Data for some African columbite-tantalite specimens

• Main Author: G.A.D. Zelt
• Format: Article
• Language: English
• Published: Geological Society of Finland 1975-12-01
• Series: Bulletin of the Geological Society of Finland
• Online Access: http://www.geologinenseura.fi/bulletin/Volume47/sgs_bt_047_pages_117_125.pdf
• ISSN: 0367-5211; 1799-4632

Twenty-three specimens from equatorial and southern Africa are investigated, primarily for X-ray powder data. Unit cell dimensions and volume, ranging from 414.7 Å3 (columbite) to 424.1 Å3 (manganotantalite) are given. A correlation between the unit cell volume and the 100 • Mn/(Mn + Fe) ratio is indicated and a further correlation to 100 • Ta/(Ta + Nb) is suggested. The degree of order is discussed according to the a0/c0 ratio. Five originally disordered specimens (a0/c0 = 2.769—2.811) and ones originally ordered specimen (a0/c0 = 2.832) were heated at 1,000° C for one hour. As a result the a0/c0 ratio in the disordered specimens increased to 2.829–2.833, causing the specimens to become ordered and the originally ordered specimen's a0/c0 ratio decreased to 2.788. Three X-ray diffractograms are presented [based on the averages of all diffractograms, and differences in: 1) disordered columbite or tantalite (110 peak may or may not be present, but when it is, it is very weak), 2) ordered Mn-tantalite (110, 200, 400, 600, and 023 peaks are primarily much stronger than in columbite or tantalite and the 711 + 512, 421, and 112 is present in Mn-tantalite but not in columbite or tantalite) and 3) disordered structures transformed into ordered structures (200, 400, 600, and 023 peaks have become more prominent, the 421, 711 + 512 are now present, and the 112 is not present]. Chemical analyses of some 108 columbite-tantalite specimens (including the specimens examined in this paper) are plotted according to their end members. The specimens approaching the Mn-tantalite end member are well represented. The atomic ratio 100 • Ta/(Ta + Nb) of 60 specimens (including the specimens investigated in this paper) are plotted against density. The ratio may be estimated according to the equation 100 • Ta/(Ta + Nb) = 37.209 d — 195 .4, where d = density. The calculated densities of the original 23 specimens are compared with the observed densities.