Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/415

Rh MINERALOGY 397 angle, as figs. 364, 368. Also occurs granular, lamellar, parallel or radiated fibrous, compact and earthy. Cl. rhombohedral along R, very perfect and easily obtained, so that the conchoidal fracture is rarely observable; brittle. H. =3 ; G. =2 6 to 2 8 ; pure transparent crystals = 2 72. Pellucid in all degrees. Very distinct double refraction. Lustre vitreous, but several faces resinous, and OR pearly. Most frequently colourless or white, but often grey, blue, green, yellow, red, brown, or black ; streak greyish white. B.B. infusible, but becomes caustic and emits a bright light. Effervesces, and is entirely sol. in h. or n. acid. The fine powder, ignited on platina-foil over the spirit-lamp, forms a somewhat connected mass, and even adheres to the platina. C.c. of the purest varieties, carbonate of lime, with 44 carbonic acid and 5(i lime, but usually contains magnesia and protoxide of iron or of manganese. Remarkable specimens of the crystallized variety or proper calc-spar are found at Alston Moor in Cumberland (flat rhombic crystals) and in Derbyshire (pale yellow Fig 366. Fig. 367. transparent pyramids), at Strontian, Elie in Fife (figs. 370, 371, 372), Andreasberg and other parts of the Harz (six-sided prisms), and at Freiberg, Tharand, and Maxen in Saxony. Certain varieties are distinguished. Iceland Spar, remarkable for its transparency and double re fraction, occurs massive and in huge crystals in a trap rock in Iceland. Slate Spar, thin lamellar, often with a shining white pearly lustre and greasy feel ; Abergairn and Glen Fig. 368. Fig. 369. Tilt in Scotland, Wicklow in Ireland, and Norway. Aphrite, fine scaly ; from Hesse and Thuringia. Marble is the massive crys talline variety of this mineral, produced by igneous action on com pact limestone. Paros, Naxos, and Tenedos furnished the chief supply to the Grecian artists ; Carrara, near the Gulf of Genoa, to those of modern times. Some of the coloured marbles Fig. 370. Fig. 371. of the ancients were impure limestones, as the Cipollino, zoned with green talc or chlorite and Verde Antique, mixed with green Fig. 372. serpentine. Ruin Marble shows irregular markings like ruins; Val d Arno (Florentine marble), and Bristol (Cotham marble). Lucullite from Egypt, and Anthraconite, from Campbelltown and Kilkenny, are black from carbon. Lnmachello, from Bleiberg in Carinthia, exhibits beautiful iridescent colours from fossil shells, sometimes deep red or orange (Fire Marble). Hislopite, from Poonah, is green, from celadonite. Limestone occurs in all formations under various names, as Oolite, egg-stone, or roe-stone, round concretions with a concentric structure like the roe of a fish ; Pisolite, or peastone, similar structure ; Chalk, soft earthy ; Lithographic Stone, yellowish and compact, from Solenhofen ; and Marl, calcareous matter more or less mixed with clay. Calcareous Tufa, generally a recent deposit from calcareous springs, has often a loose friable texture, but at other times is hard and compact; and in the neighbourhood of Rome forms the common building stone Travertine. The sandstone of Fontaine- bleau is carbonate of lime (^) mixed with quartz sand (1), and occasionally crystal lizing in rhornbohedrons. This mineral is employed in many ways : the coarser varieties, when burnt to drive off the carbonic acid, as lime, for mortar, manure, tanning ; as a flux in melting iron and other ores, or in pre paring glass, and for similar purposes ; the finer, as marbles, for sculpture, architec ture, and ornamental stone-work ; the chalk for writing, white washing, or producing carbonic acid. Plumbocalcite. Cl. 104 53. White and pearly; softer than calc-spar; but G. =2 824. Contains 2 3 to 7 8 carbonate of lead. Wanlockhead and Leadhills (Scotland). 273. DOLOMITE (Bitter-spar), CaC-f MgC. Hexagonal rhombohedral ; R 106 15 -20 ; most frequent form R. The rhombohedrons often curved and saddle-shaped ; also granular or compact, often cellular and porous. Cl. rhombohedral. H. =3 5 to 4 5; G. =2 85 to 2 95. Translucent; vitreous, but often pearly. Colourless or white, but frequently pale red, yellow, or green. B.B. infusible, but becomes caustic, and often shows traces of iron and manganese. Fragments effervesce very slightly or not at all in hydrochloric acid ; the powder is partially soluble, or wholly when heated. C.c.: 54 3 carbonate of lime and 457 carbonate of magnesia, but generally carbonate of lime with more than 20 per cent, carbonate of magnesia and less than 20 per cent, carbonate of iron. Varieties are Dolomite, massive-granular, easily divisible, white; Rhomb or Sitter-spar, larger grained, or distinctly crystallized and cleavable, often inclining to green ; and Brown-spar and Pearl-spar, in simple crystals generally curved (fig. 231), or in imitative forms, of colours inclining to red or brown, more distinct pearly lustre, and under 10 per cent, carbonate of iron. Leadhills and Charlestown in Scotland, Alston in Cumberland, in Derbyshire, Traversella in Piedmont, St Gotthard, Gap in France. Greenish, twinned ; Miemo in Tuscany (Miemitc), and Tharand in Saxony (Tharandite). The massive and compact varieties are very common, and are valued as building stones (cathedral of Milan, York Minster, and the Houses of Parliament at Westminster). The Parian marble, and also the Sutherland and lona marbles, belong to this species. 274. ANKERITE, CaC + (MgFe)C. R 106 12. Usually massive and granular. Otherwise like siderite. Unst (Shetland), Styria. 275. MAGNESITE, MgC. Rhombohedral; 107 10 -30. Reniform or massive. H. =3 5; G. =2 85 to 2 95. Subtranslucent or opaque; streak shining. Snow-white, greyish or yellowish white, and pale yellow. Tyrol, Norway, North America. 276. BREUNNERITE (Giobertitc), MgC-f (MnFe)C. Hexagonal rhombohedral ; R 1 07 10 -30. Granular or columnar. Cl. R, very perfect. H. = 4 to 4 5 ; G. = 2 &quot;9 to 3 1. Transparent or translucent on the edges ; highly vitreous. Colourless, but often yellowish brown or blackish grey. C.c. essentially carbonate of magnesia, with 517 carbonic acid and 48 3 magnesia, but often mixed with 8 to 17 carbonate of iron or manganese. Unst, Tyrol (in Fassa Valley, &c.), St Gotthard, Harz. 277. SIDERITE (Sparry Iron, Chalybite), FeC. Hexagonal and rhombohedral ; R 107. Chiefly R, often curved, saddle-shaped (fig. 232), or lenticular. Cl. rhombohedral along R, perfect; brittle. H. =3 5 to 4 5 ; G. =37 to 3 9. Translucent in various degrees, becoming opaque when weathered; vitreous or pearly. Rarely white, generally yellowish grey or yellowish brown, changing G. =2-9 to 3-1.