A synthetic fluorophlogopite mica (KMg3A1Si30 IO F 2), in which the (OR) ions that are normally found in normal micas were complet.ely replaced by fluorine, has been syn thesized. This material has essentially the same physical and electrical properties as natural phlogopite mica, except with somewhat lower flexibility. To grow large, usable sheets of mica. it is necessary to obtain pr"ferred crystal orientation, which depends primarily on controlled thermal gradients, batch composition, and rate of cooling. Platinum crucibles were found to be the best material for holding t he batch during the m elt. Data on the physical, electrical, thermal, and chemical properties of the synthetic mica are given.

In a further stud.I' of t he a lka li-aggregate reaction in high-alkali cements, 11 reactiv e a nd nonreactive experimental aggregates were examined by several procedures. On comparing t he results of each m ethod with th e standard expansion-bar test, it appears that eac h property studied has some bearing on the expansion characteristics of each aggregate. For exa,mple: (1) The chemical durabili ty as determined by the interferometer procedure was poor for t he aggregates t hat showed excessive expansion, but not a ll aggregates that are shown to be chemically reactive by this procedure will cause expansion, (2) all aggregates are suffi cie ntly hygroscopic to attract the water necessary for t he reaction if other properties arc also favorable, (3) s urface electrical-resistance measurements confirm the hygrosco picity determinations; however, fr ee ions in glasses may have considerably more effect on the surface resistance of certain glasses than do thei r hygroscopic properties, and (4) the uneve n di stribution of migratable ions bet\\'een the aggregate surface and the outer phase offers a mechan ism for the' osmotic swellin g of individual aggregate grains, with accompany ing cxp,wsioll in CO Il CI' l'{l' members.

A direct comparison of the chemical r ea ctivi ty at t he surface with t.h at in t~e bulk volume of buffer solutions over the r ange pH 7 t o pH 11.8 was m ade, u s~ng Cornll1~ 01 5 glass as the indicator. By observing in terferometrically the surface alteratIOns of opt ically flat sp ecimen s brought a bout by exposures to a q.u eous buffer solutIOns und ercontr~ll e~ conditions of t ime temperature, and pH it was possible to eva luate, at least semlquantltatlvely, t he difference b etween the che mica l react ivity of t he s urface a nd the bulk volume of the solutions. For t he buffer a t pH 7 t he surfa ce reactivity appeared to corres pond to a h ydrogen ion activity o f pH 9.4. Th e a ppare nt concentra tion different ial between th e s urfa ce a nd bulk volume falls off sharply for buffer s of in creased pH, a moun tlllg t o only 0.2 pH Ul1I t for the buffer at pH 11.0, a nd becomes nondetectable at pH 11.~. U nfo r~un a tel y t he chemical dura bili ty ch ar acteri stics of t he glass were such that no d iff erentiatio n COUld. be ma de between t he h ydrogen ion activ ity of t he surface a nd the bulk volumc III solutIOns more a cid t ha n pH 7.

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