Absorption spectra were measured in the range 5000 to 250 cm−1 for 26 analyzed zoisites and epidotes, Ca2(Al1-pFe3+p)AL2(OH)[Si2O7][SiO4], wherein 0.00< p < 0.11 (zoisites) and 0.00 < p < 0.89 (epidotes). From their IR-spectra, both polymorphs can be distinguished by the position of the main OH-valence vibration (zo.: 3260 cm−1; cl-zo. (p = 0.00) – ep. (p = 0.89): 3326-3365 cm−1), by a band at 2160 cm−1, present only in zoisite, and by completely different band shapes in the ranges 820-700 cm−1 and 540-320 cm-−1. Appreciable shifts of 5 bands were found to be a function of p, the Fe(III) content of the epidote within the epidote series. These shifts can be represented by the linear equation î = A + Bp. The values for A (representing the position of each band at p = 0.00) and B are as follows: 3325.8 cm−1, + 48.3; 1045.6 cm−1, –12.4; 741.7 cm−1, –25.7; 419.4 cm−1, –27.2; 364.2 cm−1, –13.2

Oxygen-oxygen distances within the O(10)-H-Oxygen-oxygen distances within the 0(10)-H…-O(4) bridge in the structures of zoisite and epidotes were calculated from the position of the main OH-valence vibration using a Lennard-Jones (6-12) potential function (Bellamy and Owen, 1969) zo.: Rm = 2.87Å ep.: 2.92 Å (p = 0.00) – 2.96 Å (p = 1.00). From the presence of a band at 2160 cm−1 in all zoisites–which at present can only be interpreted as a low energy OH-valence vibration–we tentatively conclude that some of the protons in zoisite are located in a strong bridge 0(x)-H …O(y) with RIR = 2.56Å. However, it is uncertain which oxygen atoms of the zoisite structure (Dollase, 1968) are involved in this bridge.

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