Small crystals of harmotome with a cruciform penetration twinning habit were found on the joint surfaces of a dike of anorthositic gabbro flanked by serpentine at Glen Riddle, Delaware County, Pennsylvania.

The dike is composed of two distinct rock types. (1) A hard gabbroic border phase consisting chiefly of sodic labradorite and 25 per cent biotite. (2) A crumbly central portion consisting chiefly of sodic oligoclase and 10 per cent biotite. Irregularly spaced crush zones parallel to the vertical foliation in the border rock contain corundum, hyalophane (Cn)15) and andesine (An)40). Similar zones occurring chiefly along the margins of the central portion contain hyalophane (Cn)10) and oligoclase (An )15-20).

The feldspars in the border rock show strain effects, and the crush zones contained therein show evidence of hydrothermal reaction and replacement such as hyalophane-plagioclase antiperthites. Similar zones in the central portion show little evidence of strain and contain patchy aggregates of hyalophane and albite-oligoclase. The most alkaline zones in the central portion reveal no evidence of strain. In these zones, sparsely distributed grains of hyalophane with indented and frayed borders are surrounded by albite-oligoclase.

The contemporaneous deposition of the hyalophane and corundum and the later deposition of the harmotome is considered to be the result of (1) deformation of a three foot wide dike of anorthositic gabbro, emplaced in pyroxenite, by forces accompanying the intrusion of a hornblende granite close by, and (2) infiltration of hydrothermal solutions (or superheated vapors) rich in potash, barium and soda, which emanated from the granite at an advanced stage of cooling, and which initiated a series of reactions along favorable channels within the dike rock, resulting in the deposition of the mineral suites described.

The hyalophane-andesine antiperthites rvith corundum are considered to have been formed at relatively high temperatures; the hyalophane-albite-oligoclase patch perthites without corundum, at lower temperatures; and the association of albite-oJigoclase with indented grains of hyalophane, as representing partial resorption of previously formed hyalophane by late soda-rich solutions which deposited the resorbed potash and barium as harmotome in the joints and fissures of the dike rock.

The value of 2V for harmotome of 79°50' (described in Section 2) is at variance with figures given in standard texts, and suggests possible oversight on the part of workers to note the effect of ordinary heat of grinding on the optical properties of the mineral.

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