DESERT ROSE

A form of calcium sulphate which is therefore a member of the sulphate group. It is a form of gypsum and is white when in it's pure form. It is however usually brown/sandy in colour due to particles of sand which become trapped within it and are an essential ingredient. (It may be darker when iron impurities are present.) It is therefore calcium sulphate with water and sand grains.

It forms when dissolved mineral matter dries out and is crystallized. Therefore it is classed as a sedimentary rock. It forms in flower like shapes and only in dry conditions. Desert rose describes this form of gypsum as well as a variety of barium (barium sulphate) which also forms in flower shapes.

It measures 1.5-2 on the mohs scale and it is easily damaged.

It can be found in the Sahara desert, particularly in Algeria and Libya. It may also be found in Morocco, Tunisia, Arizona and New Mexico (US).

DOLOMITIC ROCK

It is a form of limestone which consists mainly of dolomite which is a mixture of calcium carbonate and magnesium. Therefore it is also known as magnesium limestone and is usually cream, grey or white. Sometimes weathering may change it's colour to brown or pink. It is a silicate of calcium and magnesium and may sometimes include impurities of iron oxide. It may also contain some bitumen, clay, pyrite or quartz.

It forms from pre-existing limestone which contains calcium carbonate. During conversion into dolomitic rock this is replaced by dolomite when magnesium rich waters come into contact with the limestone and chemical reactions occur. This process is known as dolomitisation and may occur shortly after deposition or some time later. Any fossils which may have been present are destroyed by this process. It is classed as a chemical sedimentary rock and has a variable grain size.

It is named after Dolomieu, a French geologist who did a lot of work in the Alps.

It is very highly porous and is often used as a reservoir for oil or natural gas. It is a source of magnesium oxide which is used in insulating material and can also be used as a building stone.

Dolomitic rock is widespread in limestone deposits and is often found with halite or gypsum.

It may be found in Saudi Arabia and Wyoming, US.