PYROLUSITE

An oxide which can be black, grey or sandy brown. It is fairly common and is an important source of manganese along with rhodochrosite. Its hardness can vary greatly. Massive deposits are usually very soft (1-2 on the mohs scale) whereas crystals tend to be much harder (6-6.5 on the mohs scale). Crystals are also much rarer and may sometimes be mistaken for fossils. Often pyrolusite occurs in the form of a powder although it may sometimes appear in lumps, particularly in marshy areas (this is known as wad).

Pyrolusite is a deoxidizing agent and has been used for centuries to clean clear glass which has been discoloured particularly by yellow and brown iron oxide impurities. Its name is derived from the Greek words pyr, which means fire and lousis, which means washing (this refers to its use in glass cleaning). It is commonly known as glassmakers soap.

Manganese is a brittle, grey, metallic element used to reinforce steel (manganese steel may contain 11-14% manganese). It is used for railway points which need to be very resilient. Manganese can be processed directly into steel without having to be extracted. This is done by mixing pyrolusite ore directly with iron ore and removing oxygen by heating. Manganese is also used in industry to remove unwanted products such as oxygen and soulful from other materials. Therefore is is known as a deoxidizing and desulphurising agent.

Pyrolusite forms in parts of manganese deposits that have reacted with oxygen from the air or water. It can be found in bogs, some quartz veins, in shallow sea sediments and at the sea bottom. It is fairly widespread and can be found in countries such as Brazil, Canada, Czech Republic, Georgia, UK (Cornwall), India, Italy and South Africa.

PUMICE

A common igneous rock which has some very practical uses. It is mainly white but may also be yellow or brown (Hawaiian pumice is pitch black). Its colour and value depend upon the minerals it contains. Its principal component is vulcanic gases but it may also contain crystals of several different silicate minerals including hornblende, lavradoritem, orphoclase and zircon. Pumice is fine grained and very light in weight. It sometimes has surface cavities which all face the same way. This indicates the direction of the lava flow at the time of formation.

It forms when volcanic activity throws out pieces of geological debris from deep inside the earth. Rocks created in this way are known as pyroclastics. It has no crystal structure because it cools too quickly from its molten state for crystallization to take place. It is closely related to obsidian, which will turn into pumice if it is heated. The distinction between the two is that obsidian cools more slowly enabling it to develop into a form of glass. (If pumice was formed under greater pressure it would be obsidian)

As pumice is less dense than water, it floats and will only sink after it has become saturated. Due to this, during the 1883 eruption of Mt Krakatoa in Indonesia, the surface of the surrounding seas was temporarily covered with grey, buoyant pyroclastic pumice rock.

It is used as a cosmetic scourer to remove hardened skin and also in the manufacture of plaster, cement and insulation. Powdered pumice is used in cleaning and polishing.

Pumice most often occurs in rhyolites and trachytes rather than in basalt deposits and often in association with obsidian. Industrially exploitable deposits can be found in Germany, Greece, Iceland, Hungary, Hawaii, New Zealand and the Rocky Mountains (US).