Biotite-controlled linear compositional weathering trends in tonalitic to quartz dioritic saprock, Santa Margarita Ecological Reserve, southern California, USA

Identifying a reference frame for calculating mass change during weathering: A review and case study utilizing the C# program assessing element immobility and critical ratio methodology

“…The above authors focused their work on the bulk chemical effects of biotite weathering as a control on linear compositional trends at each site, but did not attempt to quantify changes in elemental or bulk mass. As documented below, the results of our work generally support the idea that the loss of K mass as biotite alters to vermiculite ± mixed-layer biotite/vermiculite ± kaolinite is the major control on the linear compositional trends calculated for each site by Girty et al (2013), and also illustrates how illuviation can produce significant additions of elemental and bulk mass. Though Girty et al (2013) report chemical data from a 10-cm thick fracture rind at one site (i.e., ZR III discussed below), we did not include the 5 analyzed rind samples in our assessment of the usefulness of AEI, as they were little weathered relative to the adjacent corestone (e.g., average alteration intensity factor = 0.08 ± 0.14).…”

“…1B). Being more prevalent and more accessible then the underlying covered basement, corestones are commonly used as a proxy for unweathered parental rock (e.g., Girty et al, 2008Girty et al, , 2013Girty et al, , 2014Nesbitt and Young, 1989).…”

“…(B) In tectonically active regions, like the Peninsular Ranges, southern California, USA, sections like those shown in (A) are commonly eroded to varying degrees, and as result exhumed corestones make up a variety of unusual landforms dotting the landscape. Based on work discussed in Nesbitt andYoung (1984, 1989), Girty et al (2013Girty et al ( , 2014, and the second authors unpublished observations.…”

“…To illustrate the usefulness of AEI, three sites studied previously by Girty et al (2013) at Santa Margarita Ecological Reserve (SMER) were selected (Fig. 3).…”

A preliminary study on the new geotechnical weathering index in the evaluation of altered biotite granite

“…As such, quantification of the degree of weathering is critical in order to evaluate ground stability before infrastructure development [4,5]. Several chemical weathering studies have been performed over the last century to measure the extent to which a rock is weathered [6][7][8][9][10][11][12][13][14]. For example, Nesbitt and Young [10] and Harnois [11] developed the Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW), respectively, which are based on changes in whole-rock chemical compositions under the assumption that these changes are similar for all rock types.…”