No. 2329. HYDROTALCITE GROUP OF MINERALS— FOSHAG. 



153 



Comparison of analyses of hrugnatclUte icitli values to suit formulas. 



Constituents. 



MgCOs.SMg 



(OH)2li 



Mg(0H)2 



Fe(0H)3. 



4H2O. 



FegOa 

 MgO. 

 CO2.. 

 H2O., 



12.40 



46. 87 



6.87 



33.75 



Which of these formulas is the correct one, or whether brug-natel- 

 lite is identical with pyroaurite can only be determined by additional 

 analyses. 



Pyrognostics. — Before the blowpipe the mineral turns golden yel- 

 low, but does not fuse and becomes magnetic. In the closed tube it 

 turns black, then golden yellow; yields considerable water, and be- 

 comes magnetic. 



Pelloux describes a mineral from Mount Kamazzo as brugnatellite, 

 but gives no analysis. Specimens from this locality in the United 

 States National Museum (No. 93008) show a light brown, pearly 

 mineral with an index of refraction of about 1.586, and very low 

 birefringence, not pleochroic. This material is too scanty and impure 

 for an analysis. 



A comparison of the properties of these minerals further brings 

 out the isomorphous relationship : 



Comparison of properties of minerals of the hydrotaleite group. 



SUMMARY. 



The formula for the hydrotalcite group of minerals is shown to be 

 MgCOs. 5Mg(0H).. 2E(OH)3. 4H2O, and the present members are 

 hvdrotalcite, MgCOs. 5Mg(6H)2. 2A1(0H)3. 4H.0; stichtite, 

 MgCOa. 5Mg(OH)3. 2Cr(0H),. 4H.0; pyroaurite, MgCOa- 

 5Mg(OH)2. 2Fe(OH)3. 4H2O. 



The chemical composition of brugnatellite and its relation to the 

 other minerals of this group remains in doubt. 



The three members have apparently the same mode of genesis and 

 result from the alteration of the corresponding members of the spinel 

 group. 



