This petro-fabric was identified and described by Torvinen (2018; Torvinen and Michelaki Forthcoming) using the modified Whitbread (C:F:V) approach (Quinn 2013). Chemical assays of thin sections selected from this petro-fabric were similar in composition to raw material samples collected from clay deposits in the Malpaso Valley of Zacatecas, Mexico. The raw chemical data were acquired using the Electron Probe Microanalyzer-Wavelength Dispersive Spectroscopy (EPMA-WDS) technique. An explanation of how the raw data were collected and processed is available in Torvinen and Michelaki (Forthcoming). Synthesized data for this petro-fabric are available at: https://core.tdar.org/dataset/458202/torvinen-2018-epma-wds-raw-data-for-sherd-samples. 

The primary inclusions of this fabric class and the closely related Altered Sand in Glassy Matrix (AS-G) Fabric are altered feldspar minerals, polycrystalline quartz, and sandstone fragments, but this fabric is differentiated by its fine-grained matrix in which glass shards are rare to absent (Torvinen and Michelaki N.d., Figure B.1-B.2). The clay matrix varies from light yellowish-brown or light grayish-brown to dark brown in PPL and is commonly very dark brown or dark yellowish-brown in XPL with weak optical activity. Voids tend to be meso- and macrovughs, some sherds have vughs with elongate extensions, and a few appear to have sericite formation in voids (PP 148, 160, 264). Some sherds assigned to this fabric have igneous rock fragments that appear to have micrographic textures (i.e., intergrowths of quartz and feldspar; PP 146, 149, 150, 151, 153, 154; Torvinen and Michelaki N.d. Figure B.2; MacKenzie and Adams 1994; Williams et al. 1954).

There are also two sets of sherds that may represent distinct sub-fabrics. Three sherds have relatively higher frequencies of igneous rock fragments (both plutonic and volcanic in origin) with single to open spacing, few mineral grains with high birefringence (suspected to be weathered amphibole), and some calcite replacement (PP 148, 152, 258; Torvinen and Michelaki N.d. Figure B.3). Another six sherds have lower amounts of the altered sand inclusions and are visually distinct due to the presence of coarse, angular to subangular feldspar (some twinned) and quartz grains in a very, very dark brown matrix (in XPL) with weak to no optical activity (PP 284, 286, 290, 293, 296, 297; Figure B.4). It is suspected that the lack of optical activity in these sherds is due to the full oxidation of the paste at a relatively high temperature. Further variation observed in inclusion grain size (coarse to fine) suggests another two potential sub-fabrics documented below, which require modal analysis to objectively verify their differentiation.

1. Coarse Altered Sand in Glassy Matrix: Predominant temper is coarse altered sand
   
   
   
   • Coarse AS-F – PP 2, 4, 26, 31, 43, 50, 53, 55, 91, 92, 93, 94, 96, 97, 98, 99, 133, 136, 137, 139, 142, 143, 144, 146, 147, 149, 150, 151, 153, 154, 155, 156, 157, 160, 184, 187, 188, 205, 222, 228, 237, 241, 245, 246, 249, 250, 252, 253, 259, 264, 266, 269, 271, 275, 277, 280, 285, 287, 289
   
   
   
   


2. Fine Altered Sand in Glassy Matrix (FAS-G): Predominant temper is fine altered sand
   
   
   
   • AS-F (fine) – PP 3, 9, 12, 13, 16, 60, 86, 87, 88, 89, 95, 100, 132, 134, 158, 159, 225, 226, 227, 229, 233, 234, 235, 236, 238, 239, 243, 244, 247, 248, 251, 291