of thought that seeks to use a separate category - "minaraloids". The definition given is that these substances are amorphous, inorganic that lack crystal structure. Along with Mercury, this group includes Water, Opal and Obsidian. I'm a traditionalist and prefer the KISS approach - mercury and water are minerals to me.

In classification terms Quartz is a trap for new players. Because people know it consists of silicon and oxygen they often mistake it for a silicate. In fact it also is an oxide - of silicon

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