HORNBLENDE 807 



at present includes a large number of minerals allied to the original hornblendes in 

 crystalline form, and, to a certain extent, in chemical composition ; but neverthe- 

 less differing widely from the typical hornblendes, not only in colour, density, and 

 other physical characters, but also in their mode of occurrence. The necessity for 

 grouping these minerals together was first recognised by the French crystallographer 

 llaiiy, who proposed to name the whole species amphibole, and to retain Werner's 

 term hornblende only for the common varieties ; a nomenclature still frequently ad- 

 hered to in France. 



The typical hornblendes crystallize in forms belonging to the oblique system, and 

 present a perfect prismatic cleavage; whilst other varieties occur in a columnar, 

 fibrous, lamellar, or granular condition. The hardness is tolerably constant, being 

 slightly below that of felspar ; but the specific gravity varies with the chemical com- 

 position. All the hornblendes are anhydrous silicates of various protoxide-bases, 

 usually lime, magnesia, and protoxide of iron ; these bases being capable of replacing 

 one another in chemical combination, without affecting the crystalline form of the 

 compound. Many of the ferruginous varieties contain alumina, the amount of which 

 may rise to upwards of 15 per cent. : the condition in which this compound exists in 

 hornblende is by no means well ascertained, but it is usually supposed to replace the 

 silica, the compounds being thus regarded as silico-alumi nates. In many varieties, 

 fluorine occurs to the extent of 2 or 3 per cent., and exists probably as a silico- 

 fluoride of calcium and magnesium. It is interesting to note that certain hornblendes 

 also contain titanic acid. 



On the chemical composition of the hornblende depends to a great extent the 

 general character of the mineral ; thus, those varieties which contain only lime and 

 magnesia have always a white or very pale colour and a low specific gravity, some- 

 times not higher than 2*9 ; while before the blowpipe they fuse to a nearly colourless 

 glass. On the other hand, the ferruginous hornblendes have a dark green or black 

 colour ; a much higher density, often reaching 3 '4 ; and are fusible with greater 

 facility, producing a glass more or less deeply coloured. The iron -hornblendes, 

 again, are partially decomposed by hydrochloric acid, which has scarcely any effect 

 on the other varieties. When exceedingly rich in iron, the mineral affects the 

 magnetic needle. 



These differences of composition, and the corresponding diversity of physical 

 characters, have given rise to numerous varieties, of which the following are the most 

 important : 



Tremolite, so named from the Swiss valley of Tremola, is a silicate of magnesia and 

 lime, containing silica, 58*87 ; magnesia, 28'19 ; lime, 1TOO ; and alumina, 1'77 = 99'83 

 (Rammelsberg). It occurs usually in slender-columnar or flat-bladed crystals of a 

 white, grey, or pale green colour ; and of specific gravity 2'93. The crystals are 

 commonly imbedded in dolomite or granular limestone, as at St. Gothard in 

 Switzerland, and Glen Tilt in Scotland. Other localities are the Tyrol, the Bannat, 

 Wermland in Sweden, New England, Maine, Massachusetts, and other of the 

 United States. Tremolite forms the Amphibole blanche of French mineralogists. 

 Grammatlte and Calamite are Swedish varieties, whilst Eaphilite is a Canadian 

 tremolite, presenting an asbestiform structure. Many of the fibrous hornblendes 

 separate on decomposition into fibres more or less delicate, and thus form varieties 

 of asbestus. 



The occasional presence of protoxide of iron in certain tremolites furnishes a 

 passage to the variety of hornblende called actinolite the actinote or Amphibole verte 

 of the French, the Strahlstein (ray-stone) of the Germans. It occurs in slender- 

 columnar crystals, often disposed in radiated groups, presenting a high lustre and 

 leek -green or black-green colour. Its specific gravity is about 3. A typical acti - 

 nolite from the Zillerthal yielded to Rammelsberg's analysis: silica, 55'50; mag- 

 nesia, 22'56 ; lime, 13-46; and protoxide of iron, 6'25 = 97'77. Actinolite is found 

 chiefly in talcose schist, serpentine, and other magnesian rocks : in Cornwall it 

 occurs at the Lizard, and at Botallack and other western mines. Among foreign 

 localities may be mentioned Greiner in the Zillerthal, St. Gothard, Salzburg, Sweden, 

 Norway, &c. 



Anthophyttite is a grey or clove-brown fibrous or lamellar hornblende, occurring in 

 Norway, Greenland, and the United States. Its compositiou is silica, 5674 ; mag- 

 nesia, 24-35 ; protoxide of iron, 14'40 ; protoxide of manganese, 2'38 ; and water, 

 1 -67 = 99-54 (Vopdius). 



Hornblende proper, the Amphibole noire of the French, includes the dark-coloured 

 iron-bearing varieties to which the name was originally applied. Common hornblende 

 occurs in dark green or black crystals, usually imperfectly formed, and readily distin- 

 guished by the silky lustre of its cleavage planes. It forms an essential constituent 

 of syenite, diorite, and hornblende -rock ; and occurs as an accessory mineral in 



