BRUCITE AND GIBBSITE. 235 



BEUCITE. 



occurrence. — BiTicite is one of the minerals which is produced in the 

 upper physical-chemical zone, especially in the belt of weathering. Brucite 

 is produced by the alterations of minerals rich in magnesia, being recorded 

 as secondary to chondrodite, clinohumite, humite, and serpentine. It is 

 especially prevalent in serpentinous rocks and veins. Doubtless in many 

 instances it forms simultaneously with the serpentine and perhaps other 

 minerals, rather than secondary to them. 



Alterations. — The one alteration of brucite noted is that of carbonation, 

 into hydromagnesite (monoclinic; sp. gr. 2.145-2.180). The reaction 

 representing the change is — 



4Mg(OH),+3CO,,=Mg 2 (C0 3 ) 3 .2Mg(OH).3H 2 0+k. 



The increase in volume is 73.08 per cent. The alteration is therefore 

 one of simple carbonation, and takes place in the zone of katamorphism, 

 especially in the belt of weathering, with expansion of volume and 

 liberation of heat. 



GIBBSITE. 



occurrence. — Gibbsite occurs as an accessory constituent in many of 

 the schists and gneisses, especially those which have been subjected to the 

 forces of the upper physical-chemical zone, and particularly in the belt of 

 weathering. As noted on subsequent pages, it may be a result of the 

 alteration of many minerals, the more important of which are as follows: 

 Anorthoclase, andalusite, biotite, cancrinite, corundum, cyanite, epidote, 

 hauynite, microcline, muscovite, nephelite, noselite, orthoclase, phlogopite, 

 plagioclases, pyrope, the scapolites, sillimanite, sodalite, topaz, tourmaline, 

 and zoisite. By reference to the discussion of these minerals and the 

 minerals which simultaneously form, the conditions of its formation may 

 be ascertained. 



Alterations. — No alterations of gibbsite are recorded in the standard text- 

 books, but where sedimentary rocks containing gibbsite are so deeply 

 buried as to pass into the zone of anamorphism it may become partly 

 dehydrated, producing diaspore (orthorhornbic ; sp. gr. 3.40), or wholly 

 dehydrated, producing corundum (rhombohedral ; sp. gr. 4.025); or the 

 aluminum may unite with other compounds, producing the same minerals 

 that are produced by corundum or diaspore. It is believed that these 



