Mica (28a) is the name given to a group of minerals differing much from each other in chemical composition and optical properties, but having as a common character an easy splitting, or cleavage, in a single direction, and thus affording plates remarkably thin, transparent, tough, and elastic. One of these minerals, Muscovite (28d), has been used in Russia in place of glass for windows, and is now in common use for lanterns and stoves, not being so easily cracked as glass by changes of temperature; it is often known in commerce as talc, a term restricted by mineralogists to a different mineral.
The group of Felspars, the most important of the rock-forming minerals, begins at case 28f. After the Felspars comes Beryl, of which the bright green variety, Emerald (29c), is one of the most valued of precious stones. It was in ancient times worked in Egypt, as is proved by specimens found in the old workings by Sir Gardner Wilkinson, and presented by him to the Museum. Emeralds occur in the Urals; but the locality for the finest stones has long been Muzo, about seventy miles from Santa Fé de Bogotá, in South America. Faceted specimens of the colourless Beryl, of the bluish-green Beryl, known in jewellery as Aquamarine, and of pink Beryl from California and Madagascar, are exhibited (30a).
In cases 30f to 32d will be found examples of the Zeolite group of minerals.
Tourmaline (33a), when free from flaws, is, in some of its varieties, to be classed with the precious stones; among these being a pink variety called Rubellite. Fine specimens of Rubellite from Burma, the Urals, and Madagascar, are shown in the case; one specimen from Burma, poor in colour but remarkable for its size and shape, was brought home by Colonel Symes, to whom it had been presented by the King of Ava in the year 1795. The pink-and-green tourmalines from Maine, U.S.A., and the magnificent crystals from San Diego Co., California, are among the more beautiful of the mineral products of the United States. Examples of the blue Tourmaline, or Indicolite, are shown in case 33b.
A rich blue mineral, the Lapis-lazuli of jewellery (34b), brought from Persia, Siberia, Bokhara, and Chili, is a mixture of various species. When powdered, Lapis-lazuli furnished the once costly pigment ultramarine; but by the discovery of a method of producing an artificial and cheap form of the latter, the use of the mineral as a pigment has ceased.
Sulphates, Phosphates, etc.
The sulphates of Strontium, Celestite (35c), of Barium, Barytes or Heavy Spar (36a), and of Lead, Anglesite (36e), are all represented by series of specimens.
Gypsum, or Selenite (36f), is a hydrated sulphate of the metal Calcium: when heated, it gives up its water of crystallisation and falls to a white powder, known as “Plaster of Paris,” which, when moistened, again combines with water and yields a coherent solid. Gypseous Alabaster, a massive variety of Gypsum (36h), owing to its whiteness, fine texture, and softness is used as a material for statuettes and other indoor ornaments. Oriental Alabaster is a harder substance, Stalagmitic Calcite, carbonate of Calcium.
Borax (37c), a borate of Sodium, is much used as a flux, also in soldering, and in the preparation of easily fusible enamels. It was formerly carried over the Himalayas on sheep and goats from lakes in Tibet, but is now obtained largely from the Borax-lakes of the United States, and is also extensively prepared from the boracic acid lagoons in Tuscany.
Nitratine or Soda-nitre (37d), found in Chili in beds of large extent, is largely used for the preparation of Nitric Acid and Saltpetre, and is also used as a fertiliser.