Page:Radio-activity.djvu/578

 varieties found are known as cleveite and bröggerite, and among the primary associated minerals are orthite, fergusonite, monazite, and thorite. The period is stated to be Post-Devonian.

4. Llano County, Texas. The variety of uraninite known as nivenite is found here in a quartzose pegmatite, associated with the primary minerals gadolinite, allanite and fergusonite, and the secondary minerals cyrtolite, yttrialite, gummite, and thorogummite.

Secondary uraninite is found at Johanngeorgenstadt, Marienberg and Schneeberg in Saxony, at Joachimsthal and Přibam in Bohemia, at Cornwall in England, and at Black Hawk, Colorado, and in the Black Hills, South Dakota, in the United States. The exact geological period of most of these secondary occurrences is somewhat uncertain, but they are undoubtedly very much later than the primary occurrences mentioned above.

As a matter of general interest the analysis of a typical primary uraninite (No. 1) and of a typical secondary uraninite (No. 2) is given below :

No. 1                   No. 2 Glastonbury, Conn.   Johanngeorgenstadt, Saxony Sp. Gr. 9·59                    6·89 UO_{3}                       26·48                    60·05 UO_{2}                      57·43                    22·33 ThO_{2}                      9·79 CeO_{2}                      0·25 La_{2}O_{3}                  0·13 Y_{2}O_{3}                   0·20 PbO                          3·26                     6·39 CaO                          0·08                     1·00 He                           und. und. H_{2}O                       0·61                     3·17 Fe_{2}O_{3}                  0·40                     0·21 SiO_{2}                      0·25                     0·50 Al_{2}O_{3}                                        0·20 Bi_{2}O_{3}                                        0·75 CuO                                                0·17 MnO                                                0·09 MgO                                                0·17 Na_{2}O                                            0·31 P_{2}O_{5}                                         0·06 SO_{3}                                             0·19 As_{2}O_{3}                                        2·34 Insoluble                    0·70

The following list comprises the more important radio-active minerals, with their approximate chemical composition and some notes on their occurrence and probable origin.