. Bulletin of the British Museum (Natural History), Mineralogy. 128 PETROLOGICAL STUDIES IN GLEN URQUHART, INVERNESS-SHIRE zoned from clinozoisite cores to epidote rims. Beautiful examples of zoning may be found, picked out between crossed nicols by variations in their bright second-order interference colours. Compositions of these minerals are most readily determined in sections cut normal to the b axis of individuals twinned on (lOo), in which the diagnostic extinction angle a, /\ c can be measured to the twin plane. No yff-clinozoisite (Johnston, 1949) has been found. Text-fig. 2 shows a cl
. Bulletin of the British Museum (Natural History), Mineralogy. 128 PETROLOGICAL STUDIES IN GLEN URQUHART, INVERNESS-SHIRE zoned from clinozoisite cores to epidote rims. Beautiful examples of zoning may be found, picked out between crossed nicols by variations in their bright second-order interference colours. Compositions of these minerals are most readily determined in sections cut normal to the b axis of individuals twinned on (lOo), in which the diagnostic extinction angle a, /\ c can be measured to the twin plane. No yff-clinozoisite (Johnston, 1949) has been found. Text-fig. 2 shows a clinozoisite-epidote individual from an impure limestone band. It is truncated by a larger clinozoisite-epidote. Johnston (1949) and Winchell (1951) show that clinozoisite has its a-vibration direction in the obtuse angle /?, whilst epidote has its a-vibration direction in the acute angle (supplementary toy^). Thus the negative extinctions in Text-fig. 2 are in a-clinozoisite, the positive extinctions in epidote. Further, oscillatory zoning is present. There is a narrow core of clinozoisite, a wider irregular series of epidote zones and a final wide clino- zoisite rim. This is the only measured crystal in which a clinozoisite rim has been Fig. 2. Twinned clinozoisite-epidote crystal, from impure limestone in the block of schists within the serpentinite 300 m. west of Loch Maolachain. The extinction angles of the numbered zones are : i, a A c — 2° ; 2, a A c; + 4° ; 3, a A c —3° ; 4, a A c -1° ; 5, a A c -2° ; 6, a A c + 3° ; 7, a A c +6° ; 8, a A c + 10° ; 9, a A c — 2°. Very similar clinozoisite-epidote crystals are prominent in the skarns to be described in Part II. Clinozoisite-epidotes in the limestones vary from saussuritic material up to crystals i to 2 mm. in length. They are almost always in close association with plagioclase. Amphiboles. The amphiboles of the Umestones apparently belong to three transitional tjrpes. Actinolite, tremolite, and "
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