FELDSPAR. 



Feldspar is the family name of sev- 

 eral minerals closely related and indeed 

 . grading- into each other, but distinguished 

 by mineralogists by separate specific 

 terms. These minerals are all silicates 

 of aluminum, with some alkali or alkali 

 earth, having a hardness of about 6 in 

 the scale in which quartz is 7 and a 

 specific gravity varying from 2.5 to 2.^. 

 They are fusible with difficulty before 

 the blowpipe, crystallize in the mono- 

 clinic or triclinic system and cleave in two 

 well-marked directions nearly or quite at 

 right angles to each other. It is this 

 latter property, probably, which led to 

 the grouping of these minerals as spar, 

 since this term is applied in common 

 language to any minerals which break 

 with bright crystalline surfaces. Thus 

 calc spar is a common name for calcite, 

 heavy spar for barite, needle spar for 

 aragonite, and so on. The term field 

 spar, of which Feldspar is probably a 

 corruption, was perhaps given the min- 

 erals of this group because of their 

 widespread occurrence. The English 

 spelling of the word, is Felspar. The 

 Feldspars form an essential part of 

 nearly all eruptive rocks and by their de- 

 composition produce clays and other soils 

 which may harden into great areas of 

 sedimentary rocks. They are thus of 

 great geological importance and interest. 

 Usually the white crystals to be seen in 

 an eruptive rock in contrast to the dark 

 green or black of the pyroxene or horn- 

 blende, or the glassy, nearly colorless 

 quartz, are Feldspar. The Feldspar may, 

 however, contain more or less iron and 

 then take on a flesh color or become even 

 darker. Feldspar crystals can best be 

 recognized by their prominent cleavage, 

 which appears as numerous bright flat 

 surfaces extending in any s^iven crystal 

 in the same direction. The crystals, 

 while they mav be of so minute dimen- 

 sions as to be visible only with the micro- 

 scope, may on the other hand reach in 

 veins in coarse-grained granites a length 

 of a foot or more. 



As ornamental stones only certain 

 varieties of Feldspar are valued and 

 their value depends on accidents of color 

 or structure. The first of the Feldspars 

 which may be mentioned as being prized 

 as an ornamental stone is amazonstone 

 or green Feldspar. This in composi- 

 tion is what is called a potash Feldspar, 

 potash being the alkali which in com- 

 bination with alumina and silica goes 

 to make up the mineral. The percent- 

 ages of each in a pure amazonstone are 

 siHca 64.7, alumina 18.4 and potash 16.9. 

 The mineralogical name of the species 

 is micro-cline, meaning small inclination, 

 and refers to the fact that the angle be- 

 tween the two cleavages of the mineral is 

 not quite a right angle. The common 

 color of miicrocline is white to pale yel- 

 low, but occasionally green and red oc- 

 cur. 



It is only to the green variety that the 

 name of amazonstone is applied, a name 

 meaning stone from the Amazon river. 

 It first referred probably to jade or some 

 such green stone from that locality and 

 then came to include green Feldspar. No 

 occurrence of green Feldspar in that 

 region is now known. 



Practically all the amazonstone now 

 used for ornamental purposes comes 

 from three localities. These are the 

 vicinity of Miask in the Ural Mountains. 

 Pike's Peak, Colorado, and Amelia Court 

 House, Virginia. In all these places the 

 amazonstone occurs in coarse-grained 

 granite and is closely accompanied by 

 quartz and Feldspar. All gradations 

 are found in color from the deep green 

 to white, only the bright green being 

 prized for ornamental purposes. The 

 Feldspar is usually well crystallized and 

 crystals of several pounds weight may 

 be found. A crystal will rarely be 

 of a uniform color, streaks of paler 

 green or white being commonly present. 

 Only the uniformly colored portions are 

 prized for ornamental purposes. The 

 green often takes on a bluish tone and 

 blue sometimes even predominates. The 



74 



