ROCKS: THEIR CONSTITUENTS AND KINDS. 69 



106° 15', and this, with crystallized specimens, is an important means of distinction. 

 Composition: Carbonate of lime 54-4, carbonate of magnesia 45-6 = 100. Tormula, 

 QCaiMg)03C. 



SiDERiTE (iron carbonate). — A valuable ore of iron, sometimes called steel ore. Crys- 

 tallizes and cleaves like the preceding, but much heavier. G = 3-7-3 -9. Color white to 

 gray, but becoming brown on exposure to the air because the iron oxidizes easily and 

 changes to limonite. Cleavage angle 107°. Occurs also massive, gray to brown, with 

 feeble luster. Formula, FeO^C (= FeO + COj). 



Aeagonite. — Like calcite in composition, but occurring in prismatic form, without the 

 cleavages of calcite. Calcite and aragonite are hence paramorphs. G = 2-9-2-94, which 

 is above that of calcite. Shells, while consisting generally of calcium carbonate, often 

 have a large part of the material in the aragonite state ; and hence aragonite is present 

 through most uncrystalline limestones. 



8. Sulphates. 



Gtpscm (or hydrous calcium sulphate). — Very soft. H = 1. One of the few minerals 

 that may be easily impressed with the teeth without producing a grating sensation. Often 

 massive and fine granular. Colors from white to black ; the white is common alabaster. 



Also occurs in crystals, with pearly luster on a cleav- 

 54. 55. age surface. Figs. 54, 55 give two of the forms of 



the crystals. It cleaves in broad pearly plates or folia, 

 which look like mica, but are softer, and not elastic. 

 Unlike limestone and other minerals, a little heat 

 reduces it to powder, making the common pilaster of 

 Paris of the shops. It consists of sulphuric acid 46-51, lime 32-56, water 20-93 = 100. 

 Formula, CaO^S + 2 aq ( = CaO. SO3 2 aq) . . 



Anhydrite (calcium sulphate, without water). — White and gray- 

 ish, reddish. H = 3-3 1. Cleavage affords rectangular blocks or plates. 

 It differs from gypsum also in affording no water when heated. 



Baeite (or heavy spar, barium sulphate, also called barytes). 

 — Occurs in tabular crystals, some of the forms of which are 

 given in Fig. 56. It is remarkable for its high specific gravity 

 (G = 4-3-4-7), whence the name, from the Greek for iveight. It 

 contains sulphuric acid 34-3, baryta 65-7 = 100. Formula, BaO^S 

 (=BaO + S03). It is ground up and used for adulterating white 

 lead paint. It is common as a gangue of different ores. 



9. PJiosphates, Fluorides. 



Apatite (calcium phosphate). — Occurs in six-sided prisms of a greenish to bluish 

 color, often looking like beryl (and this deceptive appearance led to the name from the 

 Greek, signifying to deceive)., but easily distinguished from beryl by its inferior hardness, 

 as it maybe scratched with a knife. Composition: Phosphoric acid 40-92, lime 53-80, 

 chlorine 6-82 = 101-54, for a chlorine-bearing variety. Another kind contains fluorine 

 instead of chlorine. Much used for making a fertilizer. 



Fluoeite (fluor spar, calcium fluoride). — Crystallizes in cubes, octahedrons, and 

 other related forms, which cleave easily in four directions, parallel to the faces of the 

 regular octahedron, the faces of cleavage making angles with one another of 109^ 28'. 

 Often granular-massive. Easily scratched witli a file. Colors, clear purple, yellow, blue, 

 often white, and of other shades. Massive varieties are worked into vases, etc., which 

 have much beauty. "When powdered and thrown on a shovel heated nearly to redness, it 

 phosphoresces brightly. Composition : Fluorine 48-7, calcium 51-3 = 100. 



