In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention.

Q: We have 90m² of Pakistani white onyx to install in a kitchen / utility / toilet and conservatory area. Are there any special requirements for fixing, sealing and maintaining onyx tiles? How does onyx differ from marble?

Using strict geological terminology, onyx is considered to be a layered variety of microcrystalline silica or quartz.
The layers are usually precipitated and subtle changes in the environment during this time lead to differences in the bands laid down.

The silica deposited is extremely hard. It is similar in many ways to chert, chalcedony, agate, flint and other types of microcrystalline silica. It can take a polish that is easy to maintain.

The situation suddenly becomes more complicated by the fact that there is a commercial name given to some banded stones formed from the mineral calcite and referred to as onyx marble (or onyx alabaster).

Calcite is a much softer mineral and, while these stones readily take a polish, they are much more easily scratched and can easily suffer the affects of acidic attack.

Therefore, such stones must be used more carefully in some domestic and commercial environments.
You need to determine what type of onyx you have before using it to ensure you will install it properly and treat it in the most appropriate way.

You can determine which type of onyx you have by putting a drop of hydrochloric acid on the reverse side of a tile or other sample. It is fizzes vigorously it is almost certainly calcite. If it fizzes slowly it may be dolomite based, which is a carbonate mineral similar to calcite but a little harder. If it does not fizz, it is probably silica-based. 

Hydrochloric acid is typically found in masonry cleaners and some descaling products, so you do not have to go to an industrial chemist to source it.

If the onyx is calcite based it should be treated in the same way as best quality white marble tiles.
Primarily, this means only white cement containing adhesive / mortars / jointing materials should be used to prevent staining.

A marble sealer should be used of the surface but this will not prevent scratching and acidic attack.
There may be a particular problem in toilet areas (see my ‘Ask the Expert’ column in the May issue of Natural Stone Specialist).

If the onyx is silica based it will be much more forgiving and require less treatment, though a surface impregnator will still be a good idea.

The stone ought to be laid as would a quartzite. In either case, the Stone Federation guidance for installing natural stone floors should be followed and supported by the information given in the relevant parts of BS 5385 for wall tiling, in particular the clauses dealing with intermittently damp areas such as bathrooms and toilets.