Thermal behaviour (phasetransitions andstructure deformations) of borax, tincalconite andkernite was studied by the high temperature X-raydiffraction method. These three minerals have one andthe same ratjo Na2O : B2O3 = 1 : 2 but are differingin contents of water and hydroxyl groups. Theinvestigation has given some new data. Boraxtransition into tincalconite takes place at 30-50℃,instead of 80℃ as it was indicated before. This factmakes the base to predict the behaviour of borax andtincalconite under daily temperature variations.Kenrnite, after being heated, i.e. dehydrated. turnsinto the new hydrated phase. Sharply anysotropiccharacter of the thermal expansion has been revealedfor borax (αb = -2, α11 = 77, α33 = 19 . 10(-6)℃-1)and kernite (α11 = 43, αb = 24, α33 = 10(-6) ℃-1),but the almost isotropic one - for tincalconite. Thestrongest anysotropy of borax and kernite expansionobserved in the monoclinic plane is caused by theshift deformations with β → 90°. This allows tosuggest that such anysotropy gives rise to heavily cracked areas of rocks.
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