Let me remind everyone that an email response to the MSA PIG email list comes only to me. It is not distributed to the mail list. I received this as part of an email to PIG: "After reading the description below, I think many of us would be keen to hear David's interpretation of the formation process of the massive quartz layer at Harding; we would be much obliged." Here is my response: The Jahns fence diagram that I sent shows the quartz layer to lie mostly against the upper border and wall zone at Harding. However, toward the east-southeast, Jahns portrayed that upper quartz layer pinching out above a larger, more central mass of quartz. In the fence diagram that appeared in Jahns and Ewing (1977), Jahns rendered only the upper quartz layer. In both maps, Jahns illustrated the massive quartz layer as conformable against the upper wall zone across the entire pegmatite. The quartz mass, therefore, was not cross-cutting of any units, and it was not, therefore, a late fracture-filling unit. I’ve sampled the upper border zone right at its contact with host mafic schist. There it is an exceedingly coarsely crystalline rock with abundant cm-scale crystals of beryl and apatite in a matrix of albite and lepidolite with accessory columbite; quartz is completely absent. The adjacent host mafic schist was altered in a layered sequence spanning about 15 cm outward from bityite, muscovite + spessartine, then biotite. Bityite is particularly abundant and it is a testament to the high Li and Be content of the melt that formed the border zone at Harding. Cameron et al. (1949) described the border zone at Harding as “perthite, quartz, and muscovite, with minor beryl, columbite-tantalite, microlite, apatite, spessartine, and spodumene.” (p. 27) Chakoumakos et al. (1990) described the upper border and wall unit as “quartz + albite + muscovite ± perthite”, but they noted that beryl occurs in such huge (0.5-1.0 meter) crystals and in such abundance that miners referred to the unit as the beryl zone. One could say that the crystallization of the pegmatite was anomalous from the start, but Cameron et al. (1949) observed that the border zones of pegmatites are typically the most mineralogically complex and diverse of any zone, and that the accessory minerals “may include any mineral present in the other parts of the pegmatite.” (p. 27) The disparities in the characterization of the upper border and wall zones are significant for two reasons: they imply that the units are laterally variable in composition, and that they are not granitic (i.e., close to the eutectic composition of granite) in composition anywhere along their length. Chakoumakos et al. (1990) listed quartz first as most abundant, but the places I sampled in person, and hundreds of samples from the dumps that contain the upper pegmatite-host rock contact, show it to be quartz-absent and as I described it above. These representations matter because if the initial crystallization along the upper margin at Harding produced an essentially quartz-absent assemblage, then a quartz-rich unit would follow it. There are papers I’ve written mostly derived from experiments with the Macusani obsidian, as good a proxy for the Harding pegmatite as any, that described two types of spatial segregation that match or mimic similar zonal variations seen in pegmatites. One is a spatial oscillation in mineralogy from the margins to center, wherein mineral assemblages, each of which departs from that of the bulk composition, crystallize in succession, and sometimes as a layered oscillation of the same principal minerals. Both types of variation are illustrated in London (2023) as Figure 11 (experimental) and Figure 14 (natural pegmatite).The other is a lateral variation in mineralogy along a crystallization front, which is actually more interesting and less likely to be known. In that case, one mineral or a portion of a granitic assemblage crystallized along a solidification front, whereas its complementary portion of that equilibrium assemblage crystallized elsewhere along the front. In the experiments MAC-46 and MAC-246 with the Macusani obsidian, petalite crystallized as a monominerallic solidification front alongside a graphic feldspar-quartz intergrowth. Together they formed an equilibrium assemblage but they were spatially separate “zones.” So, I could understand that a massive quartz layer would be expected to follow a previous zone that was quartz-absent, or I could envision the quartz layer as part of a primary assemblage that was crystallizing mostly feldspars elsewhere, e.g., the “blocky perthite” zone (Chakoumakos et al., 1990), which occupies the equivalent position of the massive quartz layer along the footwall. David London Pegmatite Interest Group: https://msaweb.org/pegmatites/