Document YrJ51aavMapED9xMOROabwO0O

JCCJS JCCJS Dartmouth College HANOVER NEW - HAMPSHIRE 03755 Department ofEarth Sciences - TEL 603 646-2373 R G. --S VE LEE 29 April 1977 Roger Miller Windsor Minerals Inc. Windsor Vermont 05089 Dear Roger I have read Pitfalls in ray Diffraction Methods of Analysis for Impurities in Talc Powders by J.B. Krause and W.H. Ashton If I apply the same standards that would be used in reviewing a paper for publication in a professional journal I believe that the language needs refinement and tightening The organization could be improved and referencing should be more detailed For example I see no reference to the Gandolfi method for XRD But the sciencies good The authors correctly identify speci- men flatness and placement tangent to the focussing circle as critical factors in accurate measurements of d Many accomplished diffractionists are unaware of the need for this care A displacement of 0.1 mm of the sample surface can lead to errors of 0.1 with modern diffractometers The authors correctly stress that it is fruitless to attempt amphibole species identification on the basis of the reflection at 8.4^ The amphibole group is very complicated chemically and hence in terms of nomenclature yet the structure is sufficiently similar throughout the group so that identifications based on a few lines are meaningless The authors point out the danger of identification based on a single peak Again this technique is risky Silicate minerals all have lattice planeisn great abundance whose spacings are of the order of 1.5 to 4^ However considerably more meaning can be attached to very large d single reflections such as the chlorite at 14.2^ A reflection thered can only be among the common silicate minerals vermiculite or chlorite The authors correctly stress the need to look for this reflection in separating the effects of chlorite from serpentinoer kaolinite Un- fortunately though some minerals provide only one peak of detectable in mixtures intensity These are the rhombohedral carbonates but they _ are not discussed in the paper I cannot speasko authoratatively on the morphological of talc and amphibole but their observations are in good accord own experience varieties with my JNJ 000252253 Miller Reynolds Page 2 29 April 1977 I must agree with their conclusions that diffraction patterns of mixtures of 14A chlorite and vermiculite and 7A kaolinite ser- pentine minerals are almost impossible to interpret unless the minerals happen to be well crystallized and of high abundance so that many re- flections are present Even then some varieties will give anomalous results For example rich chlorite has a very weak 001 and can be easily confused with serpentine or Kaolinite In addition theand ranges of unit cell parameters for these minerals overlap so that only some species can be confidently identified either separately or in mix- tures in talc The authors criticize using the quartz 211 samples This approach is so poor that I am to determine quartz surprised that any- one has used it In summary I find that the paper gives good evidence that the of authors know their business Each pitfall is a valid criticism ray methods I strongly agree with their conclusions that standard powder ray diffractometry cannot always provide accurate information on the kinds and amounts of minerals that are found in talc ores and talc products Sincerely RC Raynald Raynald ( Bob ) R.C. Reynolds Jr. Professor of Geology JNJ 000252254