
6 STRUCTURAL AND FIELD GEOLOGY 
form and internal crystalline structure). It is composed of silica, with 
a variable proportion of water (usually from about 3 to Io per cent.); 
the specific gravity of the mineral (1-9 to 2-3) is somewhat less than 
that of quartz, and the same is the case with the hardness (5-5 to 
6-5). The texture is colloidal or jelly-like ; and the lustre vitreous to 
resinous. The colour varies—it may be white, red, brown, yellow, green, 
or blue, and some kinds show a rich play of colours. Opal usually occurs 
in reniform, botryoidal, or stalactitic masses, occupying any irregular 
cavity in rocks. In all cases it is of secondary origin—that is, it has 
been subsequently introduced as a product of decomposition. Many 
varieties are recognised, among which the following may be mentioned : 
Szliceous Sinter or Geyserite, deposited from thermal waters, often loose 
and earthy ; //ya/z/e, usually water-clear, colourless, but sometimes white 
or translucent—it occurs in the joints, fissures, and vesicular cavities of 
some basalts ; Voble or Precious Opal, with a rich play of colours, met 
with in irregular cavities in trachyte, etc.; Common Opal, translucent, 
but showing no play of colours, occurs in veins, fissures, etc., in igneous 
rocks ; Semz-Opal is less translucent than common opal ; /asp-Ofpal is red 
or brown in colour ; W/enzlzte is an opaque greyish or brown concretionary 
opal, occurring occasionally in argillaceous rocks. 
Specular Iron or Hematite, oxide of iron (Fe,O,), crystallises in 
hexagonal forms (which are commonly combinations of rhombohedra and 
scalenohedra). It has a hardness of 5-5 to 6:5, and a specific gravity of 
5-19 to 5:28 ; crystals are bluish iron-grey in colour, while fibrous forms 
are usually brownish-red. The mineral yields a red powder when rubbed 
with a steel file. This red streak and the absence of magnetism distin- 
guish specular iron from magnetite. It occurs both crystalline and 
massive. The crystalline variety is common in veins, and is often 
accompanied by magnetite. Not infrequently it occurs as an ingredient 
of granite, syenite, gneiss, mica-schist, phyllite, etc. It is met with in 
many minerals as a microscopic inclusion (exdomorph), in the form of 
minute filmy plates or scales, the presence of which affects the colour 
of the including mineral (fevzmorph), and often imparts to it a kind of 
pearly or submetallic glimmer or iridescence. Now and again it has been 
developed in limestones at or near their point of contact with eruptive 
rocks. It occurs as a sublimation-product in volcanic regions. 
The more compact or cryptocrystalline varieties of hematite usually 
occur as veins, irregular beds, and masses. <Azdmney-ore is the name 
given to nodules and nodular masses, which often consist of concentric 
coats having a radiating fibrous structure. Hezematite frequently occurs 
in decomposing igneous rocks as an alteration-product of ferromagnesian 
minerals, and it often coats the faces of joints in these and other ferriferous 
rocks. It is probable, however, that the ferruginous mineral commonly 
seen on joint-faces is in many cases not true hematite, but Wydro-hematite 
or Zurgite, which contains a small percentage of water—only 5 per cent. 
In other respects it is so closely similar to hzmatite that it can only be 
differentiated from the latter by analysis. 
Ilmenite is an iron-black mineral, with metallic or submetallic lustre, 
